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 allosteric site that binds both substrate, linoleic acid, and inhibitors; oleic acid (OA) and oleyl sulfate (OS). OS (K(D) approximately 0.6 microM) is a approximately 30-fold more potent inhibitor than OA (K(D) approximately 20 microM) due to the increased ionic strength of the sulfate moiety. To further investigate the role of the sulfate moiety on lipoxygenase function, SLO and 15-HLO 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 not inhibited, indicating a binding selectivity of LS for the catalytic site and OS for the allosteric site. The 15-HLO, however, manifests parabolic inhibition kinetics with increasing substrate concentration, and it is irreversibly inhibited by these fatty sulfate substrates at high concentrations. The inhibition can be stopped, however, by the addition of detergent to the fatty sulfate mixture prior to the addition of 15-HLO. These results, combined with the modeling of the kinetic data, indicate that the inhibition of 15-HLO is due to a substrate aggregate. These substrate aggregates, however, do not inhibit SLO and could present a novel mode of inhibition for 15-HLO.