Rapid Reactions of Cytochromes of Hemophilus parainfluenzae on Addition of Substrates or Oxygen

Addition of oxidizable substrates to aerobic suspensions of Hemophilus parainfluenzae containing no endogenous substrate results in rapid reduction of b and c type cytochromes to the aerobic steady state. The extent of reduction depends upon the relative activities of the dehydrogenases and oxidases involved. Addition of oxygen to anaerobic cells containing substrate rapidly oxidizes the cytochromes to the aerobic steady state. The rapid kinetics observed on mixing with oxygen show no evidence that the initial rate of oxidation of part of the b or c type cytochromes is higher, even within bacteria with a large stoichiometric imbalance of these cytochrome types. Cytochrome a2 reacts rapidly enough with oxygen to be considered a terminal oxidase, and indirect evidence indicates that cytochrome o reacts as rapidly with oxygen as does the mammalian oxidase (pseudofirst order rate constant around 80 sec-1 at room temperature).