Evaluation of sera of various origin as sources of binding protein for the assay of 25-hydroxyvitamin D.

The affinity for 25-hydroxycholecalciferol (25OHD3) of transport proteins present in sera of various origin was estimated by means of Scatchard plots, using tritiated 25OHD3 as the tracer. Normal and rachitic rat sera showed the highest affinities, human sera showing lower association constants. Serum from normal rats is therefore to be considered the most suitable source of binding protein for the assay of 25-hydroxyvitamin D.