Effect of oxygen tension on the generation of alkanes and malondialdehyde by peroxidizing rat liver microsomes.

The alkanes, ethane and pentane, are often used as indices of lipid peroxidation. Because it has been indicated that O2 tension can affect the yield of these compounds, a systematic study of this was carried out. Rat liver microsomes were peroxidized using an iron-ascorbate system. The incubations were carried out in sealed flasks at 37 degrees under N2 and various concentrations of O2 up to 100%. Ethane and pentane production were measured by gas chromatography, and malondialdehyde was measured by the thiobarbituric acid reaction. Microsomal fatty acids were measured by gas chromatography. Polyunsaturated fatty acids were lost during lipid peroxidation. There was no loss of saturated or monounsaturated fatty acids. Loss of polyunsaturated fatty acids correlated with O2 tension in the flask. Half-maximal losses of docosahexaenoic acid, arachidonic acid, and linoleic acid occurred at 3, 5, and 35% O2 respectively. Malondialdehyde formation reflected polyunsaturated fatty acid loss at all O2 concentrations. Alkane formation reflected polyunsaturated fatty acid loss below 5% O2 but not above it. The ratio of alkane formed to precursor polyunsaturated fatty acid lost decreased progressively as O2 concentration was increased above 5%. For example, the molar yield of pentane formed per precursor polyunsaturated fatty acid lost was 0.3% at 5% O2 but only 0.003% at 100% O2. This indicates that quantitation of lipid peroxidation using alkane formation requires consideration of O2 tension at the site of alkane formation.