P450 and heme oxygenase-1 (HO-1) receive their necessary electrons by interaction with the NADPH-cytochrome P450 reductase (POR).  As the POR concentration is limiting when compared to P450 and HO-1, they must effectively compete for POR to function.  In addition to these functionally required protein-protein interactions, HO-1 forms homomeric complexes, and several P450s have been shown to form complexes with themselves and with other P450s, raising the question, “How are the HO-1 and P450 systems organized in the endoplasmic reticulum?”  Recently, CYP1A2 was shown to associate with HO-1 affecting the function of both proteins.  The goal of this study was to determine if CYP1A1 formed complexes with HO-1 in a similar manner.  Complex formation among POR, HO-1, and CYP1A1 was measured using bioluminescence resonance energy transfer, with results showing HO-1 and CYP1A1 form a stable complex that was further stabilized in the presence of POR.  The POR•CYP1A1 complex was readily disrupted by the addition of HO-1.  CYP1A1 also was able to affect the POR•HO-1 complex, although the effect was smaller.  This interaction between CYP1A1 and HO-1 also affected function, where the presence of CYP1A1 inhibited HO-1-mediated bilirubin formation by increasing the KmPOR•HO-1 without affecting the Vmaxapp.  In like manner, HO-1 inhibited CYP1A1-mediated 7-ethoxyresorufin dealkylation by increasing the KmPOR•CYP1A1.   Based on mathematical simulation, the results could not be explained by a model where CYP1A1 and HO-1 simply compete for POR, and are consistent with formation of a stable CYP1A1•HO-1 complex that affected the functional characteristics of both moieties.

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