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Effect of N-methyl-thiotetrazole on rat liver microsomal vitamin K-dependent carboxylation
Abstract:
The use of a number of antibiotics which contain an N-methyl-thiotetrazole (NMTT) side chain has been reported to be associated with an increased incidence of hypoprothrombinemia. The suggested role of NMTT as an inhibitor of the liver microsomal vitamin K-dependent carboxylase has been investigated. In standard incubations, NMTT had no effect on carboxylation when vitamin KH2 was a substrate but was a weak inhibitor when [vitamin K + NADH] was a substrate. Microsomal vitamin K reductases, however, were not inhibited by NMTT. Preincubation of the incubation mixture with NADH and NMTT resulted in inhibition of carboxylase activity when either vitamin KH2 or [vitamin K + NADH] was the substrate. A fraction of the microsomal membrane which was not readily solubilized by dilute detergent protected the enzyme from this inhibition. The data suggest that NMTT is metabolized to an active inhibitor or is able to covalently inactivate the enzyme in the presence of NMTT. The vitamin K responsiveness of the clinically observed hypoprothrombinemia suggests that it is not related to this in vitro inhibition of the vitamin K-dependent carboxylase.
Insights
N-methyl-thiotetrazole (NMTT) antibiotics may cause hypoprothrombinemia by inhibiting vitamin K-dependent carboxylase. This inhibition requires NMTT metabolism or covalent inactivation, and is protected by microsomal membrane fractions.
Area of Science:
- Biochemistry
- Pharmacology
- Toxicology
Background:
- Antibiotics with an N-methyl-thiotetrazole (NMTT) side chain are linked to increased hypoprothrombinemia.
- The mechanism may involve inhibition of liver microsomal vitamin K-dependent carboxylase.
Purpose of the Study:
- To investigate the role of NMTT in inhibiting vitamin K-dependent carboxylase.
- To elucidate the mechanism of NMTT-induced hypoprothrombinemia.
Main Methods:
- In vitro enzyme assays using vitamin KH2 or vitamin K + NADH as substrates.
- Assessing inhibition of vitamin K reductases.
- Investigating the effect of preincubation with NADH and NMTT.
- Examining the role of microsomal membrane fractions.
Main Results:
- NMTT weakly inhibited carboxylation with [vitamin K + NADH] but not vitamin KH2.
- NMTT did not inhibit vitamin K reductases.
- Preincubation with NADH and NMTT led to carboxylase inhibition.
- Microsomal membrane fractions protected the enzyme from NMTT inhibition.
Conclusions:
- NMTT may be metabolized to an active inhibitor or cause covalent inactivation of the carboxylase.
- The observed in vitro inhibition mechanism may not explain clinical hypoprothrombinemia due to its vitamin K responsiveness.