Kinetic involvement of acetaldehyde substrate inhibition on the rate equation of yeast aldehyde dehydrogenase

In order to evaluate the effectiveness of aldehyde dehydrogenase (ALDH) from Saccharomyces cerevisiae as a catalyst for the conversion of acetaldehyde into its physiologically and biologically less toxic acetate, the kinetics over broad concentrations were studied to develop a suitable kinetic rate expression. Even with literature accounts of the binding complexations, the yeast ALDH currently lacks a quantitative kinetic rate expression accounting for simultaneous inhibition parameters under higher acetaldehyde concentrations. Both substrate acetaldehyde and product NADH were observed as individual sources of inhibition with the combined effect of a ternary complex of acetaldehyde and the coenzyme leading to experimental rates as little as an eighth of the expected activity. Furthermore, the onset and strength of inhibition from each component were directly affected by the concentration of the co-substrate NAD. While acetaldehyde inhibition of ALDH is initiated below concentrations of 0.05 mM in the presence of 0.5 mM NAD or less, the acetaldehyde inhibition onset shifts to 0.2 mM with as much as 1.6 mM NAD. The convenience of the statistical software package JMP allowed for effective determination of experimental kinetic constants and simplification to a suitable rate expression: equation presented where the last three terms represent the inhibition complex terms for acetaldehyde, acetaldehyde-NADH, and NADH, respectively. The corresponding values of K_I-Ald, K_I-Ald-NADH, and K_I-NADH for yeast ALDH are 2.55, 0.0269, and 0.162 mM at 22 °C and pH7.8.