Vitamin D binding globulin levels and affinity in various clinical conditions.

The approximate association constants of the plasma vitamin D binding globulin (Gc-globulin) for 25-hydroxycholecalciferol (25(OH)D3) and the plasma 25(OH)D3 binding capacities were measured in samples from 123 patients with a variety of disorders. No gross differences in binding affinities were observed between different groups of patients and controls. Many patients, however, had moderately reduced, and several had grossly reduced, plasma binding capacities. The changes in Gc-globulin relative to some other proteins are also described in detail in three patients during the course of their illness. Gc-globulin concentration and hence plasma vitamin D binding capacity can undergo rapid and marked changes during illness.