Vitamin D Binding Factors in Bovine Blood

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Abstract
Both 25[25,26-hydrogen-3] hydroxycholecalciferol and [1.alpha.,2.alpha.-hydrogen-3] cholecalciferol were added to bovine plasma in vitro. Analysis by gel-filtration and ion exchange chromatography, electrophoresis, ultracentrifugation, competitive binding specificity studies and plasma stripping showed that vitamin D circulated with a protein of .alpha.-globulin mobility. This globulin had a much higher affinity for 25-hydroxycholecalciferol while vitamin D3 appeared to be associated first with an .alpha.-lipoprotein and with time became associates with the .alpha.-globulin. This .alpha.-globulin had MW of approximately 70,000 as determined by gel-filtration. Cholecalciferol appeared to bind tightly to the .alpha.-lipoprotein and resisted being stripped from the plasma. .alpha.-Globulin appears to be the major carrier of vitamin D in the blood while the .alpha.-lipoprotein may aid in the transfer of cholecalciferol from the gut to the liver via the lymph system.

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