Reactivity of mammalian 15-lipoxygenase with phospholipids in large unilamellar liposomes

The reactivity of rabbit reticulocyte 15-lipoxygenase (15-LOX) with phosphatidylcholine (PC) possessing linoleic acid (LA-PC), arachidonic acid (AA-PC), or docosahexaenoic acid (DHA-PC) was investigated by measuring oxygen uptake in the suspension of large unilamellar liposomes (LUV). The 15-LOX reaction with PC LUV was found to follow Michaelis-Menten 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). However, the maximum velocity (V-max) of DHA-PC was obviously two to three times lower than those of LA-PC and AA-PC. On the other hand, the oxidizability of PC per bis-allylic H of unsaturated fatty acids in the free radical chain reaction of LUV decreased with increasing degree of unsaturation, that is, LA-PC>AA-PC>DHA-PC. These results suggested that the oxidizability of the DHA moiety incorporated into biomembranes is not necessarily high in lipoxygenase reaction as in free radical chain reaction. (C) 1996 Academic Press, Inc.