Biochemistry

The cis-retinal turns into the trans-retinal under the influence of light, after which the rhodopsin breaks up into opsin and retinal. In the dark, these parts are reconnected, which makes it possible to see at night. The synthesis of rhodopsin is a complex process that occurs with the participation of many enzymes (Fig. 12.2). When retinol is cleaved from rhodopsin, part of it is destroyed, and therefore, new molecules of vitamin A are needed for the synthesis of rhodopsin in the dark in the light Opsin + C/sretinal ---------- > Rhodopsin -------------- ► Zhmrretinal + Opsin Enzymatic ' oxidation V Cfcretinol A Replenishment of loss Enzymatic oxidation A I Iv Enzymatic reduction 7raruretinol Irreversible loss v Esters of Vitamin A (Reserves) Figure 12.2 - Scheme of the participation of vitamin A in the act of vision Vitamin A has a positive effect on the barrier function of the skin and mucous membranes, on the permeability of cell membranes and the biosynthesis of their components (some glucoproteins). Retinol is involved in many metabolic links. It is necessary for the synthesis of nucleic acids and proteins, for normal lipid metabolism. Vitamin A affects the activity of tissue respiration enzymes and oxidative phosphorylation processes, as well as mineral metabolism, in particular, the exchange of calcium salts. A positive effect of provitamin retinol - P-carotene as an anti-radiation factor was noted. Vitamin A can be ingested as provitamins - P-carotenes. Three types of carotenes are known: a, P, ү, which differ both in structure and in biological activity. The biological activity of P-carotene is great since it contains two retinol rings; two molecules of vitamin A are formed from it upon its decay. 263