Microbial metabolism of amino ketones. l-1-aminopropan-2-ol dehydrogenase and l-threonine dehydrogenase in Escherichia coli

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Abstract
A wide range of inter-mediary metabolites and substrate analogues have no effect on the oxidation of DL-1-aminopropan-2-ol to aminoacetone by washed-cell suspensions of E. coli. Only DL-2-hydroxy-2-phenylethylamine, DL-1,3-diaminopropan-2-ol, DL-serine and L-1-(3,4-dihydroxyphenyl)-2-aminoethanol act as inhibitors. Dialysed cell-free extracts of E. coli exhibit an NAD+-dependent DL-1-aminopropan-2-ol-dehydrogenase activity of approx. 8 mumoles of amino-acetone formed/mg. of protein/ min. at the pH optimum of approx. 10. The Km values for the coenzyme and DL-amino alcohol are approx. 0.4 and 10.0 mM respectively. A smaller peak of activity occurs at pH 7.0-7.2, the Km for NAD+ at pH being approx. 0.05 m[image]. Enzyme activity in cell-free extracts is inhibited by DL-2-hydroxy-2-phenylethylamine, DL-1-aminopropane-2,3-diol and DL-serine. DL-Phenylserine and DL-1-aminobutan-2-ol are oxidized to compounds reacting as amino ketones. In fresh cell-free extracts L(+)-1-aminopropan-2-ol preparations are oxidized more rapidly than racemic or laevo-rotatory material, the D(-)-enantiomorph appearing to act as a competitive inhibitor. The Km for L(+)-1-aminopropan-2-ol appears to be approx. 1.5 m[image] when highly resolves substrate preparations are used, either in the free base form or as the L(+)-tartrate salt. L(+)-1-Aminopropan-2-ol dehydrogenase is a labile enzyme, and in appropriately treated extracts activity towards the D-enantiomorph is detectable and relatively higher than that towards the L-enantiomorph. Optimum activity of L-threonine-dehydrogenase in cell-free extracts is exhibited at pH 9.6 in the presence of NAD+. The Km values for coenzyme and amino acid substrate are approx. 0.08 and 5.0 m[image] respectively. This enzyme is distinct from 1-aminopropan-2-ol defiydrogenases on the basis of kinetic evidence, and the separation of activities by gel filtration. Both L-threonine and DL-1-aminopropan-2-ol dehydrogenases are markedly inhibited by 8-hydroxyquinoline and p-chloromercuri-benzoate, but only slightly by other chelating and thiol reagents. E. coli is incapable of growth on simple synthetic media, containing a variety of carbon sources, when DL-1-aminopropan-2-ol is supplied as the sole source of nitrogen. It appears unlikely that the microorganism can deaminate aminoacetone. The metabolic roles of L-threonine dehydrogenase, aminoacetone and l-aminopropan-2-ol dehydrogenases are discussed.

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