Effect of NO on IRP activity in monocytes from control subjects. (A) Monocytes of control subjects were treated with 100 U/mL IFN-γ plus 1 μg/mL LPS for 4 and 24 hours, in the presence and absence of 0.1 mmol/L NMMA. IRP activity and TNF-α were determined as described in Fig 1. (B) Monocytes of control subjects were incubated for 4 and 24 hours in the presence and absence of 0.1 mmol/L NMMA. (C) Monocytes of control subjects were treated with 0.5 mmol/L SNAP for 4, 8, and 24 hours. Lysates were assayed for IRP activity as described in Fig 1. The results of treatment with the iNOS inhibitor NMMA and with the NO donor SNAP indicated a role for NO in the modulation of monocyte IRP activity by cytokines.