Evidence for a catalytic role of tyrosine 383 in the peptidase reaction of leukotriene A4 hydrolase.

Leukotriene A4 (LTA4) hydrolase is a bifunctional zinc metalloenzyme which catalyzes the final step in the biosynthesis of the proinflammatory leukotriene B4 and which also possesses a peptidase activity. From sequence comparisons with aminopeptidases, a tyrosine at position 383 in LTA4 hydrolase has been suggested as a possible catalytic amino acid. To explore the potential role of this amino acid in catalysis, we replaced the tyrosine residue with phenylalanine, histidine or glutamine residues by site-directed mutagenesis. The mutated cDNAs were expressed in Escherichia coli and the resulting recombinant proteins, named [Y383F]LTA4 hydrolase, [Y383H]LTA4 hydrolase and [Y383Q]LTA4 hydrolase, were purified to homogeneity to allow assays of both the epoxide hydrolase activity, i.e. the conversion of LTA4 into leukotriene B4, and the peptidase activity. None of the mutated proteins exhibited significant peptidase activities, all of them showing activities less than 0.3% that of the wild-type enzyme. The epoxide hydrolase activity was not affected to the same degree and corresponded to 11, 16 and 17% that of the unmutated enzyme for [Y383F]LTA4 hydrolase, [Y383H]LTA4 hydrolase and [Y383Q]LTA4 hydrolase, respectively. Kinetic analysis was performed with the mutant [Y383Q]LTA4 hydrolase, which revealed an approximately 10-fold increase in Km for leukotriene A4 compared to that for the unmutated enzyme. At high concentrations of substrate, the difference in enzyme velocity was only moderate, with Vmax values of 600 nmol.mg-1.min-1 and 1000 nmol.mg-1.min-1 for [Y383Q]LTA4 hydrolase and the wild-type enzyme, respectively. No such effect of substrate concentration could be observed on the peptidase activity. As a positive control, we exchanged a glycine residue in position 386 for an alanine residue, and the recombinant protein, [G386A]LTA4 hydrolase retained 19% and 77% of the peptidase and epoxide hydrolase activities, respectively. The results from this study are consistent with a role for Tyr383 in the peptidase reaction of LTA4 hydrolase, where it may act as a proton donor in a general base mechanism. However, our data do not allow a similar interpretation for the mechanism involved in the hydrolysis of LTA4 into LTB4.