Characteristics and effects of vitamin A

A2 is mainly found in the liver of freshwater fish. Vitamin A1 is a fat-soluble light yellow flake crystal with a melting point of 64 ° C, and vitamin A2 has a melting point of 17 - 19 ° C. It is usually a golden yellow oil. Vitamin A is a polyenol containing a beta-dahurone ring. The chemical structure of vitamin A2 differs from that of A1 only by having one more double bond at the 3 and 4 positions of the β-dahurone ring. Vitamin A has unsaturated bonds in the molecule and has active chemical properties. It is easily oxidized in the air or destroyed by ultraviolet radiation and loses its physiological effect. Therefore, vitamin A preparations should be stored in a brown bottle and protected from light. Whether it is A1 or A2, it can react with antimony chloride and appear dark blue. This property can be used as a basis for quantitative determination of vitamin A. Many plants such as carrots, tomatoes, green leafy vegetables, and corn contain carotenoid substances, such as α, β, γ-carotene, cryptoxanthin, lutein, etc. Some of these carotenoids have the same ring structure as vitamin A1 and can be converted into vitamin A in the body, so they are called provitamin A. β-carotene contains two ring structures of vitamin A1, with the highest conversion rate. One molecule of beta carotene and two molecules of water produce two molecules of vitamin A1. In animals, this water oxidation process is catalyzed by β-carotene-15,15'-oxygenase and mainly takes place in the small intestine mucosa of animals. Vitamin A in food, or produced by the cleavage of beta-carotene, combines with fatty acids in small intestine mucosal cells to form esters, and is then incorporated into chylomicrons, which are absorbed into the body through lymph. The liver of animals is the main place for storing vitamin A. When the body needs it, it is released into the blood. In the blood, retinol R binds to retinol binding protein RBP and plasma prealbumin PA to form an R-RBP-PA complex that is transported to