Macrophage deactivation. Altered kinetic properties of superoxide-producing enzyme after exposure to tumor cell-conditioned medium.

Incubation of activated mouse peritoneal macrophages with tumor cell-conditioned medium (TCM) results in their deactivation, as measured by ability to release reactive oxygen intermediates and kill protozoal pathogens. The mechanism of suppression by macrophage deactivation factor (MDF) was studied. Inhibition of H2O2 release could not be overcome by increasing the concentration of phorbol diesters used to trigger the respiratory burst. Deactivated macrophages consumed H2O2 at the same rate as activated cells (t1/2, 35-40 min for 25 nmol H2O2 per 10(6) peritoneal cells). They transported glucose with the same kinetics (Km, 1 mM; Vmax, approximately 100 nmol per 6 min per milligram cell protein), and maintained similar intracellular concentrations of NADPH and NADP (approximately 0.62 mM and approximately 0.11 mM, respectively), as measured by enzymatic cycling methods and determinations of the volume of cell water (3.6 microliter/mg cell protein). To study the kinetics of the PMA-triggered NADPH oxidase in cell lysates, mixed detergents were used (deoxycholate and Tween 20). These stabilized the oxidase for approximately 3.3-fold longer than deoxycholate alone, which was used in previous studies. Incubation of activated macrophages in MDF resulted in a marked increase in the Km of the oxidase for NADPH, from 0.06 mM to 0.67 mM. The Vmax fell approximately 1.7-fold. These kinetic changes, together with the measured intracellular concentration of NADPH, account quantitatively for the suppression of H2O2 release by deactivated macrophages, and are nearly the mirror image of the kinetic changes observed during macrophage activation.