Metabolism of thyrotropin-releasing hormone in human cerebrospinal fluid. Isolation and characterization of pyroglutamate aminopeptidase activity.
Pyroglutamate aminopeptidase, which catalyzes metabolism of thyrotropin-releasing hormone (TRH) to cyclo(His-Pro), is the major enzyme of TRH metabolism in human CSF. The partially purified CSF pyroglutamate aminopeptidase has a pH optimum between 6.0 and 7.4, and a Km of 15.9 +/- 3.1 microM. A number of potential competitive inhibitors of the enzymatic activity were examined, of which luteinizing hormone-releasing hormone and bombesin were the most effective. An examination of the structure of various peptides that inhibit pyroglutamate aminopeptidase activity indicated that the enzyme generally prefers a substrate having amino-terminal pyroglutamic acid (pGlu) and a COOH-terminal that is either blocked or distant from amino-terminal pGlu. Heavy metals, EDTA and reducing agents inactivated the enyzme, whereas benzamidine, phenylmethylsulfonylfluoride, trypsin inhibitor and alkylating agents had little or no effect on the enzymatic activity. Thiol-oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid), however, considerally inhibited the enzymatic activity. We hypothesize that CSF pyroglutamate aminopeptidase may play a role in the biologic actions of TRH.
Prasad
C
C
Jayaraman
A
A
D000626:Aminopeptidases / Q000134:cerebrospinal fluid*; D002384:Catalysis; D002850:Chromatography, Gel; D004789:Enzyme Activation; D006801:Humans; D011751:Pyroglutamyl-Peptidase I / Q000037:antagonists & inhibitors / Q000134:cerebrospinal fluid* / Q000302:isolation & purification; D013379:Substrate Specificity; D013973:Thyrotropin-Releasing Hormone / Q000134:cerebrospinal fluid*
D013973:Thyrotropin-Releasing Hormone; D000626:Aminopeptidases; D011751:Pyroglutamyl-Peptidase I