Coenzyme A- and NADH-dependent esterase activity of methylmalonate semialdehyde dehydrogenase

Methylmalonate semialdehyde dehydrogenase purified to homogeneity from rat liver possesses, in addition to its coupled aldehyde dehydrogenase and CoA ester synthetic activity, the ability to hydrolyze p-nitrophenyl acetate. The following observations suggest that this activity is an active site phenomenon: (a) p-nitrophenyl acetate hydrolysis was inhibited by malonate semialdehyde, substrate for the dehydrogenase reaction; (b) p-nitrophenyl acetate was a strong competitive inhibitor of the dehydrogenase activity; (c) NAD+ and NADH activated the esterase activity; (d) coenzyme A, acceptor of acyl groups in the dehydrogenase reaction, accelerated the esterase activity; and (e) the product of the esterase reaction proceeding in the presence of coenzyme A was acetyl-CoA. These findings suggest that an S-acyl enzyme (thioester intermediate) is likely common to both the esterase reaction and the aldehyde dehydrogenase/CoA ester synthetic reaction.