Tryptic digestion of soybean lipoxygenase-1 generates a 60 kDA fragment with improved activity and membrane binding ability

Lipoxygenases are key enzymes in the metabolism of unsaturated fatty acids. Soybean lipoxygenase-1 (LOX-1), a paradigm for lipoxygenases isolated from different sources, is composed of two domains: a ∼30 kDa N-terminal domain and a ∼60 kDa C-terminal domain. We used limited proteolysis and gel-filtration chromatography to generate and isolate a ∼60 kDa fragment of LOX-1 ("mini-LOX"), produced by trypsin cleavage between lysine 277 and serine 278. Mini-LOX was subjected to N-terminal sequencing and to electrophoretic, chromatographic, and spectroscopic analysis. Mini-LOX was found to be more acidic and more hydrophobic than LOX-1, and with a higher content of α-helix. Kinetic analysis showed that mini-LOX dioxygenates linoleic acid with a catalytic efficiency approximately 3-fold higher than that of LOX-1 (33.3 × 10⁶ and 10.9 × 10⁶ M^-1·s^-1, respectively), the activation energy of the reaction being 4.5 ± 0.5 and 8.3 ± 0.9 kJ·mol^-1 for mini-LOX and LOX-1, respectively. Substrate preference, tested with linoleic, α-linolenic, and arachidonic acids, and with linoleate methyl ester, was the same for LOX-1 and mini-LOX, and also identical was the regio- and stereospecificity of the products generated thereof, analyzed by reversed-phase and chiral high-performance liquid chromatography, and by gas chromatography/mass spectrometry. Mini-LOX was able to bind artificial vesicles with higher affinity than LOX-1, but the binding was less affected by calcium ions than was that of LOX-1. Taken together, these results suggest that the N-terminal domain of soybean lipoxygenase-1 might be a built-in inihibitor of catalytic activity and membrane binding ability of the enzyme, with a possible role in physio-(patho)logical conditions.