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 serv ed as substrate for soybean lipoxygenase-l (L-l). The initial velocity of the catalytic reaction and the concentration of the substrate show ed a hyperbolic relation. The K-m values of L-l for the linoleic acids in soy-PC, DMPC, and DPPC vesicles were 0.07, 0.09, and 0.11 mM, resp ectively, being comparable with that for Tween-20 micellar linoleic ac id. Soy-PC and DMPC competitively inhibited the enzyme with K-1 values of 0.20 and 0.13 mM, respectively, whereas DPPC had no effect. DSC an alysis revealed the phase separation of linoleic acid and DPPC in vesi cles in the temperature range in which the enzyme reaction was carried out. This may account for the lack of inhibitory effect of DPPC on th e enzyme. From the temperature dependence of the specific activity of the enzyme, the E-a values of the catalytic reaction were estimated to be 26.7 and 35.3 kJ . mol(-1) for soy-PC and DPPC vesicles, respectiv ely. For linoleic acid-DMPC vesicles, a two-phase temperature dependen ce of the activity across the transition temperature of the mixed vesi cles was suggested.