Lipoxin syntheses by arachidonate 12- and 5-lipoxygenases purified from porcine leukocytes

Mutations of Triad Determinants Changes the Substrate Alignment at the Catalytic Center of Human ALOX5

For the specificity of ALOX15 orthologs of different mammals, the geometry of the amino acids Phe353, Ile418, Met419, and Ile593 ("triad determinants") is important, and mutagenesis of these residues altered the reaction specificity of these enzymes. Here we expressed wild-type human ALOX5 and its F359W/A424I/N425M/A603I mutant in Sf9 insect cells and characterized the catalytic differences of the two enzyme variants. We found that wild-type ALOX5 converted arachidonic acid mainly to 5(S)-hydroperoxyeicosatetraenoic acid (HpETE). In contrast, 15(S)- and 8(S)-H(p)ETE were formed by the mutant enzyme. In addition to arachidonic acid, wild-type ALOX5 accepted eicosapentaenoic acid (EPA) as substrate, but C18 fatty acids were not oxygenated. The quadruple mutant also accepted linoleic acid and α- and γ-linolenic acid as substrate. Structural analysis of the oxygenation products and kinetic studies with stereospecifically labeled 11(S)- and 11(R)-deutero-linoleic acid suggested alternative ways of substrate orientation at the active site. In silico docking studies, molecular dynamics simulations, and quantum mechanics/molecular mechanics (QM/MM) calculations confirmed this hypothesis. These data indicate that "triad determinant" mutagenesis alters the catalytic properties of ALOX5 abolishing its leukotriene synthase activity but improving its biosynthetic capacity for pro-resolving lipoxins.

12/15-lipoxygenase during the regulation of inflammation, immunity, and self-tolerance

12/15-Lipoxygenase (12/15-LO) catalyzes the oxidation of free and esterified fatty acids thereby generating a whole spectrum of bioactive lipid mediators. This enzyme is involved in the regulation of various homeostatic processes as well as in the pathogenesis of multiple diseases. During the innate and adaptive immune response, 12/15-LO and its products exert both pro- and anti-inflammatory effects. Likewise, this enzyme has been implicated in the pathogenesis of autoimmune disease as well as in the maintenance of self-tolerance. This review will summarize our current knowledge about the role of 12/15-LO and will try to examine the two faces of this enzyme within the context of inflammation and immunity.