Lipid peroxidation in guinea pig lung microsomes.

The effects of substances known to influence lipid peroxidation were studied in guinea pig lung microsomes by measuring the formation of malonaldehyde in vitro. Incubation of lung microsomes at 37 degrees C results in lipid peroxidation which appears to be an enzymatic process but is not dependent upon iron. Lipid peroxidation can be initiated non-enzymatically in lung microsomes by Fe2+, but ascorbate and Fe3+ have very little effect on malonaldehyde formation. The effects of NADPH on lipid peroxidation are dependent upon the concentration of Fe2+ in the incubation medium. At concentrations of Fe2+ between 0.05 mM and 1 mM, addition of NADPH causes an increase in lipid peroxidation over that produced by Fe2+ alone. This stimulation by NADPH is an enzymatic process and phosphate is required for the maximal effect. Addition of NADPH to lung microsomes in the presence of Fe3+ does not increase malonaldehyde formation over that produced by Fe3+ alone, suggesting that NADPH does not influence lipid peroxidation by maintaining iron in the reduced form. At concentrations of Fe2+ greater than 1 mM, NADPH inhibits Fe2+-induced lipid peroxidation in normal microsomes and in microsomes in which enzymes have been inactivated with heat. This latter result suggests that the inhibition by NADPH is at least partially non-enzymatic. The result suggests that the inhibition by NADPH is at least partially non-enzymatic. The results of all of these experiments are discussed and compared with those obtained during lipid peroxidation in liver microsomes. We conclude that the processes involved in pulmonary microsomal lipid peroxidation differ significantly from those in hepatic microsomes.