The identification and measurement of a folate-binding protein in human serum by radioimmunoassay.

Antiserum raised in rabbits against the FBP obtained from CML cells, and the purified binder labeled with 125I, have been used for an RIA which can measure an immunologically similar protein in human serum. The concentration of the binding protein in normal serums ranged from 1.2 to 9.3 ng/ml, with a mean +/- S.E.M. of 3.8 +/- 0.4 ng/ml. Elevated values of the binder protein were measured in the serums from patients with folate deficiency, vitamin B12 deficiency, liver disease, uremia, myeloproliferative disease, chronic lymphocytic leukemia, and various types of cancer and in the serum from pregnant women. The concentration of the binder protein and the capacity of the serum to specifically bind isotopically labeled PGA correlated poorly, indicating that the binding protein concentration and degree of saturation by endogenous serum folate vary independently in many instances.