Biosynthesis of human growth hormone-releasing hormone (hGRH) in the pituitary of hGRH transgenic mice.

We have studied the posttranslational processing of prohuman GH-releasing hormone (pro-hGRH) to the mature hormones, hGRH(l–44)-NH2 and hGRH(l–40)-OH, and its carboxyl-terminal peptide (hGCTP) in pituitary cells from transgenic mice bearing a metallothionein-hGRH fusion gene after incubation with [35S]methionine. After separation on HPLC, 35S-labeled and unlabeled hGRH in medium and cell extracts were characterized by RIA and immunoprecipitation with antisera against hGRH and against hGCTP. After a 4-h pulse, unlabeled and [35S]pro-hGRH, hGRH(l–44)-NH2, and hGRH(l–40)-OH were identified in medium and cell extracts by both RIA and immunoprecipitation with anti-hGRH serum. In cell extracts, after a 0.5-h pulse, [35S]pro-hGRH and hGRH(l–44)-NH2 but not [35S]hGRH(l–40)-OH were detectable. After a 0.5-h chase, however, 35S-labeled hGRH(l–40)-OH, pro-hGRH, and [35S]hGRH(l–44)-NH2 were all measurable. After a 4-h chase, comparable levels of [35S]hGRH(l–44)-NH2 and hGRH(l–40)-OH were present, and very little intr...