J. Biol. Chem., Vol. 259, Issue 16, 10092-10099, 08, 1984
Competition between hydrocarbon and barbiturate for spectral binding to hepatic cytochrome P-450. Inferences concerning spin state of the enzyme
WL Backes, M Means and WJ Canady
The substrates hexobarbital and ethylbenzene have been shown to compete for
the spectral binding site of phenobarbital-induced rat hepatic microsomal
cytochrome p-450. The two substrates produce different delta Absmax values,
and the presence of one substrate does not affect the delta Absmax of the
other substrate and vice versa. The respective binding constants for the
two substrates are similarly unaffected. The conclusion drawn from these
observations is that, over the concentration ranges studied, there is no
change in the availability of the enzyme as a result of substrate addition;
the difference in delta Absmax apparently being due to varying abilities of
different substrates to bring about a spin shift in the enzyme. Evidence is
presented to indicate that differences between enzymes from untreated male
rats and phenobarbital-treated male rats are attributable to differences in
the enzyme itself and not to changes in the nature of the membrane brought
about by phenobarbital administration, at least insofar as heat entropy
compensation is concerned. The enthalpy-entropy compensation observed in
the binding of a homologous series of barbiturates to the microsomal
membrane as determined from the membrane concentration dependence of their
binding constants is shown to agree surprisingly well with the direct
determination performed by Sitar and Mannering.
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