Molecular Pharmacology, Vol 8, 204-214, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

The Effect of Piperonyl Butoxide Concentration on the Formation of Cytochrome P-450 Difference Spectra in Hepatic Microsomes From Mice

¹ Department of Entomology, North Carolina State University, Raleigh, North Carolina 27607

A direct relationship is observed between the formation of the piperonyl butoxide-reduced cytochrome P-450 double Soret difference spectrum (type III) and inhibition of the formation of the carbon monoxide—cytochrome P-450 difference spectrum. This relationship appears to involve a single binding site..

The type III piperonyl butoxide—cytochrome P-450 interaction exerts a similar effect on the formation of the carbon monoxide-, pyridine-, n-octylamine-, and ethyl isocyanide— cytochrome P-450 difference spectra. Inhibition of the formation of the spectrum produce by (+)-benzphetamine is greater than that observed with the other ligands.

The relationship between the change in absorbance and the change in absorbance divided by substrate concentration is linear for the formation of the type III spectrum produced by piperonyl butoxide, inhibition by the latter of the formation of the spectrum produced by carbon monoxide, and formation of the pyridine type II substrate spectrum at low pyridine concentrations. At high pyridine concentrations and with (+)-benzphetamine this relationship is curvilinear.

The piperonyl butoxide type III interaction does not inhibit the formation of the pyridine type II substrate spectrum at low pyridine concentrations. At high pyridine concentrations the inhibition observed is similar to the inhibition of the (+)-benzphetamine type I substrate spectrum.

Our observations suggest that piperonyl butoxide binds to one form of cytochrome P-450 at a single site and that an additional form of the cytochrome is present in noninduced hepatic microsomes from mice, which does not bind type I substrates or form a type III interaction with piperonyl butoxide.