Microbial metabolism of amino ketones. Aminoacetone formation from 1-aminopropan-2-ol by a dehydrogenase in escherichia coli

Abstract

Washed-cell suspensions of E. coli, incubated at the optimum pH of 6.4 and with a saturating substrate concentration of approx. 10 mM, convert DL-1-aminopropan-2-ol into aminoacetone at a rate of approx. 4.0 m[mu]moles/ mg dry wt. of cell/min. at 30[degree]; Mg2+, Mn2+, Co2+, Zn2+, Ca2+, K+ and NH4+, as sulfates, and ethylenediaminetetraacetate (EDTA) have no effect on this rate, although Cu2+ inhibits and Fe2+ activates to some extent. Conditions of growth markedly affect the rate of aminoacetone production by cell suspensions. Dialyzed cell-free extracts of E. coli exhibit l-aminopropan-2-ol-dehydrogenase activity, the enzyme having optimum activity at pH 7.0, a requirement for nicotinamide adenine dinucleotide (NAD+) and K+, and a Km for the amino alcohol substrate of 0.8 mM, calculated for a single enantiomorph. Under optimum conditions l-aminopropan-2-ol dehydrogenase forms aminoacetone at rate of approx. 3.0 mumoles/mg of protein/min. at 37[degree]. The enzyme is only slightly inhibited by DL-3-hydroxybutyrate and DL-2-hydroxy-2-phenylethylamine. L-Threonine-dehydrogenase activity is exhibited by both whole cells and cell-free extracts. Whole cells produce aminoacetone from L-threonine more slowly than they do from DL-1-amlno-propan-2-ol, whereas the situation is reversed in cell-free extracts. Both kinetic evidence, and the fact that synthesis of 1-aminopropan-2-ol dehydrogenase, but not of threonine dehydrogenase, is repressed by compounds such as glucose and pyruvate, provide evidence that the amino alcohol is oxidized by a specific enzyme. The metabolic role of l-aminopropan-2-ol dehydrogenase is discussed.