Lysine: Functions

Following absorption, lysine is introduced into the hepatocytes (liver cells) of the liver via transport proteins. The liver is of paramount importance for intermediate protein and amino acid metabolism – similar to carbohydrates and lipids. Because the liver is anatomically located between the intestine and inferior vena cava, it is able to intervene in amino acid homeostasis and regulate amino acid supply to peripheral organs and tissues independently of food intake. All reactions of amino acid metabolism can take place in hepatocytes. The main focus is on protein biosynthesis (formation of new protein), which occurs continuously at the ribosomes of the rough endoplasmic reticulum (rER) of each cell. About 20% of the amino acids taken up are used for protein formation. The rate of synthesis is increased after a high protein intake. Lysine is required for the formation of the following proteins:

Structural proteins, such as collagen, which is a component of cellular membranes and gives the skin, bone and connective tissue in cartilage, tendons and ligaments the necessary mechanical stability.
Contractile proteins – actin and myosin allow the mobility of the muscles.
Enzymes, hormones – control of metabolism.
Ion channels and transport proteins in cell membranes – passage of hydrophobic and lipophilic molecules, respectively, through the biological cell membrane.
Plasma proteins – proteins that transport substances between tissues and organs in the blood, such as lipoproteins (transport of lipids), hemoglobin (transport of oxygen), transferrin (transport of iron), and retinol-binding protein (transport of vitamin A); in addition to transporting substances in the blood, the plasma protein albumin is also responsible for maintaining oncotic pressure
Blood clotting factors, such as fibrinogen and thrombin, which are involved in both extrinsic and intrinsic blood clotting, as well as in protective and defensive reactions of the organism
Immunoglobulins or antibodies – protection and defense against foreign substances.

In addition to protein biosynthesis, lysine is essential for the following processes:

Cross-linking of collagen fibers in the form of hydroxylysine.
Formation of biogenic amines
Synthesis of L-carnitine

Hydroxylation of lysine during collagen biosynthesis Following protein biosynthesis from mRNA – post-translationally – individual amino acids integrated in the protein can be modified enzymatically and non-enzymatically. Such structural modifications affect the functional properties of the proteins. Of particular importance is the posttranslational modification of lysine and proline in fibroblasts of connective tissue. After the biosynthesis of individual collagen polypeptide chains at the ribosomes of the rER, these enter the lumen of the ER of the fibroblasts – cells of the connective tissue. There, some lysine or proline residues of the collagen molecules are modified by hydroxygenases. Hydroxygenases represent enzymes with a divalent iron atom in the active site, which attach a hydroxyl (OH) group to their substrates, in this case lysine or proline. This OH group is quite crucial for the functionality of collagen as a structural protein. In parallel with the hydroxylation reactions, three collagen polypeptide chains are joined together in the lumen of the ER by the formation of hydrogen bonds and disulfide bonds, resulting in a three-stranded helical molecule – triple helix – called procollagen. Each collagen or triple helix can be composed of 600 to 3,000 amino acids, depending on the type of collagen. Subsequently, the procollagen, which partly contains hydroxylated lysine and proline residues, is transported from the ER to the Golgi apparatus of the fibroblasts. In the Golgi apparatus, sugar residues, such as glucose and galactose, are attached to the collagen hydroxylysine. Bonding occurs between the OH group of the hydroxylysine and the OH group of the sugar with elimination of water – O-glycosidic bond. As a result of this O-glycosylation, glycoproteins are formed, which help in protein folding or increase the stability of collagen. The hydroxylation of proline to hydroxyproline leads primarily to greater tensile strength and stability of the collagen triple helix.Procollagen is incorporated into secretory vesicles from the Golgi apparatus, transported to the cell membrane of a fibroblast and released into the extracellular space by exocytosis (fusion of the vesicles with the membrane). Subsequently, individual three-stranded collagen molecules assemble into collagen fibrils (fibrillogenesis). In a further step, covalent cross-linking of collagen fibrils occurs with the formation of collagen fibers, with cross-linking occurring at specific lysine and hydroxylysine residues. By definition, only tripelhelical molecules of the extracellular matrix are called collagens. Currently, 28 collagen types are known (type I to XXVIII), which belong to specific collagen families, such as fibrillar, reticular, or bead cord collagens. Depending on the collagen type, more or less lysine or proline residues are present in the hydroxylated state. Thus, in the basement membrane of cells, more than 60 % of the lysine molecules are modified. Up to 12 % of these are bound to carbohydrates. In cartilage, about 60 % of the lysine residues are also hydroxylated. Only a small proportion of these (4 %) are cohkinated with carbohydrates. In skin and bone, only 20 % of the lysine residues are present in the form of hydroxylysine. The carbohydrate fraction is negligible at 0.4%. For the hydroxylation of lysine and proline, the presence of vitamin C (ascorbic acid) is essential. Vitamin C influences the activity of hydroxygenase, which can only work optimally when its iron atom is in the divalent state. Various oxidizing agents, such as fluorine, oxygen, hydrogen peroxide and its adducts, are able to remove electrons from the trace element iron. Thus, iron is rapidly converted from its divalent (Fe2+) to its trivalent form (Fe3+), resulting in impairment of hydroxygenase activity. Vitamin C counteracts this. As a reducing agent, ascorbic acid maintains the divalent state of the iron atom of hydroxygenase. By transferring electrons, it reduces Fe3+ to Fe2+. Lack of vitamin C would lead to deficient hydroxylation of collagenous lysine and proline, resulting in the formation of damaged collagen molecules that cannot perform their structural protein function. Consequently, patients with the vitamin C deficiency disease scurvy often suffer from symptoms due to the defective biosynthesis of collagen. These include poor wound healing, skin problems and inflammation as well as bleeding, muscle wasting, joint inflammation, fragile blood vessels, and bone pain due to bleeding under the periosteum (subperiosteal hemorrhage). In addition, vitamin C stimulates gene expression for collagen biosynthesis and is important for both the necessary exocytosis of procollagen from the fibroblast into the extracellular matrix (extracellular matrix, intercellular substance, ECM, ECM) and for the cross-linking of collagen fibrils. Formation of biogenic amines Among many other amino acids, lysine serves as a synthesis precursor of biogenic amines. In the case of lysine, cleavage of the carboxyl group – decarboxylation – produces the biogenic amine cadaverine, which also bears the name 1,5-diaminopentane. Like all other biogenic amines, cadaverine reacts as a base due to the presence of the amino group (NH2). As a proton acceptor, it can thus absorb protons (H+) at low or acidic pH values and thus increase the pH value. Since cadaverine is produced during bacterial protein digestion (putrefaction) and has basic character, the biogenic amine is also called putrefactive base. Cadaverine synthesis from lysine is facilitated by intestinal bacteria, specifically by their enzymes, decarboxylases. These require for the cleavage of the carboxyl group (CO2) – pyridoxal phosphate (PLP) and vitamin B6, respectively. PLP thus plays the role of a coenzyme and must not be missing in the decarboxylation of amino acids to biogenic amines. Biogenic amines represent the precursors (synthesis precursors) of the following compounds.

Alkaloids
Hormones
Coenzymes – the biogenic amines beta-alanine and cysteamine are components of coenzyme A, which serves as a universal transmitter of acyl groups in intermediary metabolism
Vitamins – beta-alanine is an essential component of vitamin B5 (pantothenic acid); propanolamine represents a building block of vitamin B12 (cobalamin).
Phospholipids – ethanolamine is required for the formation of phosphatidylethanolamine and -serine, respectively, a coagulant and thrombokinase-like substance.

Some free biogenic amines can even exert physiological effects themselves. For example, gamma-aminobutyric acid (GABA), which is produced from glutamate, and histamine and serotonin function as neurotransmitters – chemical messengers – in the central nervous system. Synthesis of L-carnitine and its involvement in cellular metabolism The human body can produce L-carnitine itself from the amino acids lysine and methionine. Oral intake of lysine results in a significant increase in plasma carnitine levels. For example, after a single dose of 5 g of lysine, the plasma level of carnitine increases sixfold over a 72-hour period. For carnitine synthesis, which takes place in the liver, kidneys and brain, the essential cofactors vitamin C, vitamin B3 (niacin), vitamin B6 (pyridoxine) and iron must be available in sufficient quantities in addition to lysine and methionine. L-carnitine is a naturally occurring vitamin-like substance involved in energy metabolism and plays a key role in the regulation of fat metabolism. L-carnitine is involved in the transport of long-chain fatty acids (C12 to C22) across the inner mitochondrial membrane and provides them for the beta-oxidation (breakdown of saturated fatty acids) that occurs in the mitochondrial matrix. While long-chain saturated fatty acids can easily cross the Outer Mitochondrial Membrane, they require L-carnitine as a transport molecule to also pass the Inner Mitochendrial Membrane. At the outer mitochondrial membrane, the fatty acid residues, the acyl groups, are activated by an ATP-dependent bond to coenzyme A – acyl-coenzyme A is formed. This activation is essential because fatty acids are relatively inert and can only enter into reactions in the form of acyl-CoA. Subsequently, also at the outer mitochondrial membrane, the fatty acid residue is transferred from coenzyme A to carnitine under the influence of carnitine palmitoyltransferase I (CPT I), which is also known as carnitine acyltransferase I. The resultant acyl carnitine is then converted into carnitine. The resulting acyl carnitine is now transported to the interior of the mitochondrion by a C-acylcarnitine translocase. There, carnitine palmitoyl or acyl transferase II transfers the acyl residue from carnitine to CoA, so that acyl-CoA is again present. The L-carnitine released in this process is returned to the cytosol of the cell in the antiport with acyl-carnitine by the translocase. The resulting acyl-CoA remains in the mitochondrial matrix and is now ready for degradation. Beta-oxidation, or the degradation of activated fatty acids, occurs stepwise in a repeating sequence of 4 individual reactions. The products of a single sequence of the 4 individual reactions include a fatty acid molecule that is two carbon atoms shorter in the form of acyl-CoA and an acetyl residue bound to coenzyme A, which is composed of the two split-off C atoms of the fatty acid. The fatty acid, which is smaller by two C atoms, is returned to the first step of beta-oxidation and undergoes another shortening. This reaction sequence is repeated until two acetyl-CoA molecules remain at the end. Acetyl-CoA flows into the citrate cycle for further catabolism. There, energy is produced in the form of GTP (guanosine triphosphate), reduction equivalents (NADH, FADH2) and carbon dioxide. NADH2 and FADH2 provide the necessary electrons for the subsequent mitochondrial respiratory chain. The result of the respiratory chain is again the production of energy, this time in the form of ATP (adenosine triphosphate), which is essential as an energy source for basic, energy-consuming processes in the organism. It is needed, for example, for the synthesis of organic molecules, active mass transport across biomembranes, and muscle contractions. Acetyl-CoA can also be used for the synthesis of ketone bodies or fatty acids. Both fatty acids and the ketone bodies acetoacetate, acetone and beta-hydroxybutyrate (BHB) represent important energy suppliers to the body. Ketone bodies are formed in the mitochondria of the hepatocytes (liver cells), especially during periods of reduced carbohydrate intake, for example during fasting diets, and serve as a source of energy for the central nervous system. In starvation metabolism, the brain can obtain up to 80% of its energy from ketone bodies.Meeting the energy demand from ketone bodies during dietary restriction serves to conserve glucose. As a substrate of carnitine palmitoyltransferase, carnitine is involved in the regulation of carbohydrate metabolism in addition to fat metabolism. Sufficiently high carnitine plasma levels are a prerequisite for an optimal reaction rate of CPT, which is active especially under physical stress and receives the fatty acids released from the fat depots at the mitochondria of energy-requiring cells and makes them available for L-carnitine. As carnitine acyltransferase I transfers acyl residues from acyl-CoA to carnitine, the pool of free coenzyme A in the mitochondrial matrix increases. The free CoA now enters glycolysis (carbohydrate catabolism), in which the monosaccharide (simple sugar) glucose is gradually degraded to pyruvate – pyruvic acid. For further catabolism of pyruvate, the free CoA is transferred to an acetyl residue to form acetyl-CoA, which is used to provide energy. Since pyruvic acid is continuously converted to acetyl-CoA by the presence of unbound CoA, it is only present in low concentrations. If lactate (lactic acid) accumulates in muscle tissue during intense exercise due to anaerobic conditions, lactic acid is metabolized to pyruvate due to concentration differences. Thus, excess lactate is degraded and the pool of pyruvate is maintained, which in turn is oxidatively decarboxylated to acetyl-CoA by the action of pyruvate dehydrogenase in the mitochondrial matrix. In addition, as a result of lactate catabolism, a drop in pH in muscle fibers is prevented, thus preventing premature fatigue. Other effects of L-carnitine:

Cardioprotective effect – carnitine improves the performance of the heart muscle in heart failure (inability of the heart to distribute the amount of blood required by the body as needed).
Lipid-lowering effect – carnitine lowers plasma triglyceride levels.
Immunostimulatory effect – carnitine is able to improve the function of T and B lymphocytes, as well as macrophages and neutrophils.

Limitations in the availability of L-carnitine, either due to inadequate intake or low plasma levels of lysine and methionine, lead to disturbances in energy metabolism. Low concentrations of carnitine, due to its carrier function, reduce both the passage of long-chain fatty acids across the inner mitochondrial membrane and the degradation of fatty acids in the mitochondrial matrix. As a result of the accumulation of long-chain, non-utilizable acyl-CoA esters in the cytosol of cells and the deficient beta-oxidation, the ATP supply and thus the energy supply of the cells suffers. This particularly affects the cardiac muscle, which is dependent on fatty acid breakdown as the main source of energy production due to its low stores of glycogen – storage form of glucose. The energy deficit caused by carnitine deficiency leads to circulatory disturbances that significantly reduce oxygen transport to the heart. This increases the risk of suffering angina pectoris symptoms, which are characterized by a burning, tearing, or cramping sensation in the heart region. The mismatch between oxygen demand and oxygen supply results in myocardial ischemia (undersupply of oxygen to the myocardium), which is not infrequently the trigger for myocardial infarction (heart attack). Finally, sufficient availability of L-carnitine plays an important role in the prevention and therapy of metabolic disorders in the poorly perfused myocardium. Carnitine deficiencies also affect protein and carbohydrate metabolism. Due to the reduced utilization of fatty acids in carnitine deficiency, other substrates must be increasingly called upon to maintain energy supply. We are talking about glucose and proteins. Glucose is increasingly transported from the blood into the cells when energy is required, causing its plasma concentration to drop. Hypoglycemia (lowered blood glucose level) is the result. Deficient acetyl-CoA synthesis from fatty acids causes limitations in gluconeogenesis (new glucose formation) and ketogenesis (formation of ketone bodies) in the hepatocytes of the liver. Ketone bodies are particularly important in starvation metabolism, where they serve as an energy source for the central nervous system.The energy-rich substrates also include proteins. When fatty acids cannot be used to obtain ATP, there is increased protein breakdown in muscle and other tissues, with far-reaching consequences on physical performance and the immune system.

L-carnitine in sports

Carnitine is often recommended as a supplement to those individuals who are seeking body fat reduction through exercise and diet. In this context, L-carnitine is said to lead to increased oxidation (burning) of long-chain fatty acids. In addition, the intake of carnitine is expected to increase endurance performance and speed up regeneration after intense exercise. Studies have shown that an increased carnitine intake with food only leads to an increase in performance or a decrease in body weight via the stimulation of fat breakdown if there was previously a reduced L-carnitine concentration in the muscle fibers, either as a result of insufficient intake, increased losses or genetically or otherwise caused restrictions in carnitine synthesis. In addition, L-carnitine supplementation also benefits individuals with body fat loss who regularly engage in endurance exercise and those with increased energy requirements. The reason for this is the mobilization of triglycerides from the fat depots, which is increased during aerobic endurance exercise as well as during energy deficiency. The breakdown of fatty acids in adipose tissue and the subsequent transport of free fatty acids in the bloodstream to the energy-requiring myocytes (muscle cells), is an essential prerequisite for the effectiveness of L-carnitine. In the mitochondria of the muscle cells, carnitine can finally perform its function and make the free fatty acids available for beta-oxidation by transporting them into the mitochondrial matrix. Consequently, sufficiently high plasma levels of carnitine are important to ensure the priority utilization of fatty acids as the main energy suppliers of skeletal muscle at rest, in the postabsorptive phase, during starvation, and during long-term endurance exercise, and thus to lose excess body fat. By primarily utilizing fatty acids, L-carnitine has a protein-sparing effect during catabolic conditions, such as endurance training or starvation. It provides protection from important enzymes, hormones, immunoglobulins, plasma, transport, structural, blood clotting and contractile proteins of muscle tissue. Thus, L-carnitine maintains performance and has immunostimulatory effects. Among other studies, scientists from the University of Connecticut in the USA also found that L-carnitine intake significantly improves average endurance performance and results in faster recovery after major physical exertion. These effects are presumably due to the good energy supply of the cells by L-carnitine, which results in increased blood flow and improved oxygen supply to the muscles. Furthermore, a sufficiently high L-carnitine concentration in the blood of healthy recreational athletes leads to significantly lower production of free radicals, less muscle soreness and less muscle damage after exercise. These effects can be explained by the increased breakdown of lactate, which accumulates during intense exercise as a result of a lack of oxygen. Drinking caffeinated beverages, such as coffee, tea, cocoa or energy drinks, can support oxidative fatty acid catabolism in the mitochondria and contribute to body fat reduction. Caffeine is able to inhibit the activity of the enzyme phosphodiesterase, which catalyzes the breakdown of cAMP – cyclic adenosine monophosphate. Thus, a sufficiently high concentration of cAMP is available in the cells. cAMP activates lipase, which leads to lipolysis – cleavage of triglycerides – in adipose tissue. This is followed by an increase in free fatty acids in adipose tissue, their removal in plasma to the liver or muscles with the help of the transport protein albumin, and subsequent cellular beta-oxidation. It has been known for some time that the consumption of coffee prior to endurance exercise has benefits for fat loss. However, coffee should be avoided before long-term endurance exercise. Due to its diuretic effect, caffeine promotes fluid loss via the kidneys, which is increased in endurance athletes anyway. Athletically active people should pay attention to a high lysine intake in order to maintain carnitine plasma levels at a high level.Likewise, regular intake of methionine, vitamin C, vitamin B3 (niacin), vitamin B6 (pyridoxine) and iron must not be ignored to ensure sufficient endogenous carnitine synthesis. During physical exertion or in a state of starvation, L-carnitine is inevitably lost from the muscle and the excretion of L-carnitine esters in the urine increases. The losses increase the more free fatty acids (FFS) from adipose tissue are offered to the muscle. Consequently, there is an increased need for L-carnitine for individuals who exercise or diet a lot. The losses can be compensated by an increased endogenous synthesis from lysine, methionine and the other essential cofactors as well as by an increased carnitine intake via food. L-carnitine is mainly absorbed via meat. Rich in carnitine is red meat, especially sheep and lamb. In contrast to athletically active people, an increased carnitine intake does not lead to increased fatty acid oxidation in non-athletes or physically inactive people. The reason for this is that physical inactivity results in insufficient or no fatty acid mobilization from the fat depots. As a result, neither beta-oxidation in the mitochondria of cells nor reduction of body fat tissue can occur. Other functions of lysine and their applications.

Enhancing effect on arginine – By delaying the transport of arginine from the blood into cells, lysine provides an increased arginine plasma concentration. Arginine belongs to the semi-essential – conditionally indispensable – amino acids and is found in almost all proteins. It can be synthesized in the organism from glutamate or ornithine, citrulline and aspartate, respectively, and is integrated in the ornithine cycle, which is localized in the liver. In the ornithine cycle, the cleavage of arginine results in the biosynthesis of urea. In this way, the ammonia released from amino acids can be detoxified. In addition, arginine is the sole precursor of nitric oxide (NO), which plays a crucial role in vasodilation and inhibition of platelet aggregation and adhesion. NO counteracts endothelial dysfunction (impaired vascular function) and thus atherosclerotic changes. Sufficiently high plasma arginine levels continue to be important for STH secretion. Somatotropic hormone (STH) stands for somatotropin, a growth hormone produced in the adenohypophysis (anterior pituitary gland). It is essential for normal length growth. Its production is particularly pronounced during puberty. STH affects almost all tissues of the body, especially bones, muscles and liver. Once the genetically determined body size is reached, somatotropin mainly regulates the ratio of muscle mass to fat.
Increased absorption and storage of calcium in bones and teeth – the intake of lysine-rich foods or supplementation with lysine is beneficial for osteoporosis patients.
Increased absorption of iron – one study found that increased lysine intake positively affected hemoglobin levels in pregnant women. Hemoglobin is the iron-containing red blood pigment of erythrocytes (red blood cells).
Herpes simplex – Lysine may help cure herpes infections. Thus, a study of herpes simplex patients who received 800 to 1,000 mg of lysine daily during the acute phase of infection and 500 mg per day for maintenance resulted in significantly accelerated healing. By some experts, the use of lysine is also considered exceedingly useful in genital herpes.
Wound healing – as an essential component of collagen, adequate intake of lysine-rich foods optimizes the healing of wounds. Lysine, together with proline in the hydroxylated state, is responsible for the formation of collagen fibers through cross-linking of collagen fibrils and for the stability of collagen molecules.
Atherosclerosis (arteriosclerosis, hardening of the arteries) – lysine can be used for the prevention and treatment of atherosclerosis. Atherosclerosis is an arterial occlusive disease in which there are deposits of blood fats, thrombi, connective tissue and calcium in the arterial or vascular walls. Lysine prevents the deposition of lipoprotein (a) – Lp(a) – and thus renders it ineffective.L(a) represents a fat-protein complex and is structurally similar to LDL (low density lipoprotein), the so-called “bad cholesterol“. Because Lp(a) is a particularly “sticky” lipoprotein, it is responsible for the majority of fatty deposits in the arterial wall. Finally, Lp(a) is an independent risk factor for atherosclerosis and its sequelae. Separately, Lp(a) promotes thrombus (blood clot) formation by inhibiting fibrin cleavage in the vessel lumen via displacement of plasmin. Fibrin is an activated, cross-linked “glue” of plasmatic blood clotting and leads to the closure of wounds via formation of a blood clot. In addition, lysine can degrade pre-existing atherosclerotic plaque by removing deposited Lp(a) and other lipoproteins in the arterial wall. Studies have elucidated the importance of lysine in the treatment of atherosclerosis. Over a period of 12 months, 50 men and 5 women in different stages of the disease were given 450 mg of lysine and proline per day in combination with vitamins, minerals, trace elements and 150 mg of cysteine, L-carnitine and arginine per day. After these 12 months, ultrafast computer tomography revealed that the progression of atherosclerosis had been clearly slowed down or almost halted. Hardly any new plaques were formed in the vessel walls of the patients. In all subjects, the rate of growth of atherosclerotic deposits in the coronary vessels was reduced by an average of 11%. Patients in the early stages of the disease responded significantly better to the therapy. In these patients, the rate of plaque growth was reduced by 50 to 65 %. In one case, the calcification of the coronary vessels was even reversed and the disease cured. It is assumed that the significantly reduced formation of further atherosclerotic deposits is based on the synergistic effect of all administered vital substances.

Biological valence

The biological protein value (BW) means the nutritional quality of a protein. It is a measure of the efficiency with which a dietary protein can be converted into endogenous protein or used for endogenous protein biosynthesis. The similarity between dietary and endogenous protein depends on the amino acid composition. The higher the quality of a dietary protein, the more similar it is to the body’s protein in its amino acid composition, and the less of it needs to be ingested to maintain protein biosynthesis and meet the organism’s requirements – provided the body is adequately supplied with energy in the form of carbohydrates and fats, so that dietary proteins are not used for energy production. Of particular interest are the essential amino acids, which are important for endogenous protein biosynthesis. All of these must be present simultaneously for protein formation at the site of synthesis in the cell. An intracellular deficit of only one amino acid would bring the synthesis of the protein in question to a halt, requiring the degradation of the sub-molecules already built. The essential amino acid that is the first to limit endogenous protein biosynthesis due to its insufficient concentration in dietary protein is called the first-limiting amino acid. Lysine is the first-limiting amino acid in proteins, especially in glutelins and prolamins of wheat, rye, rice, and corn, as well as in linseed and rapeseed protein. To determine the biological value of proteins, the two nutrition researchers Kofranyi and Jekat developed a special method in 1964. According to this method, for each test protein, the amount sufficient to maintain the balance of nitrogen balance is determined – determination of the N-balance minimum. The reference value is whole egg protein, whose biological value was arbitrarily set at 100 or 1-100 %. Among all individual proteins, it has the highest BW. If a protein is utilized by the body less efficiently than egg protein, the BW of this protein is below 100. Proteins from animal foods have higher BW than the proteins from plant sources due to their more similar amino acid composition to body protein. Consequently, animal protein generally meets the needs of humans better.To give an example, pork has a BW of 85, while rice has a BW of only 66. By cleverly combining different protein carriers, foods with a low biological value can be enhanced. This is known as the complementary effect of different proteins. Cornflakes, for example, have a very low BW because they contain only small amounts of the essential amino acid lysine. They are almost worthless as protein suppliers. Mixing them with milk, however, significantly increases the BW of the cornflakes protein, as the protein fractions of milk, such as casein and lactatalbumin, contain abundant lysine and are therefore of high biological value. With the help of the supplementation effect of individual proteins, it is possible to achieve a BW higher than that of whole egg protein. The greatest supplementation effect is achieved by combining 36% whole egg with 64% potato protein, which achieves a BW of 136.

Lysine degradation

Lysine and other amino acids can in principle be metabolized and degraded in all cells and organs of the organism. However, the enzyme systems for catabolism of the essential amino acids are found predominantly in hepatocytes (liver cells). During the degradation of lysine, ammonia (NH3) and an alpha-keto acid are released. On the one hand, alpha-keto acids can be used directly for energy production. On the other hand, since lysine is ketogenic in nature, they serve as a precursor for the synthesis of acetyl-CoA. Acetyl-CoA is an essential starting product of lipogenesis (fatty acid biosynthesis), but can also be used for ketogenesis – synthesis of ketone bodies. From acetyl-CoA, the ketone body acetoacetate is formed via several intermediate steps, from which the other two ketone bodies acetone and beta-hydroxybutyrate are formed. Both fatty acids and ketone bodies represent important energy suppliers to the body. Ammonia enables the synthesis of nonessential amino acids, purines, porphyrins, plasma proteins, and proteins of infection defense. Since NH3 in free form is neurotoxic even in very small amounts, it must be fixed and excreted. Ammonia can cause serious cell damage by inhibiting energy metabolism and pH shifts. Ammonia fixation occurs through a glutamate dehydrogenase reaction. In this process, ammonia released into extrahepatic tissues is transferred to alpha-ketoglutarate, producing glutamate. The transfer of a second amino group to glutamate results in the formation of glutamine. The process of glutamine synthesis serves as a preliminary ammonia detoxification. Glutamine, which is mainly formed in the brain, transports the bound and thus harmless NH3 to the liver. Other forms of transport of ammonia to the liver are aspartic acid (aspartate) and alanine. The latter amino acid is formed by binding of ammonia to pyruvate in the muscles. In the liver, ammonia is released from glutamine, glutamate, alanine and aspartate. NH3 is now introduced into the hepatocytes (liver cells) for final detoxification using carbamyl-phosphate synthetase in urea biosynthesis. Two ammonia molecules form a molecule of urea, which is non-toxic and is excreted via the kidneys in the urine. Via the formation of urea, 1-2 moles of ammonia can be eliminated daily. The extent of urea synthesis is subject to the influence of diet, especially protein intake in terms of quantity and biological quality. In an average diet, the amount of urea in daily urine is in the range of about 30 grams. Persons with impaired kidney function are unable to excrete excess urea via the kidneys. Affected individuals should eat a low-protein diet to avoid increased production and accumulation of urea in the kidney due to amino acid breakdown.