The mechanism of action of aldolases.

was discovered in 1952 by Racker (1) in mammalian liver and as a constitutive enzyme in Escherichiu coli. In 1958, Domagk and Horecker (2) reported the presence of a similar enzyme in extracts of cells of Lactobacillus plantarum adapted to growth on 2-deoxy-n-ribose. The properties and partial purification of this inducible enzyme were subsequently described by Pricer and Horecker (3). Previous investigations have demonstrated the interaction of rabbit muscle aldolase and yeast transaldolase with their respective substrates, dihydroxyacetone phosphate and fructose 6-phosphate, to yield Schiff base intermediates (4-7). These have been shown to involve e-amino groups of specific lysine residues of these enzymes. Reduction of the Schiff base intermediates with borohydride, followed by acid hydrolysis, permitted the isolation and identification of the reduced lysine substrate derivative, N6-/%glyceryllysine. Recently, evidence has also been obtained for similar Schiff base intermediates with the enzymes 2-keto-3-deoxy 6-phosphogluconate aldolase from Pseudomonas flzwrescens and 2-deoxy-nribose B-phosphate aldolase from L. plantarum (8). In the latter case, the reduced lysine-acetaldehyde complex has been identified as W-ethyllysine (9). In each instance it was essential to demonstrate a loss of enzyme activity following reduction with borohydride which was dependent upon the presence of the specific enzyme substrate and which did not occur with other aldehydes, including those, such as n-glyceraldehyde a-phosphate, which were capable of condensing with the Schiff base intermediates to form the aldol condensation products. In the process of testing this specificity of deoxyribose-5-P aldolase, partial enzyme inactivation after reduction with borohydride in the presence of propionaldehyde was encountered. This suggested that propion-