Formation of acetaldehyde from threonine by lactic acid bacteria

Group N streptococci were found to cleave threonine to form acetaldehyde and glycine. Threonine aldolase, the enzyme catalysing this reaction, was found in all strains except Streptococcus cremoris Z8, an organism which had been shown previously to have a nutritional requirement for glycine. The enzyme was strongly inhibited by glycine and cysteine. The inhibition showed characteristics of allosteric inhibition and was pH-dependent. Inhibition by glycine, but not by cysteine, was highly specific. Analogues and derivatives of cysteine which contained a thiol group and a free amino group inhibited the activity of threonine aldolase. The presence of a carboxyl group was not necessary for inhibition. The cleavage of threonine by whole-cell suspensions was stimulated by either an energy source to aid transport or by rendering the cells permeable to substrate with oleate. Threonine did not appear to be degraded by enzymes other than threonine aldolase, as threonine dehydratase activity was low and NAD- and NADP-dependent threonine dehydrogenases were absent.