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 of calciferol (vitamin D2). Some time later it was recognized that 7-dehydrocholesterol was a provitamin D and vitamin D3 was eventually obtained pure.