Corticotropin-releasing factor and its binding protein in human plasma.

CRF is unusual in that it is synthesized and released from the placenta into the circulation in humans, reaching levels in the third trimester that would normally be expected in the hypothalamic portal system during stress. This rise is even more pronounced in pregnancy-induced hypertension and preterm labour. Paradoxically, there is no associated rise of either ACTH or cortisol. This lack of biological response and the stability of the peptide in human (but not rat) plasma in vitro initiated a search for the human CRF-binding plasma protein. This CRF-BP proved to have a molecular mass in the region of 40 kDa, and has been purified to homogeneity. It has an affinity constant in the nanomolar range and when mixed with appropriate amounts of CRF completely inhibits the ACTH-releasing activity of the peptide in vitro. With the cloning of the cDNA for CRF-BP, sufficient pure material has become available for the development of a radioimmunoassay. Although CRF-BP levels in pregnant women are normal in the second trimester, they begin to fall by week 35, reaching approximately 50% of normal values by term. The net effect of this would be an accelerated increase in free, potentially biologically active CRF.