Abstract
A dehydrogenase catalyzing the NADH-dependent reduction of diacetyl to (+)-acetoin was found in Lactobacillus kefir. The enzyme could be purified by heat treatment, ion exchange chroraatography and gel filtration (130 U/mg). Further purification resulted in two isoenzymes, E1 (1061 U/mg) with a molecular weight of 66 kD and E2 (366 U/mg) of 74 kD. Important biochemical data concerning the application of the enzyme are: pH-optimum at 4.3 (E1,) and 5.0 (E2), temperature optimum at 70d`C, stable at 57d`C for 10 min. Besides diacetyl several other diketones were reduced. For diacetyl, a KM-value of 310 mM for E1 and 67 mM for E2 was measured. The reverse reaction of acetoin oxidation in the presence of NAD could not be observed, whereas D(—)-lactate was oxidized by the enzyme. In exploratory experiments conversions of 200 mM diacetyl in batch reactions with coenzyme regeneration resulted in 170 mM product concentration. The formation of (+)-acetoin was confirmed by determination of its molar optical rotation ...
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