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 (Vmax) 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.