Reactivity of soybean lipoxygenase-1 to linoleic acid entrapped in phosphatidylcholine vesicles.

The linoleic acids embedded in the SUVs of soy-PC, DMPC, and DPPC served as substrate for soybean lipoxygenase-1 (L-1). The initial velocity of the catalytic reaction and the concentration of the substrate showed a hyperbolic relation. The Km values of L-1 for the linoleic acids in soy-PC, DMPC, and DPPC vesicles were 0.07, 0.09, and 0.11 mM, respectively, being comparable with that for Tween-20 micellar linoleic acid. Soy-PC and DMPC competitively inhibited the enzyme with Ki values of 0.20 and 0.13 mM, respectively, whereas DPPC had no effect. DSC analysis revealed the phase separation of linoleic acid and DPPC in vesicles in the temperature range in which the enzyme reaction was carried out. This may account for the lack of inhibitory effect of DPPC on the enzyme. From the temperature dependence of the specific activity of the enzyme, the Ea values of the catalytic reaction were estimated to be 26.7 and 35.3 kJ.mol-1 for soy-PC and DPPC vesicles, respectively. For linoleic acid-DMPC vesicles, a two-phase temperature dependence of the activity across the transition temperature of the mixed vesicles was suggested.