Enzymatic formation of 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acid: kinetics of the reaction and stereochemistry of the product

The enzymatic conversion of leukotriene A4 into 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acid, catalyzed by mouse liver cytosolic epoxide hydrolase (EC 3.3.2.3), was recently described (Haeggstr?m, J., Meijer, J. and R?dmark, O. (1986) J. Biol. Chem. 261, 6332-6337). In the present study, we report analytical data confirming the stereochemistry of this novel enzymatic metabolite of leukotriene A4. By steric analysis of the vicinal diol and comparison with synthetic material, the structure was established as (5S,6R)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acid. Apparent kinetic constants of this reaction were determined and found to be 5 microM and 550 nmol.mg-1.min-1, for Km and Vmax, respectively. Also, a semipurified preparation of human liver cytosolic epoxide hydrolase avidly catalyzed the same hydrolysis of leukotriene A4 (apparent Km was 8 microM). The enzyme was not inactivated by leukotriene A4, as judged by time-course experiments with a second substrate addition.