Oxygen reduction metabolism by the digestive gland of the common marine mussel, <i>Mytilus edulis</i> L.

Abstract

Abstract Partial reduction of molecular oxygen (O 2 ) produces reactive oxygen species such as the superoxide anion radical (O − 2 ) and H 2 O 2 . Redox cycling of iron promotes the conversion of O − 2 and H 2 O 2 to the highly reactive and potentially destructive hydroxyl radical (·OH). These processes of one‐electron metabolism of O 2 were studied in digestive gland microsomes of the marine mussel, M. edulis L. by using the free radical scavengers benzoate and 2‐keto‐4‐methiolbutyric acid (KMBA) to detect ·OH. Benzoate was decarboxylated and 2‐keto‐4‐methiolbutyric acid (KMBA) was oxidized to ethylene by M. edulis microsomal electron transport (MET) in a reaction that was stimulated by ferric‐EDTA and sodium azide (an inhibitor of catalase), suggesting that H 2 O 2 was the precursor of the oxidizing species. In contrast to previous reports on rat liver microsomes (Jewell and Winston, '89), NADH supported a two to three times higher rate of oxyradical production than did NADPH. After a 10 min lag phase, NADH‐dependent KMBA oxidation was linear up to at least 60 min. NADPH‐dependent KMBA oxidation showed no lag phase and was linear for 30–40 min. In the presence of ferric‐EDTA, NADH‐dependent KMBA oxidation was weakly inhibited by superoxide dismutase (approximately 15%) whereas NADPH‐dependent KMBA oxidation was inhibited nearly 40%. As ferrous iron is required for ·OH from H 2 O 2 , ferric iron is at least in part reduced directly by MET and to a greater extent with NADH as electron donor than with NADPH. These data show that M. edulis microsomes generate superoxide/H 2 O 2 during MET and in the presence of ferric‐EDTA can oxidize ·OH scavengers, albeit via different mechanisms. Cytosol accounted foremost of the oxyradical‐generating capacity of digestive gland post 9,000 g supernatant. A substantial nonpyridine nucleotide‐dependent oxyradical‐generating activity was found in cytosol that was removed by G‐25 Sephadex filtration. Protective antioxidant enzymes were present in digestive gland, indicating an in vivo scenario for oxyradical production.