Metabolism of Vitamin D. I. Preparation of Radioactive Vitamin D and Its Intestinal Absorption in the Rat*

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Abstract
Methods are described for the chemical preparation of H3-vitaniin D3 by catalytic exchange in tritiated acetic acid, and of uniformly labeled C14-vitamin D2 from C14-ergosterol synthesized by yeast. The radioactive vitamins were recrystallized as the dinitrobenzoyl esters. Intestinal absorption of the labeled vitamins was studied in the rat. Vitamin D is absorbed maximally in the jejunum and transferred chiefly into lymph. It is found mainly in the chylomicron fraction as the free, biologically active sterol. Bile salt, specifically taurocholate as compared to taurodeoxycholate and taurochenodeoxycholate, is required for optimal absorption. Two steps can be distinguished in the transport across the intestine: uptake at the mucosal surface, relatively rapid, and transfer to lymph, relatively slow.