Inhibition studies of soybean and human 15-lipoxygenases with long-chain alkenyl sulfate substrates

Lipoxygenases are currently potential targets for therapies against asthma, artherosceloris, and cancer. Recently, inhibition studies on both soybean (SLO) and human lipoxygenase (15-HLO) revealed the presence of an allosteri c site that binds both substrate, linoleic acid, and inhibitors; oleic acid (OA) and oleyl sulfate (OS). OS (K-D approximate to 0.6 muM) is a approxim ate to 30-fold more potent inhibitor than OA (K-D approximate to 20 muM) du e to the increased ionic strength of the sulfate moiety. To further investi gate the role of the sulfate moiety on lipoxygenase function, SLO and 15-HL O were assayed against several fatty sulfate substrates (linoleyl sulfate ( LS), cis-11,14-eicosadienoyl sulfate, and arachidonyl sulfate). The results demonstrate that SLO catalyzes all three fatty sulfate substrates and is n ot inhibited, indicating a binding selectivity of LS for the catalytic site and OS for the allosteric site. The 15-HLO, however, manifests parabolic i nhibition kinetics with increasing substrate concentration, and it is irrev ersibly inhibited by these fatty sulfate substrates at high concentrations. The inhibition can be stopped, however, by the addition of detergent to th e fatty sulfate mixture prior to the addition of 15-HLO. These results, com bined with the modeling of the kinetic data, indicate that the inhibition o f 15-HLO is due to a substrate aggregate. These substrate aggregates, howev er, do not inhibit SLO and could present a novel mode of inhibition for 15- HLO.