The polyphasic nature of the respiratory process at the mitochondrial level

The kinetics of oxygen consumption by rat liver mitochondria, respiring under a variety of metabolic conditions, have been studied. Respiration was initiated by injecting oxygen into anaerobic suspensions of mitochondria. It was found that, irrespective of the metabolic state of the mitochondria and the nature of the respiratory substrate, the rates of electron flow and oxygen consumption follow the pattern of a polyphasic reaction. The rates of oxygen uptake during the first phase are extremely fast and depend on oxygen concentration. The second phase represents a transition in which net oxidation of cytochrome-c oxidase stops and the rates of oxygen consumption suddenly decrease. The third phase is characterized by its changeability. Depending on initial conditions the rates may increase, decrease, or remain constant, although the reaction is not one of zero order. During the last phase, the rates decrease and the oxidase becomes increasingly reduced. It is postulated that the mitochondrial respiratory process is basically a cyclic event in which the redox state of the membrane and the rates of oxygen consumption oscillate with amplitudes and frequencies conditioned by the energy demand and energy-yielding capacity of the cell.