Metabolism of ethyl methyl sulphide and sulphoxide in rat microsomal fractions

Abstract

1. Gas chromatographic (GC) methods for the analysis of ethyl methyl sulphide (EMS) and its corresponding sulphoxide (EMSO) and sulphone (EMSO₂) in rat microsomes and aspects of the in vitro metabolism of EMS and EMSO are described. 2. EMS and the internal standard (dimethyl sulphide) were extracted by a headspace procedure and separated satisfactorily using a column packed with 4% Carbowax® 20 M/0.8% KOH on Carbopack® B. EMSO, EMSO2 and the internal standard n-propyl sulphone were separated satisfactorily using a 2-m column packed with 10% Carbowax® 20 M on Chromosorb® W. 3. Under the optimum conditions (incubation of 10 min and microsomal protein content of ~4 mg/ml), 10% of the initial EMS concentration (2.5 mM) was converted to the corresponding sulphoxide in rat liver microsomal incubations. However, < 0.1% of the sulphone was detected when rat liver microsomes were incubated with EMS. Similarly, 2.5% of the initial EMSO concentration (2.5 mM) was converted to the corresponding sulphone by rat liver microsomes (~4 mg/ml/protein) during an incubation of 30 min. However, no EMS was detected after incubation with EMSO under these conditions. 4. The estimated apparent V_max and K_m for the sulphoxidation of EMS were 3.8±0.02 nmol/mg/protein/min and 1.9±0.10 mM respectively. V_max1, V_max2 and K_m1 and K_m2 for the S-oxidation of EMSO were 0.5±0.01 and 0.2±0.01 nmol/mg/protein/min and 0.7±0.02 and 0.1±0.00 mM respectively. 5. Studies with selective inducers and inhibitors of microsomal monooxygenases indicated that the sulphoxidation of EMS is mediated mainly by FMO, whereas both FMO and cytochrome P450 are involved in the S-oxidation of EMSO.