Guanethidine N-oxide formation as a measure of cellular flavin-containing monooxygenase activity

The usefulness of guanethidine N-oxide formation as a measure of cellular flavin-containing monooxygenase activity was assessed using the purified hog liver enzyme, rat liver microsomes and hepatocytes. The apparent Km and Vmax for this reaction in hepatocytes were 0.30 +/- 0.20 mM and 0.81 +/- 0.36 nmole per 10(6) cells min-1 respectively. The Km for the purified enzyme was 0.31 mM and the Vmax was 0.56 nmole per microgram enzyme min-1. 2-Diethylaminoethyl-2,2-diphenyl valerate (SKF-525A) at a concentration of 0.5 mM had no effect on guanethidine N-oxide formation by either rat liver microsomes or the purified enzyme. In contrast 2,4-dichloro-6-phenylphenoxyethylamine (DPEA) at the same concentration caused greater than a 100% increase in the microsomal production of guanethidine N-oxide. The tertiary amines imipramine, chloropromazine and methylpyrilene inhibited N-oxide formation by both hepatocytes and the purified enzyme. These data indicate that guanethidine N-oxide formation can be used as a measure of cellular flavin-containing monooxygenase activity.