DETERMINATION OF HUMAN HEPATIC CYTOCHROME P4501A2 ACTIVITY IN-VITRO -USE OF TACRINE AS AN ISOENZYME-SPECIFIC PROBE

Oxidative metabolism of the cognition activator tacrine (1,2,3,4-tetra hydro-9-aminoacridine) is thought to be catalyzed by cytochrome P4501A 2 (CYP1A2). In this study, the use of tacrine as a specific substrate to measure CYP1A2 activity in vitro was investigated. Tacrine metaboli sm was assessed in 16 human liver microsomal samples. Initially, the p ercentage conversion of tacrine to stable metabolites (i.e. 1-, 2-, 4- , and 7-hydroxytacrine) at a single time point was correlated with lev els of CYP1A2 apoprotein. Apoprotein was detected by immunoquantificat ion using a monospecific CYP1A2 antipeptide antibody. Significant corr elations were seen between CYP1A2 content and the degree of l-hydroxyl ation (r = 0.81, p < 0.001), 7-hydroxylation (r = 0.70, p < 0.001), an d metabolism to all stable products (r = 0.82, p < 0.001). The major m etabolite formed in all livers was 1-hydroxytacrine. The conversion of tacrine to this metabolite was examined in more detail, The rate of f ormation varied from 19.2 pmol min(-1) mg(-1) to 101.0 pmol min(-1) mg (-1). There was a significant correlation (r = 0.84, p < 0.001) betwee n the rate of formation and CYP1A2 levels. Tacrine metabolism was also compared with the rate of formation of 3-methylxanthine, from theophy lline, a reaction previously shown to be catalyzed by CYP1A2. Signific ant correlations were found between 3-methylxanthine formation and all quantified tacrine metabolites. The rate of 3-methylxanthine generati on also correlated with CYP1A2 apoprotein levels. It is concluded, the refore, that tacrine is a valuable probe for the determination of huma n hepatic CYP1A2 activity in vitro.