Singlet oxygen production by soybean lipoxygenase isozymes

The oxidation of linoleic acid catalyzed by soybean lipoxygenase isozymes was accompanied by 1268 nm chemiluminescence characteristic of singlet oxygen. The recombination of peroxy radicals as first proposed by Russell (Russell, G.A. (1957) J. Am. Chem. Soc. 79, 3871-3877) is a plausible mechanism for the observed singlet oxygen production. Lipoxygenase-3 was the most active isozyme. Under the optimal aerobic conditions of p2H 7, 100 micrograms/ml lipoxygenase-3, 100 microM linoleic acid, 100 microM 13-hydroperoxylinoleic acid, and air-saturated buffer, the yield of singlet oxygen was 12 +/- 0.4 microM or 12% of the amount predicted by the Russell mechanism. High yields of singlet oxygen required the presence of 13-hydroperoxylinoleic acid. Systems containing lipoxygenase-2 and lipoxygenase-3 produced comparable yields of singlet oxygen without added 13-hydroperoxylinoleic acid, since the lipoxygenase-2 served as an in situ source of hydroperoxide. Lipoxygenase-1 was active only at low oxygen concentrations. Its singlet oxygen-producing capacity was greatly increased by the addition of acetone to the system. Lipoxygenase-2 did not produce detectable quantities of singlet oxygen.