Oxygen activation and defence against oxygen toxicity in a psychrophilic bacteroidaceae

Abstract

When suddenly exposed to air the growth of the obligate anaerobic bacterium of the bacteroidaceae type, strain B6, continues for a few hours before coming to a complete stop. When air is shut off soon after growth has ceased, the organism is able to reestablish anaerobic conditions due to an ability to reduce O₂, and resumes normal growth after another few hours. The O₂ reducing ability of the organism is due to the presence in the cells of a particlebound NADH oxidase, a soluble NADPH oxidase and a soluble pyruvate oxidase. The two pyridine nucleotide oxidase reduce O₂ to H₂O₂, the pyruvate oxidase reduces O₂ to H₂O. Catalase and peroxidase were not detected in anaerobically grown cells. Kinetic studies with cell-free extracts showed that the pyruvate oxidase had a considerably greater affinity (smaller K _m) for O₂ and capacity (higher V _max) for O₂ reduction than the two other oxidases. It is postulated that the pyruvate oxidase acts as a scavenger for O₂, leading to the non-toxic reduction product H₂O, and thus functions as a defense mechanism against oxygen toxicity when the organism is exposed to aerobic condition.