Some Aspects of Vitamin A Metabolism

Abstract

The existence of a fat-soluble Factor A was postulated in 1913, and 40 later it could be claimed that the purely chemical problems of structure and synthesis in the field of vitamin A and its carotenoid precursors had, in the main, been solved. In contrast, the state of knowledge concerning the physiology and biochemistry of vitamins A is much less satisfactory. Progress has certainly been made in describing the deficiency syndrome and in partially elucidating many separate aspects of it; the special subject of the mode of action of vitamin A in visual processes has been in large measure reduced to order. It must be admitted, however, that the mode of action of vitamin A in promoting growth and in maintaining normal states in epithelial tissues, in short its main systemic action, is hardly understood at all. There is as yet no master key to avitaminosis A, no biochemical common denominator to the changes which occur in bone and nerve, to hyperkeratosis (including xerophthalmia), and to retarded growth. This chapter is concerned with two divisions of the field, the conversion of carotene to vitamin A and the systemic mode of action.