Purification, partial characterization, and possible role of catalase in the bacterium Vitreoscilla

Vitreoscilla is a gram-negative bacterium that contains a unique bacterial hemoglobin that is relatively autoxidizable. It also contains a catalase whose primary function may be to remove hydrogen peroxide produced by this autoxidation. This enzyme was purified and partially characterized. It is a protein of 272,000 Da with a probable A2B2 subunit structure, in which the estimated molecular size of A is 68,000 Da and that of B, 64,000 Da, and an average of 1.6 molecules of protoheme IX per tetramer. The turnover number for its catalase activity was 27,000 s-1 and the Km for hydrogen peroxide was 16 mM. The peroxidase activity measured using o-dianisidine was 0.6% that of the catalase activity. Cyanide, which inhibited both catalase and peroxidase activities, bound the heme in a noncooperative manner. Azide inhibited the catalase activity but stimulated the peroxidase activity. An apparent compound II was formed by the reaction of the enzyme with ethyl hydrogen peroxide. The enzyme was reducible by dithionite, and the ferrous enzyme reacted with CO. The cellular content of Vitreoscilla hemoglobin varies during the growth cycle and in cells grown under different conditions, but the ratio of hemoglobin to catalase activity remained relatively constant, indicating possible coordinated biosynthesis and supporting the putative role of Vitreoscilla catalase as a scavenger of peroxide generated by Vitreoscilla hemoglobin.