Enterohepatic circulation of water soluble metabolites of 25-OH vitamin D

Abstract

ENTEROHEPATIC CIRCULATION OF WATER SOLUBLE METABOLITES OF 25-OH VITAMIN D. M. Sitrin, M. Bolt, I. Rosenberg. Department of Medicine. University of Chicago, Chicago, Illinois. Vitamin D metabolites ha;e been obr the secretion was greatest in the first 2 hrs. No differences were noted in the biliary secretion of 25-OH-D in replete vs deficient rats. Biliary metabolites were separated by lipid-water partitioning and Sephadex LH-20 chromatography. Only 20% of biliary vitamin was lipid soluble with less than 10% chromatographing as unchanged 25-OH-D. The remaining metabolites were watar soluble with a single peak accounting for 60% of the biliary radioactivity. Treatment of this peak with Bglucurididase indicates that 25-OH-D is secreted into bile mainly as a glucuronide derivative. Ifien these tritiated water soluble 25-OH-D metabolites are administered intraduodenally to a rat, 30% of the radioactivity appears in bile and 10% in urine in 24 hrs. In viva, isolated rat intestinal sacs were used to identify the site of absorption of 25-OH-D and water soluble biliary metabolites. 25-OH-D, like vitamin D, is absorbed predominantly in the proximal jejunum while absorption of the water soluble matabolites is maximal in the distal ileum. Summary: 1) 25-OH-D is rapidly secreted into bile and secretion rate is not altered by vitamin D deficiency. 2) Little unchanged 25-OH-D is secreted into bile; a major fraction of biliary vitamin is a glucuronide derivative of 25-OH-D. 3) Water soluble biliary metabolites of 25-OH-D are absorbed maximally in the ileum, while 25-OH-D is predominently absorbed in the jejunum. Conclusion: Vitamin D undergoes an enterohepatic circulation in the form of water soluble metabolites; disturbances in this process wst be considered in the pathogenesis of vitamin D deficiency.