A NEW MECHANISM FOR THE AEROBIC CATABOLISM OF DIMETHYL SULFIDE

Aerobic degradation of dimethyl sulfide (DMS), previously described fo r thiobacilli and hyphomicrobia, involves catabolism to sulfide via me thanethiol (CH3SH). Methyl groups are sequentially eliminated as HCHO by incorporation of O2 catalyzed by DMS monooxygenase and methanethiol oxidase. H2O2 formed during CH3SH oxidation is destroyed by catalase. We recently isolated Thiobacillus strain ASN-1, which grows either ae robically or anaerobically with denitrification on DMS. Comparative ex periments with Thiobacillus thioparus T5, which grows only aerobically on DMS, indicate a novel mechanism for aerobic DMS catabolism by Thio bacillus strain ASN-1. Evidence that both organisms initially attacked the methyl group, rather than the sulfur atom, in DMS was their conve rsion of ethyl methyl sulfide to ethanethiol. HCHO transiently accumul ated during the aerobic use of DMS by T. thioparus but not with Thioba cillus strain ASN-1. Catalase levels in cells grown aerobically on DMS were about 100-fold lower in Thiobacillus strain ASN-1 than in T. thi oparus T5, suggesting the absence of H2O2 formation during DMS catabol ism. Also, aerobic growth of T. thioparus T5 on DMS was blocked by the catalase inhibitor 3-amino-1,2,4-triazole whereas that of Thiobacillu s strain ASN-1 was not. Methyl butyl ether, but not CHCl3, blocked DMS catabolism by T. thioparus T5, presumably by inhibiting DMS monooxyge nase and perhaps methanethiol oxidase. In contrast, DMS metabolism by Thiobacillus strain ASN-1 was unaffected by methyl butyl ether but inh ibited by CHCl3. DMS catabolism by Thiobacillus strain ASN-1 probably involves methyl transfer to a cobalamin carrier and subsequent oxidati on as folate-bound intermediates.