Comparisons between two receptor assays for 1, 25-dihydroxyvitamin D

Abstract

We present a competitive protein binding assay (CPBA) for 1,25(OH)₂D employing 1,25(OH)₂D receptor from calf thymus, which was compared with a CPBA-employing receptor from rachitic chick intestine. The thymus receptor assay was more sensitive, specific and precise than the intestinal receptor assay. The thymus receptor assay measured both 1,25(OH)₂D₂ and 1,25(OH)₂D₃ with equal affinity, whereas 1,25(OH)₂D₂ was 1.1 times less potent than 1,25(OH)₂D₃ in the displacement from the chick intestinal receptor. Mean serum values of 1,25(OH)₂D in normal subjects, post-menopausal women, pregnant women, and patients with chronic renal failure measured by the two assay systems did not differ. Furthermore, both assays showed that 1,25(OH)₂D was unchanged in post-menopausal women after treatment with vitamin D₂ or vitamin D₃, 4000 IU/day for 8 weeks. We conclude that the high sensitivity of the thymus receptor and the equal affinity for the D₂ and D₃ analogue make the thymus receptor assay a reliable alternative to the chick intestinal receptor assay.

Key Words:

• competitive protein assay
• vitamin D₂
• vitamin D₃