Provitamin D3 in tissues and the conversion of cholesterol to 7-dehydrocholesterol in vivo.

The discovery that the absorption bands at 293, 281-5 and 271 m,u. (Heilbron, Kamm & Morton, 1926, 1927) present in fractions obtained in the recrystallization of cholesterol were shown very strongly by ergosterol (Rosenheim & Webster, 1927) focused interest on the latter and culminated in the isolation ofcalciferol (vitamin D2). Some time later it was recognized that 7-dehydrocholesterol was a provitamin D and vitamin D3 was eventually obtained pure. Vitamin D. in fact appeared to be the principal, if not the only 'natural', vitamin D. With the achievement of commercial production interest in the field declined,although many problems remained unsettled. Perhaps the most important of these is the biogenesis ofprovitamin D. In the herbivorous animal, the dietary sterols, sitosterol, stigmasterol, etc., are unlikely to be responsible for the appearance of 7dehydrocholesterol. Ergosterol, which is characteristic of yeasts and fungi, could scarcely account for the wide distribution of provitamin D3 in animals. The present work provides evidence ofa reversible enzymic dehydrogenation of cholesterol mainly in the gut wall. The 7-dehydrocholesterol is thus not of dietary origin. It may have biological functions independent of its role as provitamin D3.