Mechanism and kinetics of the hemin-catalyzed oxidation of linoleate in the oil-water interface.

The purpose of this investigation was to find out why hemin catalyzes the oxidation of linoleic acid or its esters by O2 only in the biphasic oil-in-water system but not in homogeneous solutions. We find that only those molecules of linoleic acid or methyllinoleate which form a monomolecular ti at the oil-water interface react with hemin and OZ. The high affinity of hemin and hemoglobin for the xylenewater interface (HAUROWITZ, F., DICKS, M., AND THERRIAULT, D. (1957) Nature 180, 437438) facilitates their combination with the molecules of the interfacial oil film. Microscopic observation revealed that increase in the amount of methyllinoleate results in coalescence of small oil spherules and a concomitant decrease in the area of the oil-water interface. Consequently, increase in the amount of methyllinoleate by a factor, R, leads to an increase in the rate of 02 uptake by a factor smaller than n*‘3, close to nl/*. The initial rate of O2 uptake, V, in the continually shaken manometric flasks at 37” is strictly proportional to pOz up to pOZ = 0.06 atm; further increase in pOZ does not raise V. Kinetic analysis reveals that the rate-limiting step is the transfer of linoleate molecules from the inside of oil spherules into the interfacial film. During the catalytic process hemin is degraded to non-heme iron which consists of free or loosely bound ferrous ions ; their catalytic effect is lower than that of hemin. No evidence for the presence of radicals or for chain reactions in the hemin-catalyzed oxidation was found. We conclude that hemin acts in the oxidation of linoleate not only as an oxygen-oxidoreductase but also as a peroxidase which catalyzes the destruction of linoleate hydroperoxides and thus prevents the formation of free radicals. Since hemin can be replaced by hemoglobin, cytochrome c, or ferrous ions, the mechanism of the oxidation of unsaturated fatty acids in uiuo is most likely identical with that proposed for the hemin-catalyzed process in vitro.