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Quasi lip oxygenase activity of hemo globin a model for lip oxygenases ec

, : Quasi lip oxygenase activity of hemo globin a model for lip oxygenases ec European Journal of Biochemistry 120(1): 161-168

Hb catalyses at low concentrations (0.01-1 .mu.M) a quasi-lipoxygenase reaction with remarkably high substrate specificity. Formation of lipohydroperoxides was demonstrated only with free 9-cis,12-cis-octadecadienoic acid (linoleic acid), its glycerol monoester, or 9-cis,12-cis-octadecadienol (linoleoyl alcohol). All-cis-9,12,15-octadecatrienoic acid (.alpha.-linolenic acid), all-cis-5,8,11,14-eicosatetraenoic acid (arachidonic acid) and all-cis-9,12,15-eicosatrienoic acid (dihomo-.gamma.-linolenic acid), phospholipids and biological membranes were not attacked. Saturated and unsaturated free fatty acids other than linoleic acid were strong inhibitors of the Hb-catalyzed oxygenation of linoleate. The reaction was also inhibited by cyanide, carbon monoxide, phenolic antioxidants and the lipoxygenase inhibitor salicylhydroxamic acid, as well as by some antiiinflammatory drugs (prednisolone, dexamethasone, acetylsalicylic acid); indomethacin did not inhibit. Protein inhibitors of lipoxygenase reactions were found in lung homogenates of various species [beef, sheep, pig, rabbit]. The molecular activity of the Hb-catalyzed oxygenation of linoleate was comparable with those of true lipoxygenases [rabbit reticulocyte, rat liver mitochondria, beef heart electron transfer particles] and was 8-fold higher than that of free hemin. Accordingly, the quasi-lipoxygenase activity of Hb was strongly suppressed by denaturation (heating, urea, pronase treatment, acid splitting). The apparent S0.5, the activation energy and the pH-optimum did not differ from those of most of the true lipoxygenases. The reaction product hydroperoxylinoleic acid but not hydrogen peroxide was an activator of the Hb-catalyzed oxygenation of linoleate. Hydroxylinoleic acid was a competitive inhibitor of the activation. At Hb concentrations higher than 1 .mu.M the quasi-lipoxygenase activity was completely suppressed presumably owing to the formation of inhibitory concentrations of hydroxylinoleate via the lipohydroperoxidase activity of Hb. The quasi-lipoxygenase reaction of Hb exhibits a suicidal behavior, caused by destruction of heme groups. The properties of the free hemin-catalyzed reaction were similar to those of Hb. Among other hemoproteins tested myoglobin and cytochrome P-450LM show comparable activities. The mechanism of the Hb-catalyzed oxygenation of linoleate seems to involve a change of valency of the heme iron.


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