Haemoglobin potentiates the respiration-inhibitory action of lipoxygenases via its pseudolipohydroperoxidase activity.

The inhibition of the NADH oxidase and of the succinate-cytochrome c oxidoreductase activities of beef heart submitochondrial particles (ETP) induced by reticulocyte lipoxygenase is strongly potentiated by haemoglobin added simultaneously or subsequently to lipoxygenase. Half-maximal potentiation was observed with 0.9 microM haemoglobin. Haemoglobin augments also the lipoxygenase-induced loss of acid-labile sulphur in ETP not related to respiratory inhibition. Soybean lipoxygenase produces only weak respiratory inhibition even at high concentrations in the absence of haemoglobin; strong inhibition is obtained, however, in the presence of haemoglobin. Among other haem compounds tested only free haemin but not cytochrome c and catalase exhibit identical effects. The action of haem compounds on the system ETP plus lipoxygenase is accompanied by their destruction. The experimental evidence indicates that the effects of haemoglobin described here are due to its ability to decompose catalytically hydroperoxy lipids generated by lipoxygenase. This catalytic property (pseudolipohydroperoxidase activity) was demonstrated with 13L-hydroperoxylinoleic acid as substrate. The relations between the various catalytic actions of haemoglobin related to lipid peroxidation are discussed.