HYDROXYLATION AND DEMETHYLATION OF THE TRICYCLIC ANTIDEPRESSANT NORTRIPTYLINE BY CDNA-EXPRESSED HUMAN CYTOCHROME-P-450 ISOZYMES

The metabolism of nortriptyline was studied in vitro using cDNA-expres sed human cytochrome P450 isozymes 1A2, 3A4, 2C19, and 2D6, CYP2D6 was the sole isozyme mediating hydroxylation of nortriptyline, the quanti tatively most important metabolic pathway, and only (E)-10-OH-nortript yline was formed. CYP2D6, 2C19, and 1A2, mentioned in decreasing order of significance, mediated the demethylation reaction of nortriptyline , whereas 3A4 did not participate in the metabolism of nortriptyline. Concerning the quantitative relations, CYP2D6 exhibited a high affinit y with respect to hydroxylation and demethylation (K-m 0.48-0.74 mu mo l/l), a high hydroxylation capacity (V-max 130 mol/hr/mol CYP) and a s omewhat lower demethylation capacity (V-max 19 mol/hr/mol CYP). The af finities of 1A2 and 2C19 were 100-fold lower (K-m 54-118 mu mol/l), Th e capacity of 1A2 was low (V-max 6.8 mol/hr/mol CYP), whereas 2C19 had the highest demethylation capacity (V-max 93 mol/hr/mol CYP). Taking into account the relative amounts of CYP isozymes present in the liver , about 90% of the metabolism was estimated to depend on CYP2D6, with CYP2C19 and 1A2 mediating the remaining 10%. In subjects lacking the 2 D6 isozyme, CYP2C19 and 1A2 are expected to be of major importance for elimination of nortriptyline.