A NOVEL-APPROACH TO STUDY LINOLEIC-ACID AUTOXIDATION - IMPORTANCE OF SIMULTANEOUS DETECTION OF THE SUBSTRATE AND ITS DERIVATIVE OXIDATION-PRODUCTS

In this paper we have proposed a novel approach for studying the react ion of lipid oxidation by using the simplest chemical system available . Neat linoleic acid was incubated for 24 hours at 37 degrees C in the air. The course of lipid oxidation was followed by measuring simultan eously by HPLC with a diode array detector 1) linoleic acid decrease, 2) the products formed by radical attack, namely four hydroperoxy-octa decadienoic acid (HPODE) isomers, two c,t (c,t) and two trans, trans ( t,t). 3) the byproducts formed by HPODE degradations, the four oxo-oct adeca-dienoic acid (oxo-ODE) isomers. In HPODEs the presence of conjug ated diene chromophore was confirmed by second derivative spectrophoto metry. c,t HPODEs were also identified for their positional isomerism, while for t,t molecules the lack of suitable reference compound makes unfeasible the identification of their positional isomerism. As in th e case of the latter two c,t and two t,t oxo-ODE isomers were characte rized. This simple system appears to be useful for studying the activi ty exherted by lipophilic molecules that, like alpha-tocopherol, may a ct as antioxidants and/or as hydrogen atom donating molecules. The pre sence of alpha-tocopherol in different concentration for 24 hours in t he reaction environment, shifts the reaction of linoleic acid autoxida tion towards different byproduct formations. From the results obtained it is evident that alpha-tocopherol acts as hydrogen atom donor at al l concentration tested, shifting the reaction toward a prevalent forma tion of c.t isomer of both HPODEs and oxo-ODEs. At concentration lower than 40 nmoles, when the ratio between alpha-tocopherol and linoleic acid was 1:100, the reaction of autoxidation is strongly inhibited, wh ile at higher concentration alpha-tocopherol acted as a prooxidant. In these experimental conditions, alpha-tocopherylquinone was spectropho tometrically identified as the predominant oxidation product of alpha- tocopherol.