The iron regulatory proteins IRP1 and IRP2 are involved in posttranscriptional regulation of cellular iron metabolism by binding to mRNA iron-responsive elements (IREs). IRPs respond to stress mediators, iron concentration and signaling factors, including nitrogen monoxide, cytokines and hydrogen peroxide. IRP-1 is a bifunctional soluble protein that functions as an IRE-binding protein or as the cytoplasmic isoform of aconitase.