Significant Differences in the Degradation of Pro-Leu-Gly-NH 2 by Human Serum and that of Other Species 1

Abstract

Summary

The acyclic C-terminal tri-peptide of oxytocin, H-Pro-Leu-Gly-NH₂, is not degraded upon incubation with human (male, female or pregnant female) plasma or serum for 1 hr at 37°. However, the sera of other species tested, including rat, chicken and carp, degrade this tripeptide 100%, 4% and 30 %, respectively, in 1 hr, as determined by quantitative amino acid analysis of released products. Among the species studied there seems to exist a correlation between the anatomic development of the pars intermedia and the ability of the serum to hy-drolyze H-Pro-Leu-Gly-NH₂, which has been proposed to be a MSH-release-in-hibiting factor. The only identified degradation products are Pro, Leu and H-Gly-NH₂, with no detectable levels of H-Leu-Gly-NH₂. The dipeptides H-Leu-Gly-NH₂ and H-Pro-Leu-OH are each cleaved at similar rates in either human or rat serum, although the rate of hydrolysis of both peptides is lower in human than in rat. Thus, it does not appear that the dipeptide, H-Leu-Gly-NH₂, can accumulate as one of the breakdown products of the tripeptide. The arylamidase present in rat serum has different characteristics from the enzyme in rat brain which can degrade H-Pro-Leu-Gly-NH₂.

The authors wish to thank Drs. P. L. Hoffman and Abba Kastin for critical evaluation of the manuscript.