Thyrotropin-releasing hormone-degrading enzyme in human serum is classified as type II of pyroglutamyl aminopeptidase: influence of thyroid status

The characteristics of thyrotropin-releasing hormone (TRH)-degrading enzyme in human serum were studied. Serum was incubated in 0.1 M phosphate buffer containing proline-3H]TRH at 37 degrees C. A thin layer chromatography analysis of TRH degradation did not show any radioactive peak located in an acid TRH position, but apparent radioactive peaks corresponding to His-Pro and His-ProNH2 occurred in the presence of p-hydroxymercuriphenyl sulfonic acid, an inhibitor of proline dipeptidase. With ion exchange paper chromatography, the formation of 3H-labeled His-Pro and His-ProNH2 was estimated as an end point in the measurement of pyroglutamyl aminopeptidase (pGlu-peptidase) activity. An assay using p-hydroxymercuriphenyl sulfonic acid was developed to sensitively quantitate the pGlu-peptidase. Neither bacitracin nor p-chloromercuribenzoic acid increased the activity of pGlu-peptidase. The addition of EDTA, dithiothreitol, and o-phenanthroline significantly inhibited pGlu-peptidase activity, but neither iodoacetamide nor ethylmaleimide altered its activity. The pGlu-peptidase had a stereotypic specificity for the tripeptide, pGlu-His-ProNH2 of TRH, and its Km was 44.9 microM. The pGlu-peptidase activity was not changed by either hyper- or hypothyroidism. The present data indicate that a TRH-degrading enzyme in human serum possesses a nature identical to type II of pGlu-peptidase which is not altered by thyroid status.