The so called "soluble" oxidase(s) are not involved in the respiratory burst of guinea pig and human granulocytes and of guinea pig peritoneal resident and elicited macrophages. 2. The activation of the oxidation of NADPH by a membrane bound NAD(P)H oxidase is the main mechanism responsible for the activation of the respiration of phagocytes. 3. The oxidase is inactive in resting cells and the activated form works on the plasma membrane. 4. More than one mechanism is operative in the oxidation of NAD(P)H by cell free particles in vitro. These mechanisms vary in relation to the conditions of assay (pH and concentration of substrate). 5. Under optimal conditions in vitro the enzymatic oxidation of NADPH practically involves the univalent pathway of oxygen reduction with stoichiometry of two nanomoles of O2 formed for one nanomole of NADPH oxidized. 6. Also in intact cells all O2 is first univalently reduced to O2 and then discharged outside the cell or in the phagocytic vacuoles. 7. The main reactions involved in the O2 balance in intact cells are the univalent reduction of O2, the dismutation of O2 to H2O2 and the degradation of the peroxide through catalatic and peroxidatic mechanisms. 8. The total oxygen univalently reduced by the activated oxidase is 2-4 folds the net oxygen consumed by the cells, depending on the mechanism of H2O2 degradation. 9. All the rate of extrarespiration is accounted for by the rate of oxidation of physiological concentration of NADPH by the membrane-bound enzyme. This adequacy can be observed only under appropriate experimental conditions, because the high activity of the oxidase is not a permanent state.
