Conversion of Cholesterol to Provitamin D₃ in vivo

Abstract

THERE is now evidence in the literature that steroid hormones¹ and bile acids² are formed from cholesterol. A similar origin for provitamin D₃ (7-dehydrocholesterol) in the animal body, certainly of Herbivora, is now accepted; but there is little or no conclusive evidence as to the site or mode of formation of the provitamin. The part of the body richest in 7-dehydrocholesterol is generally thought to be the skin³. Recently, however, we have detected a sterol showing the ultra-violet absorption (max. 271·5 mμ, 281·5 mμ and 292 mμ inflexion 260–265 mμ and min. 276 mμ) characteristic of the provitamins D, and thought to be provitamin D₃, in the lining of the small intestine of the guinea pig, rat and ox. It is most concentrated in the duodenum and is associated largely with the mucosa and lamina propria. In the guinea pig, the concentration of the '7-dehydrocholesterol' in the non-saponifiable matter of the whole of the small intestine is c. 3·6 per cent and is very much greater than the 0·5 per cent observed in the skin; whereas in the rat both organs contain about 0·5 per cent.