The mechanism of inhibition by 2,2'-pyridylisatogen tosylate of NADPH-linked enzyme activities in microsomes isolated from rat liver.

Microsomal preparations isolated from rat liver were used to study the action of 2.2'-pyridylisatogen tosylate (PIT) on aniline hydroxylation, cytochrome c reduction and NADPH oxidation. PIT was found to inhibit both the NADPH-dependent (5-100 microM, PIT) and the NADPH-independent (0.05-2.5 mM, PIT) hydroxylation of aniline, but had no significant effect on either the NADPH-dependent oxidation of hexobarbital, or the NADPH-independent hydrolysis of glucose-6-phosphatase. PIT was also found to inhibit cytochrome c reductase competitively (Ki = 35 microM) and to stimulate NADPH oxidation (ED50 = 6.5 microM) PIT and aniline were both found to bind to the microsomal haemoprotein cytochrome P-450 and produce Type II spectral changes. It is proposed that PITs ability to bind to the haemoprotein and its ability to accept electrons from the microsomal NADPH-cytochrome c reductase system leads to the inhibition of aniline hydroxylase activity.