Studies of NADH displacement in the presence of excess aldehyde
dehydrogenase confirmed that a conformational change of the enzyme. NADH
binary complex occurs as an essential step in the reaction mechanism.
Modification of sulphydryl groups on the enzyme by the thiol
reagent p-(chloromercuri) benzoate (PCMB) produced either activation or
inhibition of the enzyme activity, depending upon the relative
concentrations of PCMB and aldehyde dehydrogenase, the mixing
conditions, and on the concentration of the aldehyde substrate. There
was no direct evidence to support the widely held view that a
sulphydryl group is catalytically essential.
Studies of the pH dependence of the steady state and presteady
state phases of the reaction indicated that there was a change in the
rate limiting step as the pH was increased from acyl-enzyme hydrolysis
at low pH to release of NADH from the enzyme at high pH. At low pH the
release of NADH may occur before acyl-enzyme hydrolysis. Activation by
high concentrations of propionaldehyde was shown to occur over the
entire pH range (5 to 10).
The reaction could be reversed when acid anhydrides were used to
acylate the enzyme. NADH complex but the binding of the substrates for
the reverse reaction did not appear to be ordered. Under these
conditions other groups on the enzyme were acylated with resultant
inhibition or activation of the dehydrogenase activity of the enzyme
depending on the relative concentrations of the substrates and
reactants, and on the mixing conditions.
The enzyme catalysed the hydrolysis of p-nitrophenylacetate in the
presence of NADH but with no significant production of acetaldehyde.
It was conclude d that ester hydrolysis does not occur at the site of
Preliminary studies on the reaction of the
diethylpyrocarbonate indicated that the enzyme may
catalytically essential histidine residue.
Irregular pagination: missing page 61.
Content removed due to copyright restrictions:
MacGibbon, A. K., Motion, R. L., Crow, K. E., Buckley, P. D., & Blackwell, L. F. (1979). Purification and properties of sheep-liver aldehyde dehydrogenases. European Journal Of Biochemistry, 96(3), 585-595.
Motion, R. L., Blackwell, L. F., & Buckley, P. D. (1984). Activating effect of p-(chloromercuri)benzoate on the cytoplasmic aldehyde dehydrogenase from sheep liver. Biochemistry, 23(26), 6851-6857.