Alteration of human leukotriene A4 hydrolase activity after site-directed mutagenesis:: serine-415 is a regulatory residue

Leukotriene A(4) hydrolase (LTA-H) is a bifunctional protein that has aminopeptidase activity, but also contains an epoxide hydrolase activity that converts leukotriene (LT)A(4) to LTB4. The lipid metabolic activity of this enzyme plays a central role in the control of polymorphonuclear leukocyte function and in the development of inflammation. LTA-H is widely spread in many mammalian tissues, although it appears to be inactive in many cases. Regulation of this enzyme's activity by phosyhorylation of a serine at residue 415 has recently been described. Since the activation of LTA-H in the presence of activated PMNL would likely lead to a substantial increase in the production of inflammatory lipids, regulation of LTA-H presents a novel potential target for anti-inflammatory therapy. We have now made a series of site-directed mutants at this site to test the importance of this residue to the activity of LTA-H. Replacement of the critical serine with threonine or glutamine has little effect on either the epoxide hydrolase or aminopeptidase activities. However, replacing serine with a negatively charged amino acid (either aspartate or glutamate), intended to mimic phosphorylation at that site, causes significant reduction in epoxide hydrolase activity (50-70%). These mutations have little effect on the aminopeptidase activity of the LTA-H, suggesting that the mutation models the regulatory event and is not simply due to improper folding of the protein. (C) 1994 Elsevier Science B.V. All rights reserved.