4‐Phenyl‐3‐butenoic acid, an in vivo inhibitor of peptidylglycine hydroxylase (peptide amidating enzyme)

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Abstract
The ability of a series of non‐peptide carboxylic acids to act as substrates or inhibitors of the peptide‐amidating enzyme (peptidyl‐glycine hydroxylase) was assessed by determining their ability to reduce the rate of enzymic conversion of d‐tyrosyl‐valyl‐glycine or d‐tyrosyl‐phenylalanyl‐glycine to the corresponding dipeptide amide. The inclusion of a phenyl substituent in a position distal to the carboxyl group promoted the inhibitory action. The inhibition was found to be irreversible when an olefinic double bond, α or β to the carboxyl group, was present in the molecule; the inhibition appeared to be associated with a covalent interaction between the amidating enzyme and the inhibitor. With 4‐phenyl‐3‐butenoic acid the inhibitory properties were manifest only in the presence of cofactors of the enzyme. When 4‐phenyl‐3‐[2‐14C]butenoic acid was used, the radioactivity was shown to be incorporated into protein that co‐chromatographed with active enzyme.Incubation of rat thyroid carcinoma CA 77 cells in the presence of 4‐phenyl‐3‐butenoic acid led to a decrease in the levels of intracellular amidating activity and of thyrotropin‐releasing hormone, an amidated peptide produced by these cells. The inhibitory effects reached a maximum at approximately 15 h after which the enzyme levels returned to the control values even though the concentration of 4‐phenyl‐3‐butenoic acid in the cells remained unchanged. The results indicate that a mechanism exists in these cells for regulation of amidating activity.