Decarboxylation of alpha-keto acids by Streptococcus lactis var. maltigenes.

Decarboxylation rates for a series of C-3 to C-6 alpha-keto acids were determined in the presence of resting cells and cell-free extracts of Streptococcus lactis var. maltigenes. The C-5 and C-6 acids branched at the penultimate carbon atom were converted most rapidly to the respective aldehydes in the manner described for alpha-carboxylases. Pyruvate and alpha-ketobutyrate did not behave as alpha-carboxylase substrates, in that O(2) was absorbed when they were reacted with resting cells. The same effect with pyruvate was noted in a nonmalty S. lactis, accounting for CO(2) produced by some "homofermentative" streptococci. Mixed substrate reactions indicated that the same enzyme was responsible for decarboxylation of alpha-ketoisocaproate and alpha-ketoisovalerate, but it appeared unlikely that this enzyme was responsible for the decarboxylation of pyruvate. Ultrasonic disruption of cells of the malty culture resulted in an extract inactive for decarboxylation of pyruvate in the absence of ferricyanide. Dialyzed cell-free extracts were inactive against all keto acids and could not be reactivated.