Function or effect of vitamin A derivatives.
| Substance group | Function or effect |
| Retinol | Transport form, bound in serum to retinol binding protein (RBP) and transthyretin (TTR). |
| 11-cis and all-trans retinal | In the rhodopsin cycle of the eye |
| Retinoic acid | Inhibits tumor promoters and important for proliferation and differentiation of various tissues (intestinal mucosa/intestinal mucosa, respiratory epithelium, skin)Various tumor cells, premyeloid and myeloid forms, embryonic forms |
| Retinyl ester | Storage form of vital substance, occurs in liver, testes, retina, lungs |
| Glucuronated compounds – retinoic acid and retinol. | Excretory products affecting differentiation and growth |
Visual process
- Rhodopsin forms the visual pigment in the retina (retinal) and is a compound of the protein opsin and retinal
- 11-cis retinal can absorb light and is converted to the all-trans form when exposed to light
- Cleavage of retinal from rhodopsin.
- Activation of rhodopsin molecules, which subsequently activate transducin molecules.
- The result is hyperpolarization – increase in membrane potential – resulting in a nerve impulse that leads to sensory perception
- Conversion of the all-trans retinal to the 11-cis retinal, which binds to opsin and is thus reintroduced into the rhodopsin molecule.
Embryogenesis
- Retinoic acid-dependent receptors are expressed at very early stages of development and regulate the development of the skeletal system, neural tube, various organs and tissues
- Both too high and too low vitamin A intake can lead to malformations in the unborn child
Cell proliferation and differentiation
- Vitamin A derivatives are able to inhibit or promote the growth of various cells, or to induce differentiating or dedifferentiating effects – this occurs either by attack of vitamin A at the retinoid receptor, or independently of it
- Often the effect on growth and differentiation is coupled – retinoic acid inhibits the growth of a neoplastic cell and at the same time it comes to the differentiation of normal cells
- Vitamin A mainly regulates the regular differentiation of the mucous membrane of the respiratory tract and the skin
- Vitamin A also influences the growth and differentiation of epithelial, dental, and bone tissue, as well as placental and embryonic tissue.
By influencing cell growth and differentiation, vitamin A is essential for healthy development of skin, hair, eyes, mucous membranes, lymphatic vessels, gametes, bones, and teeth.
Immune System
- Retinol and its derivatives protect (by maintaining the structure and function of cell membranes) skin and mucosa (mucosal) cells of the airways, digestive tract, and urinary tract, which provide a barrier to bacteria, viruses, and parasites, and thus to infection
- Retinol and retinyl esters are involved in the production of antibodies – increased stimulation of antibody formation in leukocytes (white blood cells) – and activation of T lymphocytes (the main regulatory cells of the immune system).
- The carotenoids are attributed antioxidant and immunomodulatory properties as well as anticarcinogenic properties
The protection of skin and mucosal cells and increased antibody formation by vitamin A are prerequisites for a functioning immune system.
Other functions of vitamin A
- Initiation and control of the production of steroid hormones including corticosteroids.
- Erythropoiesis (red blood cell production) – retinoids are required for differentiation of stem cells into erythrocytes (red blood cells)
- Iron transport – vitamin A mobilizes iron from the stores, for incorporation into the hemoglobin (blood pigment) of erythrocytes (red blood cells).
- In addition to the synthesis of proteins and fats, vitamin A also plays an essential role in the formation of androgens and estrogens – a normal sperm count, shape and motility requires an optimal vitamin A status
- Essential for hearing, tasting and smelling
- Myelin synthesis in the nervous system
- Regeneration of bone fractures
- Anticarcinogenic effect of retinoic acid by influencing gene expression in the promotion phase of skin cancer.