Abstract
A mutant strain of Salmonella typhimurium (SL 1634 dml-51) capable of growth on d-malate as sole carbon source was shown to produce d-malic enzyme. This enzyme was absent in the parent wild-type strain which was unable to grow on d-malate. Growth of the mutant on d-malate also resulted in a greatly increased level of β-isopropylmalic enzyme compared with its level in the wild-type strain grown on citrate or l-malate. The d-malic and β-isopropylmalic enzymes, both of which catalyze a nicotinamide adenine dinucleotide- and Mg++-dependent oxidative decarboxylation of their respective substrates, were shown to be distinct enzymes by selective inhibition with erythro-dl-β-hydroxyaspartate and by other methods. Cell extracts of the mutant strain also oxidized dl-β-methyl-, dl-β-ethyl-, dl-β-propyl- and dl-ββ-dimethylmalates, in order of decreasing activity. dl-β-Methyl-malate was shown to be oxidized by both the d-malic and the β-isopropylmalic enzymes, whereas the oxidation of the other β-alkylmalates appeared to be effected exclusively by the β-isopropylmalic enzyme. β-Isopropylmalic enzyme activity was induced by d-malate but not by l-malate, showing that it behaved as a d-malictype enzyme. Growth of Aerobacter aerogenes on d-malate, which caused induction of d malic enzyme, resulted in only a small increase in the activity of β-isopropylmalic enzyme.
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