Analysis of vitamin D, its metabolites and structural analogues

Abstract

Vitamin D*, a secosteroid derived from a sterol precursor by opening of the steroid B ring between carbons 9 and 10, exists in a number of different chemical forms. In the clinical environment, only vitamins D2 and D3 are important. Vitamin D3 (cholecalciferol) arises by the action of UV light on the sterol precursor, 7-dehydrocholesterol, which is found in the upper layers of the skin. The chemical reactions involved are shown in Figure 10.1. It can be seen that apart from the production of the provitamin and subsequent conversion into vitamin D3, a number of other isomers are formed. The proportion of these different products from 7-dehydrocholesterol varies depending upon the conditions under which they are formed. Vitamin D2 (ergocalciferol) arises in a similar fashion in that it derives from the UV irradiation of the 5,7-diene sterol precursor, ergosterol. Ergosterol is widely distributed in plants, yeasts and a number of animals other than humans. Vitamin D2 has an extra methyl (C-28) attached to carbon 24 on the vitamin D side-chain together with an extra double bond at C-22(23), and its structure is given in Figure 10.1. Because the further metabolism of vitamin D involves hydroxylations on the side chain, it is inevitable that the physiological effects of vitamin D2 and vitamin D3 may well be slightly different. For historical reasons, vitamin D2 is more readily available commercially and is used in a number of countries throughout the world to supplement milk and other foodstuffs, and calciferol BP is in fact vitamin D2.