Evidence that Peroxidation of Lysosomal Membranes Is Initiated by Hydroxyl Free Radicals Produced during Flavin Enzyme Activity

Since evidence had been published that lysosomes are disrupted by a free radical-like agent produced during the oxidation of NADPH by liver microsomes, the nature of this free radical was investigated. The following data are presented.

1. The oxidation of NADPH by liver microsomes in the presence of ADP and Fe³⁺ produced a factor with properties of a free radical which caused the lysis of lysosomes. The addition of superoxide dismutase enhanced the lysis.

2. Lysis of lysosomes also was mediated by the oxidation of NADPH by a partially purified NADPH-cytochrome P₄₅₀ reductase.

3. The oxidation of xanthine by xanthine oxidase in the presence of ADP and Fe³⁺ produced a radical capable of causing lysosomal lysis. Here again superoxide dismutase caused an enhancement of this lysis.

4. It was demonstrated that the superoxide free radical per se was not the agent causing lysis. Evidence is presented that the free radical causing the lysis of the lysosomes is the hydroxyl free radical (OH·).

5. During lysosomal lysis by the OH· radical, peroxidation of the lysosomal lipids was observed as assayed by the formation of malondialdehyde.

6. The mechanisms whereby superoxide and hydroxyl free radicals may be produced and interact with lysosomal membranes are discussed.