The nature and mechanism of formation of the thiobarbituric acid (TBA)-reaction material produced in the autoxidation of polyunsaturated fatty acids (PUFA) or their esters has been studied. On the basis of chemical studies and spectroscopic evidence, it is concluded that the TBA test detects malonaldehyde which arises at least in part from the acid-catalyzed or thermal decomposition or endoperoxides (2,3-dioxanorbornane compounds). These endoperosides have structures related to those of the endoperoxides produced in the biosynthetic sequence leading to prostaglandins. A mechanism is proposed in which these endoperoxides are formed in a free radical cyclization process operating in competition with hydroperoxide formation during the autoxidation of PUFA or their esters containing three or more double bonds. When 20:3 or 20:4 PUFA undergo autoxidation, some of the natural, physiologically active prostaglandins would be produced, although in very low yield, along with many other stereo- and positional isomers. Thus, it is possible that some of the complex symptoms of lipid peroxidation in vivo could be due to nonenzymatically produced prostaglandins or their steroisomers.

Neat samples of polyunsaturated fatty acids were exposed to ozone in air in a flow system, and the formation of peroxides, conjugated dienes and thiobarbituric acid (TBA)-reactive material was followed as a function of time. The effect of ozone is to shorten the induction period normally observed in autoxidation studies, but the ozone, at the concentrations used here (0-1.5 ppm), appears to have no effect on the rates of product formation after the induction period. During the induction period, increasing ozone concentrations give rise to substantially increased rates of peroxide (or materials which titrate like peroxide) formation, a slightly increased rate of conjugated diene formation, and no significant increase in the rate of production of TBA-reactive material. Vitamin E lengthens the induction period but appears to have no other effect. Some of these data are in conflict with earlier reports of Menzel et al.