Nonenzymatic acetolactate oxidation to diacetyl by flavin, nicotinamide and quinone coenzymes.
Acetolactate nonenzymatically reduced flavins, quinones and nicotinamide coenzymes in a time-dependent manner at physiological pH and moderate temperature. In the presence of excess acetolactate, the reduction of FAD and NAD+ followed pseudo-first-order kinetics. The rate of reduction was proportional to the concentration of acetolactate, and the rate constants at 37 degrees C and pH 7.5 were 4.8 x 10(-2) M-1 s-1 and 7.4 x 10(-3) M-1 s-1 for FAD and NAD+, respectively. In contrast, ubiquinone reduction followed pseudo-zero-order kinetics in the presence of excess acetolactate. At 37 degrees C and pH 7.5, the rate of reduction was proportional to the acetolactate concentration, and the apparent rate constant was 8.3 x 10(-6) s-1. In contrast to FAD, the rate of reduction of ubiquinone was higher at low pH. The kinetics of ubiquinone reduction suggested that the rate-limiting step was acetolactate decarboxylation and formation of the enolate anion, whereas the rate of FAD reduction was governed by the second-order reaction of the enolate anion. Following the oxidation, acetolactate was converted to diacetyl. Reduced FAD formed by the reaction with acetolactate generated a low rate of O2 consumption during assays of the oxygenase activity of acetohydroxy acid synthase. The reaction of acetolactate with quinones may provide a mechanism for the nonenzymatic formation diacetyl in whole milk.
Department of Microbiology, University of Georgia, Athens 30602-2605, USA.
H S
HS
Park
R
R
Xing
W B
WB
Whitman
D016227:Benzoquinones / Q000378:metabolism*; D002855:Chromatography, Thin Layer; D003931:Diacetyl / Q000378:metabolism*; D005415:Flavins / Q000378:metabolism*; D007773:Lactates / Q000378:metabolism*; D009536:Niacinamide / Q000378:metabolism*; D010084:Oxidation-Reduction
D016227:Benzoquinones; D005415:Flavins; D007773:Lactates; D009536:Niacinamide; C004532:quinone; C006359:alpha-acetolactate; D003931:Diacetyl