Posttranscriptionally IRP1 regulates the expression of proteins involved in the maintenance of homeostasis in mammals Fe ions . IRP1 is a bifunctional protein that occurs in the cytoplasm of aconitase , an enzyme containing a catalytic [ 4Fe -4S ] ( holo - IRP1 ) or trans - regulatory protein , devoid of the center (apo - IRP1 ) . Two IRP1 activity are mutually exclusive and are inversely regulated by Fe ions contained in the LIP , the labile iron pool .