The Turnover and Transport of Vitamin D and of a Polar Metabolite with the Properties of 25-hydroxycholecalciferol in Human Plasma

Four normal men were injected intravenously with physiological doses (6 mug) of vitamin D(3)-1,2-(3)H. Serial samples of plasma were collected for 50 days. Total lipid extracts were chromatographed on silicic acid columns or thin-layer plates in order to characterize the radioactive components. Labeled vitamin D(3) disappeared rapidly from plasma (initial half-life approximately 12 hr); after 7 days unchanged vitamin D(3) represented less than 1% of circulating radioactivity. Coincident with vitamin D(3) disappearance a more polar labeled metabolite appeared with chromatographic and other properties identical with those of 25-hydroxycholecalciferol. The disappearance of the more polar metabolite was relatively slow with a half-life of 19.6 +/-0.6 days. A similar half-life was seen in a fifth subject, injected with 80 mug of vitamin D(3)-(3)H. Most (approximately 92%) of the plasma total radioactivity was represented by this component throughout the study. Plasma samples collected at various times were adjusted to density (d) 1.21 and were ultracentrifuged to separate plasma lipoproteins from proteins with d > 1.21. In all samples, almost all (mean 94%) of the radioactivity was found in association with proteins of d > 1.21. This observation was confirmed by bioassay, measuring uptake of (45)Ca by intestinal slices. All plasma bioassayable vitamin D was found in association with proteins of d > 1.21; 55% of bioactivity was found in the chromatographic fraction corresponding to 25-hydroxycholecalciferol and 44% in the fractions representing vitamin D(3). Since both vitamin D(3) and its 25-hydroxy metabolite are lipid-soluble sterol derivatives, the finding that these compounds do not circulate in association with the known plasma lipoproteins provides presumptive evidence for the existence of a specific transport protein of d > 1.21. The transport protein for the polar metabolite has been partly characterized by gel filtration on Sephadex G-200 and by electrophoresis on polyacrylamide gel. The protein has an apparent size slightly smaller than plasma albumin (approximate mol wt 50,000-60,000) and an electrophoretic mobility very slightly greater than that of albumin. Studies are in progress to fractionate further and to characterize the transport protein.