Circulating plasma corticotrophin-releasing factor-like immunoreactivity.

Abstract

Normal human peripheral plasma, examined by direct radioimmunoassay (RIA) for human corticotrophin-releasing factor (CRF) using seven different antisera, showed marked inhibition of tracer binding to antiserum. This apparently high CRF-like immunoreactivity (CRF-LI) did not dilute in parallel with synthetic human CRF-41. In comparison, most equine and ovine plasma samples had low CRF-LI concentrations. Plasma fractionation of both normal and late pregnancy human plasma on Sephadex G-75 at neutral pH indicated the majority of the CRF-LI to be of large molecular size. The molecular weight was estimated to be approximately 29,000 following chromatography of normal human plasma on Sephacryl S-200. The high CRF-LI in normal plasma was reduced to barely detectable levels by methanol treatment despite the recovery of added human CRF-41 of greater than 80%. Methanol extracts of late pregnancy human plasma, however, retained high CRF-LI which exhibited the immunoreactive and Sephadex elution characteristics of synthetic human CRF-41. 125I-Labelled human CRF-41 added to human plasma showed a reversible, time-dependent alteration in molecular size and reduction in binding to excess antiserum. These findings indicate that direct RIA of human CRF in unextracted plasma leads to spurious results and suggest the presence in human plasma of an interfering factor and/or binding substance that is not generally apparent in equine or ovine plasma under the conditions described.