REACTIVITY OF MAMMALIAN 15-LIPOXYGENASE WITH PHOSPHOLIPIDS IN LARGE UNILAMELLAR LIPOSOMES

The reactivity of rabbit reticulocyte 15-lipoxygenase (15-LOX) with ph osphatidylcholine (PC) possessing linoleic acid (LA-PC), arachidonic a cid (AA-PC), or docosahexaenoic acid (DHA-PC) was investigated by meas uring oxygen uptake in the suspension of large unilamellar liposomes ( LUV). The 15-LOX reaction with PC LUV was found to follow Michaelis-Me nten kinetics. The apparent values of the Michaelis constants (K-m) of LUV of LA-PC, AA-PC, and DHA-PC were nearly the same (1.0-1.2 mM). Ho wever, the maximum velocity (V-max) of DHA-PC was obviously two to thr ee times lower than those of LA-PC and AA-PC. On the other hand, the o xidizability of PC per bis-allylic H of unsaturated fatty acids in the free radical chain reaction of LUV decreased with increasing degree o f unsaturation, that is, LA-PC>AA-PC>DHA-PC. These results suggested t hat the oxidizability of the DHA moiety incorporated into biomembranes is not necessarily high in lipoxygenase reaction as in free radical c hain reaction. (C) 1996 Academic Press, Inc.