In vitro production of a TRH-homologous peptide and His-Pro diketopiperazine by human semen.

Fresh human semen, diluted 1:1 v/v with 0.15 M NaCl- 0.05 M phosphate, pH 7.5, undergoes a 3-fold increase in total thyrotropin-releasing hormone (TRH) immunoreactivity on incubation at 4 C for 8 to 16 hours. To identify the mechanism for this increase, pooled human semen was incubated at 4, 37, and 60 C, and the change in composition of the immunoreactive TRH peptides was quantitated by high pressure liquid chromatography and radioimmunoassay of TRH. His-Pro diketopiperazine, a biologically active metabolite of TRH consisting of a cyclic dipeptide of histidine and proline, also was measured by specific RIA. The concentration of TRH (pGlu-His-Pro-NH2) dropped precipitously within the first hour after dilution and incubation at all temperatures studied. A hydrophobic TRH-homologous peptide with the amino acid composition (Glu,X,Y,Pro), where X and Y are neutral, nonaromatic amino acids, increased 8-fold during 16 hours of incubation at 4 C. This TRH-homologous peptide is not derived from TRH because it lacks the histidine is not derived from TRH because it lacks the histidine residue. A 3- to 23-fold increase in His-Pro diketopiperazine levels occurred after 4 hours at 37 C. This was not due primarily to enzymatic removal of the pyroglutamyl residue from TRH by pyroglutamate aminopeptidase, since about 1 hour after ejaculation the initial His-Pro diketopiperazine levels were 9.7 +/- 5.1 micrograms/ml, or approximately 1000-fold greater than the corresponding levels of seminal TRH. Because cycloheximide, which blocks ribosomal protein biosynthesis, did not inhibit in vitro production of the TRH-homologous peptide and the His-Pro cyclic dipeptide, these peptides, like TRH, most likely arise from post-translational cleavage and processing from pre-existing macromolecular precursor proteins.