Effect of N-Methyltetrazolethiol on Liver Microsomal gamma-Glutamylcarboxylation: Modification of the In Vitro Action of N-Methyltetrazolethiol

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Vitamin K-dependent gamma-glutamylcarboxylation activity in rat liver microsomes was monitored using the incorporation of ¹⁴CO₂ into exogenous pentapeptide and endogenous protein substrates as indicators. Detergent solubilization of the microsomal enzymes was required for the activity to develop, but higher concentrations of detergent inhibited the enzymatic reaction. Pyridoxal-5'-phosphate (PAL-P) and dithiothreitol (DTT) stimulated the enzyme activity. The enzyme activity was observed when the hydroquinone form of vitamin K or the quinone form plus NADH was employed as the cosubstrate, but little activity was detected in the reaction system containing vitamin K-epoxide plus NADH plus DTT. N-Methyltetrazolethiol (NMTT), the substituent at the 3'-position of several beta-lactam antibiotics, inhibited the enzyme activity in vitro only in the reaction system containing NADH. Addition of DTT diminished the in vitro action of NMTT, while PAL-P and detergent did not affect it. The results indicate that the in vitro inhibitory action of NMTT is observable under some specific and restricted assay conditions. This paper also discusses the differences between the in vitro action and the in vivo effect of NMTT.