STEREOSELECTIVE REVERSIBLE KETONE FORMATION FROM 10-HYDROXYLATED NORTRIPTYLINE METABOLITES IN HUMAN LIVER

1. E- and Z-10-hydroxynortriptyline are major metabolites of amitripty line and nortriptyline in man. Upon incubation with human liver micros omes or cytosol, these metabolites were oxidized to the corresponding ketones, E- and Z-10-oxonortriptyline. (+)-E- and (+)-Z-10-hydroxynort riptyline were distinctly preferred over the (-)-isomers as substrates . NADP(+) supported the oxidation in cytosol whereas in microsomes NAD (+) was the best cofactor. 2. Incubation of E- and Z-10-oxonortriptyli ne with NADPH and cytosol resulted in the nearly exclusive formation o f (+)-E- and (+)-Z-10-hydroxynortriptyline. Kinetic analysis revealed high-affinity reduction (K-m 1-2 mu M) of the two ketones and an addit ional low-affinity component with the E-isomer. 10-Oxonortriptyline re duction was also catalysed by rabbit, but not by rat or guinea pig liv er cytosol. 3. With [4-H-3]NADPH as cosubstrate, tritium was incorpora ted into E- and Z-10-hydroxynortriptyline preferentially from the pro- 4R position. Redox cycling of (+)-E- and (+)-Z-10-hydroxynortriptyline in cytosol in the presence of NAD(+) and NADPH was indicated by H-3 i ncorporation from [pro-4R-H-3]NADPH. 4. Recombinant human carbonyl red uctase catalysed low-affinity reduction of E-10-oxonortriptyline with preferential transfer of the pro-4S-H-3 of labelled NADPH. 5. Ketone r eduction in cytosol was strongly inhibited by 9,10-phenanthrenequinone and dehydrolithocholic acid and moderately by other 3-oxo steroids an d some antiinflammatory drugs. 6. The high-affinity reduction of E- an d Z-10-oxonortriptyline and the oxidation of the alcohols in cytosol a re probably mediated by a member of the aldo-keto reductase family of enzymes.