Phagocytes employ ROS such as, superoxide, H2O2, hydroxyl radical, and HOCl (hypochlorous acid) as major components of the chemical and enzymatic arsenal used to destroy ingested cells. Production of ROS begins shortly (15-60 seconds) after internalization of an encapsulated cell, using O2 and electrons derived from NADPH. The accompanying surge in oxygen consumption has been termed the respiratory burst. Producing the large quantities of NADPH required via the pentose phosphate pathway is facilitated by the phagocyte’s heavy reliance on aerobic glycolysis to generate ATP, a consequence of the low number of mitochondria contained within them.
The first step in the formation of microbicidal ROS during the respiratory burst is the synthesis of superoxide, which is catalyzed by the NADPH oxidase system. Catalysis proceeds via a two-step mechanism, the reduction of molecular oxygen to form superoxide (see Table 1):
followed by the spontaneous dismutation of hydrogen peroxide from two molecules of superoxide:
The NADPH oxidase system is comprised of cytochrome b558, a plasma membrane–associated heterodimer, and two cytoplasmic polypeptides of 47 and 67 kDa. On activation, the cytoplasmic peptides are recruited to the plasma mem brane where they associate with cytochrome b558 to form the active complex. Flux through the pentose phosphate cycle, the cell’s primary source of NADPH, also increases markedly during phagocytosis. The cell is protected from any super oxide that may escape from the phagosomes by superoxide dismutase, which catalyzes the disproportionation of two superoxide radical anions into one molecule each of H2O2 and O2. The hydrogen peroxide can be used as a substrate for myeloperoxidase (see later) or disposed of by the action of glutathione peroxidase or catalase.
Table1. Enzymes & Proteins of the Granules of Phagocytic Leukocytes
