VITAMIN A is an unsaturated cyclic alcohol.1 It has been extracted from natural sources and reduced to crystalline form,2 and its synthesis has been reported.3 The precursors of vitamin A are synthesized only by plants, as far as as is known,1 and these substances, classed as carotenoids, function as photosensitive structures in plants.4 In the animal kingdom, carotenoids are oxidized to vitamin A, and the vitamin is stored in the body. In some fish and mammals tremendous amounts of vitamin A are found stored in the liver and in the intestinal fat.5 The purpose of these great stores is not known.
In the human organism, vitamin A is stored in the liver and carotene, after oxidization to vitamin A, is also stored. The conversion of carotene to vitamin A has been assumed to occur in the liver. However, it has been shown that in rats