Studies on a ketone reductase in human and rat liver and kidney soluble fraction using warfarin as a substrate.

The enantiomers of warfarin were reduced to warfarin alcohols on incubation with human and rat liver and kidney cytosol. The metabolism was NADPH-dependent with NADH being only one tenth as effective at supporting the reduction. R-warfarin was reduced at a greater rate than S-warfarin by each of the tissues examined. At a concentration of 2·08 mM the rate of S-warfarin reduction ranged from 5 to 50% of the rate occurring with R-warfarin in the same tissue fraction. R-warfarin (1·3 mM) was mainly reduced to warfarin alcohol 1 (RS) (77%) the remainder being warfarin alcohol 2 (RR). S-warfarin formed slightly more warfarin alcohol 2 (SS) (57%) than warfarin alcohol 1 (SR). The apparent K_m values for the reduction of R-warfarin by human and rat liver and kidney cytosol ranged from 0·54 to 1·55 mM. The human liver reductase, even 3·5–8·5 hr after death, was significantly more active (2 times) than the rat liver reductase. Stability studies using rat liver indicated that about half of the reductase activity activity may be lost during the first 6 hr after death. This would suggest that human liver had more than four times the warfarin reductase activity of rat liver.