Assessment of the Free Fraction of 25-Hydroxyvitamin D in Serum and Its Regulation by Albumin and the Vitamin D-Binding Protein *

Abstract

We measured the free fraction of 25-hydroxyvitamin D (25OHD) in human serum and determined that 25OHD bound to a component with an affinity constant of 7 X 10(8) M-1 and a concentration of 4.5 X 10(-6) M. This concentration was equal to that of the vitamin D-binding protein (DBP) in the same serum sample. We removed DBP from the serum using actin affinity columns and found that the affinity for 25OHD of the remaining serum components was equivalent to that of human serum albumin (6 X 10(5) M-1). We then measured the free fractions of 25OHD, DBP, and albumin in normal and cirrhotic subjects. We calculated that 88 +/- 3% (+/- SD) and 83 +/- 8% of the 25OHD were bound to DBP in the serum of normal and cirrhotic subjects, respectively. We compared previously reported data for the free fraction and the free concentration of 1,25-dihydroxyvitamin D in these subjects with the current data for the free fraction and free concentration of 25OHD. The total concentrations and free fractions of both metabolites correlated to each other and to the DBP and albumin concentrations in these subjects, but the free concentrations of these metabolites did not. We conclude that 25OHD, like 1,25-dihydroxyvitamin D, is transported in blood bound primarily to DBP and albumin. Changes in the concentrations of DBP and albumin affected the total and free fractions of 25OHD in serum, but the actual free concentration of 25OHD was independent of such changes.

A Predictive Equation to Guide Vitamin D Replacement Dose in Patients

“…Low serum albumin concentrations may also indicate hepatocellular dysfunction and the inability of the liver to convert vitamin D into 25-hydroxyvitamin D. Albumin is also a carrier protein for 25-hydroxyvitamin D, and low serum concentrations of albumin may result in low serum concentrations of 25-hydroxyvitamin D because of a deficiency of a carrier protein in a manner similar to the low serum concentrations of 25-hydroxyvitamin D caused by lower levels of a specific vitamin D binding protein among blacks in America. 80,81 The RDA for vitamin D was revised from 400 to 800 IU/day for most adults. 12,17,31 A similar dose has been found to be sufficient to prevent fractures.…”

Vitamin D-binding protein: one more piece in the puzzle of acromegalic osteopathy?

“…Consistently with the free hormone hypothesis, only hormone not bound to protein vectors can exert biological actions [11]. More than 80 % of circulating vitamin D (i.e., both 25(OH)-vitamin D and 1,25-dihydroxy-vitamin D3) is bound to vitamin D-binding protein (VDBP) which acts as regulator of hormone bio-availability to target tissue [12]. Indeed, VDBP could be either a reservoir of vitamin D prolonging its half-life or a modulator of hormone biological activity [9,10,13].…”

“…Because of their low concentrations, direct measurement of free vitamin D by laborious ultrafiltration and dialysis methods may be challenging. For this reason, Altinova et al performed an indirect estimation of free vitamin D based on the measurement of 25(OH)-vitamin D, albumin, and VDBP values [12]. This method is easily applied in the clinical practice but some methodological shortcomings may affect the reliability of results.…”

Cellular uptake of steroid carrier proteins—Mechanisms and implications

“…It exhibits highest affinity for 25-OH vitamin D 3 (K d 10 -10 -10 -12 M) (Haddad, 1995;Haddad et al, 1981). Because DBP is present in 100-fold molar excess in the circulation, virtually all 25-OH vitamin D 3 is bound by this carrier (Bikle et al, 1986). …”