Turnover of the Plasma Binding Protein for Vitamin D and Its Metabolites in Normal Human Subjects

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Abstract
The turnover of the plasma binding protein for vitamin D and its metabolites (DBP) was investigated in 4 normal young men. 125I-Labeled DBP was injected i.v. into each subject, and the plasma specific radioactivity-time curve was determined for the following 15-18 days. All urine was collected during the study, and the cumulative urinary 125I excretion curve was determined. The serum and urinary radioactivity data were analyzed by compartmental analysis using a weighted, least squares, nonlinear regression technique. In each of the subjects studied, a 3 pool model provided the best fit to the serum specific radioactivity-time data. A 3 pool, catenary model was selected on the basis of physiological considerations. The urinary data were fit by assuming a single outflow from pool 1 into an iodide compartment. Total body exchangeable DBP averaged 2.89 g. The production rate of DBP varied from 0.69-0.93 g/day in the subjects studied, with a mean of 10.1 mg/kg body wt per day. Inferences could be drawn about the anatomical localization of the DBP molecules that comprise each of the 3 pools in the model. The mass of pool 1 was very close to the calculated mass of DBP in the plasma compartment and comprised a mean of 50% of the total body exchangeable DBP. DBP in pool 1 comprises DBP in the intravascular plasma compartment. Pool 2, slightly smaller than pool 1, probably represents DBP in the extravascular, extracellular compartment. Others have demonstrated that human DBP is present in many tissues. Pool 3, identified from the kinetic analysis reported here and .apprx. 300 mg in size, may represent the intracellular compartment of DBP.

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