Factors Affecting the Affinity for Oxygen of Cytochrome Oxidases in Hemophilw parainfluenzae

The rate of oxygen utilization by the electron transport system at low oxygen concentrations has been difficult to explain mechanistically (l-3). As in most enzymatic reactions the rate of oxygen utiliiation is independent of oxygen concentration when the oxygen concentration is high, but becomes dependent upon the oxygen concentration at some critical value of the oxygen tension. Chance (4) has provided evidence from spectrophotometric studies that the rate of oxygen uptake is proportional to the rate of production of fumarate from succinate and to the ratio of reduced cytochrome oxidase to oxidized cytochrome oxidase. By use of an analogue computer, Chance (4) has explained the behavior of this system and accounted for both the influence of the oxygen tension and the influence of the reduced oxidase concentration on the rate of oxygen utilization. Certain characteristics of Hemophilus parainjIuenzae permit the study of the effect of the composition of the electron transport system on the rate of oxygen utilization at low oxygen concentrations. The intact bacteria are permeable to NADH which reacts rapidly with the electron transport system. The electron transport system of these bacteria contains flavoproteins, cytochromes bl, c, and cl, and cytochrome oxidases ai, a2, and “0” (5). The proportions of the intermediate cytochromes to the oxidases in the electron transport chain can be changed markedly by varying growth conditions (6). Several substrates react directly with the isolated electron transport system to reduce all the cytochrome oxidases but different proportions of cytochrome bl (7). This study indicates that the concentration at which the oxygen tension affects the rate of oxygen utilization can be raised by reducing the proportion of cytochrome oxidase to cytochrome bl, by using submaximal amounts of substrate, or by using respiratory inhibitors. The critical oxygen concentration is a function of the rate of electron transfer to the oxidases as well as of the rate of reoxidation of the oxidases by oxygen.