Evidence for the B₁₂-dependent enzyme ethanolamine deaminase in Salmonella

Abstract

THE enteric bacteria Escherichia coli and Salmonella typhimurium, like man, do not synthesise significant amounts of cobalamin (B₁₂) compounds and thus depend on exogenous vitamin B₁₂ for their B₁₂-dependent enzymes¹. In E. coli and S. typhimurium the only reported B₁₂-dependent enzyme catalyses the final step in methionine biosynthesis—the methylation of homocysteine to give methionine. These bacteria do not depend on exogenous B₁₂ for growth, however, for they have an alternative B₁₂-independent homocysteine transmethylase (non-B₁₂ enzyme) which can catalyse the same reaction. The non-B₁₂ transmethylase is much less efficient than the corresponding B₁₂-dependent enzyme: in bacteria grown in the absence of vitamin B₁₂, the non-B₁₂-dependent enzyme comprises 3–5 % of the total soluble protein². If this enzyme is blocked by mutation, the mutant bacteria require either methionine itself or exogenous B₁₂, which allows them to utilise the B₁₂-dependent enzyme for methionine biosynthesis³. We have sought other B₁₂-dependent enzymes in S. typhimurium and found evidence for an ethanolamine deaminase. We did not find other examples of parallel non-B₁₂-dependent and B₁₂-dependent enzymes in S. typhimurium. E. coli also seems to have an ethanolamine deaminase.