Substrate Characteristics of Human Liver Aldehyde Dehydrogenase

Abstract

Human liver NAD+-dependent aldehyde dehydrogenase oxidizes several aldehydes which differ in the extent of their hydration in aqueous solution. Results of experiments indicate that the free carbonyl form is the operative substrate. When nonenzymatic hydration of acetaldehyde was slower than the enzymatic oxidation reaction, acetaldehyde which initially was anhydrous was completely oxidized at an almost linear rate. By contrast, oxidation of acetaldehyde already in equilibrium with hydrate was biphasic. An initial rapid reaction accounted for approximately half the total aldehyde, i.e. the free aldehyde. During the second stage the acetaldehyde, initially in hydrate form, was oxidized at a reduced rate expected for a prior rate-limiting dehydration step. Small amounts of hydroxylamine, capable of reacting rapidly with free formaldehyde, caused transient inhibition of oxidation of aqueous formaldehyde (which is almost completely hydrated). Furthermore, Michaelis constants for crotonaldehyde and benzaldehyde (which are largely unhydrated in aqueous solution) were very low.