Abstract
BackgroundBioavailable 25-hydroxyvitamin D (25OHD) may be a better indicator of vitamin D sufficiency than total 25OHD. This report describes a novel assay for measuring serum bioavailable 25OHD.MethodsWe developed an assay for 25OHD % bioavailability based on competitive binding of 25OHD tracer between vitamin D-binding protein (DBP)-coated affinity chromatography beads and serum DBP. Bioavailable 25OHD, total 25OHD, albumin, and DBP protein concentrations were measured in 89 samples from hospitalized patients and 42 healthy controls to determine how the DBP binding assay responds to differences in concentrations of DBP and compares to calculated bioavailable 25OHD values.ResultsDBP binding assay showed a linear relationship between DBP-bound 25OHD tracer recovered from bead supernatant and DBP calibrator concentrations (y = 0.0017x +0.731, R2 = 0.9961, p<0.001). Inversion of this relationship allowed interpolation of DBP binding equivalents based upon 25OHD tracer recovered. The relationship between DBP binding equivalents and % bioavailability fits a non-linear curve, allowing calculation of % bioavailable 25OHD from DBP binding equivalents (y = 10.625x-0.817, R2 = 0.9961, p<0.001). In hospitalized patient samples, there were linear relationships between DBP protein concentrations and DBP binding equivalents (y = 0.7905x + 59.82, R2 = 0.8597, p<0.001), between measured vs. calculated % bioavailability (y = 0.9528 + 0.0357, R2 = 0.7200, p<0.001), and between absolute concentrations of measured vs. calculated bioavailable 25OHD (y = 1.2403 + 0.1221, R2 = 0.8913, p<0.001).ConclusionsThe DBP-binding assay for bioavailable 25OHD shows expected changes in 25OHD % bioavailability in response to changes in DBP concentrations and concordance with calculated bioavailable 25OHD concentrations.
Highlights
The “free hormone hypothesis” claims that only hormone not bound by high affinity binding proteins are bioavailable to target tissues [1,2,3,4]
The D binding protein (DBP)-binding assay for bioavailable 25OHD shows expected changes in 25OHD % bioavailability in response to changes in DBP concentrations and concordance with calculated bioavailable 25OHD concentrations
The fraction of 25OHD not bound by DBP has been called “bioavailable” which includes both free 25OHD and 25OHD loosely bound by albumin and other plasma proteins, and it has been proposed that only this unbound fraction is bioavailable for conversion to active 1,25-dihydroxyvitamin D [1, 2]
Summary
The “free hormone hypothesis” claims that only hormone not bound by high affinity binding proteins are bioavailable to target tissues [1,2,3,4]. The fraction of 25OHD not bound by DBP has been called “bioavailable” which includes both free 25OHD and 25OHD loosely bound by albumin and other plasma proteins, and it has been proposed that only this unbound fraction is bioavailable for conversion to active 1,25-dihydroxyvitamin D [1, 2]. This role for DBP may imply that the concentration of circulating DBP is a limiting factor for how much 25OHD can be carried in circulation and may influence the half-life and circulating concentrations of total 25OHD. This report describes a novel assay for measuring serum bioavailable 25OHD
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
