Degradation of luteinizing hormone-releasing hormone by serum and plasma in vitro.

Luteinizing hormone-releasing hormone (LH-RH) is degraded in vitro by serum and plasma from several species (human, rat, guinea-pig and cattle). Separation of the degradation products by high-performance liquid chromatography (HPLC) followed by amino acid analysis and radioimmunoassay showed that the main sites of cleavage are the Trp3-Ser4 and Tyr5-Gly6 bonds. Two peptidases are responsible since the cleavage at Trp3-Ser4 can be selectively inhibited by EDTA. In human plasma, the peptidase responsible for Trp3-Ser4 hydrolysis has a Km of 2.9 . 10(-4) M and V of 30 nmol/h per ml plasma. The half-life in vitro of LH-RH in serum and plasma from various species ranges from 3 h (guinea-pig) to 9.8 h (human). The peptidase cleaving LH-RH at Tyr5-Gly6 is present as an impurity in some commercial bovine serum and plasma albumins. Such contamination may have important practical implications for work involving peptide assays where albumins are used as carrier proteins.