Competitive inhibition of the 5-lipoxygenase-catalysed linoleate oxidation by arachidonic and 5-hydroperoxy-eicosatetraenoic acids.

Linoleic and arachidonic acids are competing substrates for 5-lipoxygenase from barley. When these two substrates are added simultaneously, arachidonic acid acts as a competitive inhibitor of linoleic acid oxidation with Ki of 20 microM, the same value as the Michaelis constant for arachidonate oxygenation by this enzyme (22 +/- 3 microM). Linoleic acid hydroperoxide accumulated in the reaction mixture does not inhibit the enzymatic process, while arachidonic acid hydroperoxy product (5-hydroperoxy-6,8,11,14-eicosatetraenoic acid) inhibits it with very low Ki equal to 0.5 microM.