Lipoxygenase catalysis products

Furthermore, in the system with coupled lipase and lipoxygenase, the production rate of HP is governed by the first enzymatic reaction and mass transfer. When TL,- is small (0 to 1 mM equiv. 3 mM LA), the kinetic curve has a sigmoid shape due to surface active properties of LA and HP [25]. Hydrolysis of TL and the increase of LA favor the transfer of LA. Such a transfer allows the lipoxygenase reaction to progress. Since lipox-ygenation consumes LA and produces HP, catalysis and transfer demonstrates a reciprocal influence. 

Therefore, the nature of the oxygenated cholesterol products in atherosclerotic human aorta does not exclude the fact, that they have been formed during lipoxygenase catalysis. Really, our results demonstrated that the activity of C-15 animal lipoxygenase may be greatly stimulated by addition of the atherogenic LDL to the incubation media [21,22] (Figure 6). 

It has been reported that a single methionine residue of rabbit reticulocyte 15-lipoxygenase can be oxidized to its sulphoxide by treatment of the enzyme with 13-hydroperoxy-octadecadienoic acid (a 15-lipoxygenase product from linoleic acid) under anaerobic conditions [59]. Since under this condition the enzyme functioned as lipohydroperoxidase , splitting the hydroperoxide, and resulted in self-inactivation , a central role of the methionine residue was presumed for the catalysis of the enzyme. Recently, this particular methionine has been identified as Met-590 in human 15-lipoxygenase and as Met-591 in rabbit 15-lipoxygenase. When Met-590 in the human enzyme is replaced by leucine by the site-directed mutagenesis, the mutant enzyme is still inactivated by 13-hydroperoxyoctadeca-dienoic acid, 15-HpETE or AA. The result shows that the enzyme inactivation is not attributable to methionine oxidation [60]. 

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