In vivo production of ethylene from 2-keto-4-methylthiobutyrate in mice

The use of 2-keto-4-methylthiobutyric acid (KMB), the alpha-keto analog of methionine, was studied as a potential means of detecting free radical generation in vivo. KMB-dependent ethylene production (presumably from free radical interception), and ethane production from in vivo lipid peroxidation, were monitored simultaneously by measuring the rate of exhalation of these hydrocarbons by mice. Injection of KMB (1 g/kg) into mice resulted in an 8-fold increase in ethylene production above endogenous levels seen in saline-injected controls (1.47 +/- 0.35 vs 0.17 +/- 0.02 nmoles/100 g/hr respectively). Administration of CCl4 (3.0 g/kg) to initiate hepatic lipid peroxidation, 20 min prior to KMB injection, augmented the production of ethylene (2.37 +/- 0.10 nmoles/100 g/hr). Lipid peroxidation following injection of CCl4 was monitored via the increased exhalation of ethane. Pretreating the mice with vitamin E (100 mg/kg daily for 3 days), an inhibitor of lipid peroxidation, did not result in a significant change in ethylene production from KMB by itself or after prior injection of CCl4. However, vitamin E did suppress ethane production initiated by CCl4. Similar results were obtained with mouse liver slices studied in vitro. Metyrapone (150 mg/kg), an inhibitor of hepatic mixed function oxidase activity, also suppressed significantly the CCl4-stimulated production of ethane, but not the CCl4-stimulated production of ethylene from KMB. It appears that ethylene production from KMB does not derive from free radicals generated during in vivo lipid peroxidation since suppression of lipid peroxidation by vitamin E or metyrapone did not suppress ethylene production.