title: Metabolism of thyrotropin releasing hormone in brain extracts. Isolation and characterization of an imidopeptidase for histidylprolineamide.
abstract: An extract of porcine brain acetone powder incubated with thyrotropin-releasing hormone (TRH; pGlu-His-ProNH2) produces acid TRH (pGlu-His-Pro), histidine, and prolineamide. Fractionation of the brain extract by DEAE-cellulose chromatography produces three protein fractions which metabolize TRH. The activity of these fractions was characterized using TRH with a 3H-label on the histidine or proline as well as [His-3H]His-ProNH2. Fraction I contains pyroglutamate aminopeptidase and Fraction II contains TRH deamidase. Fraction III was found to contain a previously unrecognized enzyme which cleaves His-ProNH2 to histidine and proline. The histidylprolineamide imidopeptidase has been characterized. A competition study using a variety of compounds containing histidine or proline suggests that the best substrates for the imidopeptidase contain a free alpha-amino group on histidine and a blocked carboxyl group on proline, as is found in His-ProNH2. A survey of a variety of polypeptide hormones indicates that many of them inhibit the imidopeptidase activity. A kinetic study of the inhibition of the enzyme by adrenocorticotropic hormone (1-24) shows that the inhibition by polypeptide hormones is noncompetitive. We hypothesize that pituitary hormones may stimulate the production of (cyclo)-His-Pro by inhibiting alternate routes of TRH metabolism.
mesh_terms: D000818:Animals; D001921:Brain / Q000201:enzymology*; D004150:Dipeptidases / Q000302:isolation & purification* / Q000378:metabolism; D004151:Dipeptides; D006639:Histidine / Q000031:analogs & derivatives; D007700:Kinetics; D011392:Proline / Q000031:analogs & derivatives; D013973:Thyrotropin-Releasing Hormone / Q000378:metabolism*
chemical_list: D004151:Dipeptides; C019184:histidylprolineamide; D006639:Histidine; D013973:Thyrotropin-Releasing Hormone; D011392:Proline; D004150:Dipeptidases; C019190:imidopeptidase