A Novel Approach to Study Linoleic Acid Autoxidation: Importance of Simultaneous Detection of the Substrate and its Derivative Oxidation Products

Abstract

In this paper we have proposed a novel approach for studying the reaction of lipid oxidation by using the simplest chemical system available. Neat linoleic acid was incubated for 24 hours at 37°C in the air. The course of lipid oxidation was followed by measuring simultaneously by HPLC with a diode array detector 1) linoleic acid decrease, 2) the products formed by radical attack, namely four hydroperoxy-octadecadienoic acid (HPODE) isomers, two c,t (c,t) and two trans, trans (t,t). 3) the byproducts formed by HPODE degradations, the four oxo-octadeca-dienoic acid (oxoODE) isomers. In HPODEs the presence of conjugated diene chromophore was confirmed by second derivative spectrophotometry. 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 the case of the latter two c,t and two t,t oxo-ODE isomers were characterized. This simple system appears to be useful for studying the activity exherted by lipophilic molecules that, like α-tocopherol, may act as antioxidants and/or as hydrogen atom donating molecules. The presence of α-tocopherol in different concentration for 24 hours in the reaction environment, shifts the reaction of linoleic acid autoxidation towards different byproduct formations. From the results obtained it is evident that α-tocopherol acts as hydrogen atom donor at all concentration tested, shifting the reaction toward a prevalent formation of c,t isomer of both HPODEs and oxo-ODEs. At concentration lower than 40 nmoles, when the ratio between α-tocopherol and linoleic acid was 1:100, the reaction of autoxidation is strongly inhibited, while at higher concentration α-tocopherol acted as a prooxidant. In these experimental conditions, α-tocopherylquinone was spectrophoto-metrically identified as the predominant oxidation product of α-tocopherol.

Key Words:

• linoleic acid
• linolelaidic acid
• conjugated dienes
• linoleic hydroperoxide isomers
• oxo linoleic isomers
• HPLC
• diode array detector
• second derivative UV spectrophotometry.