Oleyl sulfate reveals allosteric inhibition of soybean lipoxygenase-1 and human 15-lipoxygenase

Inhibition of lipoxygenase (LO) is currently an important goal of biomedical research due to its critical role in asthma, atherosclerosis, and cancer regulation. Steady-state kinetic data indicate that oleic acid (OA) is a simple competitive inhibitor for soybean lipoxygenase; however, kinetic isotope effect (KIE) data suggest a more complicated inhibitory mechanism. To investigate the inhibitory effects of fatty acids on lipoxygenase more thoroughly, we have synthesized a novel inhibitor to lipoxygenase, (Z)-9-octadecenyl sulfate (oleyl sulfate, OS), which imparts kinetic properties that are inconsistent with simple competitive inhibition for both SLO-1 and 15-HLO. The KIE exhibits a hyperbolic rise with addition of OS, indicating the formation of a catalytically active ternary complex with K-D values of 0.6 +/- 0.2 and 0.4 +/- 0.05 mu M for SLO-1 and 15-HLO, respectively. The steady-state kinetics show that SLO-1 proceeds through a hyperbolic mixed-type inhibition pathway, where OS binding (K-i = 0.7 +/- 0.3 mu M) causes an approximate 4-fold increase in the K-m(app) (alpha = 4.6 +/- 0.5) and a decrease in the k(cat) by approximately 15% (beta = 0.85 +/- 0,1). 15-HLO also exhibits a hyperbolic saturation of k(cat)/K-m consistent with the observed rise in its KIE. Taken together, these findings indicate the presence of an allosteric site in both SLO-1 and 15-HLO and suggest broad implications regarding the inhibition of LO and the treatment of LO-related diseases.