EPR DEFINITION OF THE NONHEME FERRIC ACTIVE-SITES OF MAMMALIAN 15-LIPOXYGENASE - MAJOR SPECTRAL DIFFERENCE RELATIVE TO HUMAN 5-LIPOXYGENASES AND PLANT LIPOXYGENASES AND THEIR LIGAND-FIELD ORIGIN

Lipoxygenases (LOs) are non-heme iron containing enzymes which catalyz e the hydroperoxidation of polyunsaturated fatty acids. Mammalian LOs have great physiological and pathological importance as they play key roles in the biosynthesis of leukotrienes and Lipoxins. Electron param agnetic resonance (EPR) studies have shown that ferric active sites of soybean lipoxygenase-1 (SLO-1) and human 5-lipoxygenase (5-HLO) exhib it axial EPR patterns which convert to rhombic spectra upon addition o f hydroperoxide product (13-HPOD). In this report, we extend EPR studi es to rabbit (15-RLO) and human (15-HLO) mammalian 15-LOs. The spectra of 15-RLO and 15-HLO have rhombic high-spin ferric EPR signals which convert to axial upon interaction with product, opposite to the behavi or reported for SLO-1 and 5-HLO, which is indicative of a significant structural difference between the ferric sites of these mammalian 15-L Os and those of SLO-1 and 5-HLO. This appears to relate to the substit ution of an asparagine ligand in the SLO-1 and 5-HLO active sites for a histidine ligand in the rabbit and human 15-LOs. A ligand field mode l of the zero-field splitting is presented which accounts for this opp osite EPR spectral behavior in terms of ligand strength differences as sociated with the Asn (weak) --> His (strong) ligand substitution.