A specific carrier substance for human corticotrophin releasing factor in late gestational maternal plasma which could mask the ACTH-releasing activity.

Late gestational maternal plasma contains a carrier substance for corticotrophin releasing factor (CRF-41) with a molecular weight in the region of 40,000. Using gel chromatography and CRF-41 immunoradiometric assay (IRMA), we show that binding of the peptide to its plasma carrier can be disrupted by treatment with urea. The binding capacity of the carrier substance is not saturated, since time-dependent incorporation of synthetic CRF-41 occurs at 4 degrees C. The carrier substance is also present in normal male plasma. It is specific for human CRF-41, not binding to ACTH, GnRH, vasopressin or ovine CRF-41. Most of the high concentration of CRF-41 in late gestational maternal plasma is bound to the carrier. Dilutions of pooled fractions containing carrier-bound CRF-41 after chromatography of maternal plasma had ACTH-releasing activity as did the synthetic peptide. However, the more concentrated chromatographic fractions at the apex of the carrier-bound CRF-41 peak showed reduced bioactivity, indicating that the higher concentrations of carrier in the original maternal plasma could mask the ACTH-releasing activity of CRF-41.