In Vitro Effects of Various Heterocyclic Thiol Compounds and β-Lactam Antibiotics on Vitamin K-Dependent γ-Glutamylcarboxylation Activity in Liver Microsomes

The in vitro effect of N-methyltetrazolethiol (NMTT), one of the common substituents at the 3'-position of the cephem in various β-lactam antibiotics, on liver microsomal γ-glutamylcarboxylation (γ-carboxylation) activity was examined using solubilized rat liver enzyme. The enzyme activity was inhibited by coexisting with NMTT and NADH, and this inhibitory activity could be suppressed by the addition of a sulfhydryl compound such as dithiothreitol (DTT), glutathione or cysteine. Various five-membered heterocyclic thiol compounds exhibited concentration-dependent inhibition of microsomal γ-carboxylation activity. These inhibitory actions diminished markedly in the presence of 1 mM DTT. In vitro γ-carboxylation activity also decreases upon addition of various β-lactam antibiotics at 1 or 10 mM, depending upon the concentration of the drug. Among the heterocyclic thiol compounds, there is a correlation between their inhibitory activities and hydrophobicities. Thus, the in vitro inhibitory activity of heterocyclic thiol compounds and β-lactam antibiotics on microsomal γ-carboxylation activity is not correlated with their molecular structures, but rather depends on their hydrophobicities and with the concentrations in the reaction mixture.