Abstract
It has been demonstrated that cysteine has proxidative activity in the system containing linoleic acid (LA system). In order to clarify the mechanism of proxidative action of cysteine in the autoxidation of linoleic acid, the effects of cysteine in the systems containing linoleic acid hydroperoxide (LAHPO system) and secondary degradation product of linoleic acid (LASP system) as well as LA system were further studied. The following results were obtained.
1) The presence of cysteine in LA system increased POV, TBA value and oxygen absorption, especially at pH 4-5. The oxygen uptake of LA system with cysteine was immediately observed without any lag period after addition of cysteine.
2) No significant change in oxygen uptake was observed in LASP system with or without cysteine, whereas a slight increase of oxygen uptake was observed in LAHPO system containing cysteine. However, it was assumed that the slight increase of oxygen uptake in LAHPO system with cysteine might be effected by a small quantity of linoleic acid contaminated in this system.
3) Cysteine inhibited the decreases of POV in both LAHPO and LASP system.
4) It was confirmed by thin layer chromatography that cysteine in both LA and LAHPO system was transformed into cystine in the course of the reaction.
From these results, the following mechanism on proxidative action of cysteine is proposed: Cysteine takes plays an essential role in initiating the reaction which includes the oxygen uptake in autoxidation of linoleic acid and the acceleration of free radical formation, and then inhibits degradation of hydoperoxide by reacting with ROO. Consequently, cysteine is oxidized into cystine after the reactions.
