ROSs like superoxide are produced by NADPH oxidase activity when white blood cells, primarily neutrophils, phagocytose pathogens. For decades it has been thought that high concentrations of the toxic ROSs resulting from this respiratory burst are directly responsible for killing microorganisms. But the new results prove otherwise.
Segal's group showed that mice deficient in protease activity have increased sensitivity to microbes. Additionally, purified neutrophils from the protease-deficient mice are impaired in their antimicrobial activity in vitro. “Our results are conclusive,” says Segal. “By taking out the proteases, we do not interfere with the respiratory burst, but do interfere with killing.”
The proteases are activated through increases in both pH and hypertonicity in the phagocytotic vacuoles. These changes arise when NADPH oxidase activity drives electrons into the vacuole. To compensate for the charge transfer across the membrane, K+ also enters. The resulting hypertonic conditions release the cationic proteases from their bound state on an anionic proteoglycan matrix. K+ entry also results in alkaline conditions that stimulate protease activity. The proteases are then free to bind to and digest the target microbes. ▪