Abstract

Theophylline is metabolized in the liver by one or more cytochrome P-450 enzymes. To assess the amounts and types of these human cytochromes P-450, we incubated theophylline with microsomes prepared from 22 different human livers in the presence of NADPH, and measured simultaneous rates of 1- and 3-N-demethylations to 3-methylxanthine (3-MX) and 1-methylxanthine (1-MX), respectively; and 8-hydroxylation to 1,3-dimethyluric acid (1,3-DMU). Under optimal conditions, 3-MX, 1-MX, and 1,3-DMU formation proceeded with mean Km values of 2.05, 1.93, and 5.34 mM and Vmax values of 2.28, 2.48, and 23.4 pmol/mg/min, respectively. Formation of 3-MX and 1-MX correlated best with amounts of the immunoreactive protein HLd (P-450IA2) (p less than 0.05), whereas formation of 1,3-DMU correlated with the microsomal content of HLp (P-450IIIA3) and HLj (P-450IIE1). In immunoinhibition experiments, incubations conducted with a polyclonal anti-rat P-450c/d antibody, the formation of all the three theophylline metabolites (p less than 0.05) was significantly inhibited. However, addition of isoform-specific anti-rat-P-450d antibodies to the microsomal mixture significantly inhibited 1-N-demethylation, selectively, with little (if any) inhibition of 3-N-demethylation or 8-hydroxylation. Nonspecific cytochrome P-450 inhibition was ruled out by showing that erythromycin N-demethylation, an activity catalyzed by HLp, was unaffected by either anti-P-450c/d (P-450IA1/IA2) or anti-P-450d. Anti-rat-P-450p antibodies failed to block formation of theophylline metabolism, but did inhibit erythromycin N-demethylase.(ABSTRACT TRUNCATED AT 250 WORDS)