Abstract
The steady-state kinetics of formaldehyde dehydrogenase from human liver were explored. Non-linearities were obtained in v [velocity] vs. v/[S] (substrate concentration) plots. It was necessary and sufficient to consider 2 reactants of the equilibrium mixture of formaldehyde, glutathione and their hemimercaptal adduct for a complete description of the kinetics. A random sequential reaction scheme is proposed in which adduct and .beta.-NAD+ are the substrates. Glutathione can bind to an allosteric regulatory site and only the glutathione-containing enzyme is considered productive. Various alternative reaction models were examined but no simple alternative was superior to the model chosen. The discrimination was largely based on results of non-linear regression analysis. Several S-substituted glutathione derivatives were tested as activators or inhibitors of the enzyme, but all were without effect. Thio-NAD+, nicotinamide-hypoxanthine dinucleotide and 3-acetylpyridine-adenine dinucleotide could substitute for .beta.-NAD+ as the nucleotide substrate, .alpha.-NAD+ and ADP-ribose were competitive inhibitors with respect to .beta.-NAD+ and non-competitive with glutathione and the adduct. When used simultaneously, the inhibitors were linear competitive vs. each other, indicating a single nucleotide-binding site or, if more than one, non-cooperative binding sites.