Influence of hyperosmotic environment comparable to the renal medulla upon membrane NADPH oxidase of human polymorphonuclear leukocytes.

Hyperosmotic environment in the renal medulla seems important for bacterial pyelonephritis because it exerts inhibitory influences upon the function of polymorphonuclear leukocytes (PMN). Urea and NaCl primarily contribute to high osmolarity in the renal medulla. We previously reported that PMN function was actually suppressed in phagocytosis, intracellular bacterial killing and superoxide generation in the hyperosmotic solution of urea and NaCl. In the present report, to verify the mechanism of this inhibitory effect, a kinetic study for NADPH oxidase in the cell membrane, the key enzyme complex of superoxide generation, was carried out in the cell membrane-solubilizing system under the hyperosmotic condition caused by urea or NaCl. Urea directly denaturated NADPH oxidase, and its inhibitory mechanism was reversible and uncompetitive with a decrease in Vmax and Km, while NaCl had no effect upon it, maintaining Lineweaver-Burk plots in the same position as those of the control. This result suggests that urea at least produces an inhibitory effect upon PMN through the direct inactivation of NADPH oxidase, although NaCl was unable to do so.