Simplified competitive protein binding assay for 1,25-dihydroxycholecalciferol, the most active metabolite of vitamin D.

Simplified competitive protein binding assay for determination of 1,25-dihydroxycholecalciferol, with chicken intestinal cytosol receptor as binding protein and tritiated 1,25-dihydroxycholecalciferol as a competitive radioligand was developed using the same outline procedure as for determination of the other biologically active metabolites of cholecalciferol. Binding characteristics (binding capacity and affinity) of a cytosol receptor were determined.