Complex Formation of Heme Oxygenase-2 with Heme Is Competitively Inhibited by the Cytosolic Domain of Caveolin-1

The mechanism and physiological functions of heme oxygenase-2 (HO-2)-mediated carbon monoxide (CO) production, accompanied by heme metabolism, have been studied intensively in recent years. The enzymatic activity of constitutively expressed HO-2 must be strictly controlled in terms of the toxicity and chemical stability of CO. In this study, the molecular interaction between HO-2 and caveolin-1 and its effect on HO action were evaluated. An enzyme kinetics assay with residues 82-101 of caveolin-1, also called the caveolin scaffold domain, inhibited HO-2 activity in a competitive manner. Analytical ultracentrifugation and a hemin titration assay suggested that the inhibitory effect was generated by direct binding of caveolin-1 to aromatic residues, which were defined as components of the caveolin-binding motif in the HO-2 heme pocket. Herein, we developed a HO-2-based fluorescence bioprobe, namely EGFP-Delta 19/D159H, which was capable of quantifying heme binding by HO-2 as the initial step in the CO production. The fluorescence of EGFP-Delta 19/D159H decreased in accordance with 5-aminolevulinic acid-facilitated heme biosynthesis in COS-7 cells. In contrast, expression of the N-terminal cytosolic domain of caveolin-1 (residues 1101) increased the probe fluorescence, suggesting that the cytosolic domain of caveolin-1 potently inhibits the binding of heme to the heme pocket of EGFP-Delta 19/D159H. Taken together, our results suggest that caveolin-1 is a negative regulator of HO-2 enzymatic action. Moreover, our bioprobe EGFP-Delta 19/D159H represents a powerful tool for use in future studies addressing HO-2-mediated CO production.