The aromatic and heme chromophores of rabbit hemopexin: Difference absorption and fluorescence spectra

Abstract

Spectrophotometric and fluorimetric techniques were employed to characterize the environment of the heme chromophore of rabbit hemopexin and to monitor changes in the environment of aromatic amino acid residues induced by the interaction of hemopexin with porphyrins and metalloporphyrins. Difference spectra showed maxima at 292 and 285 nm when hemopexin binds heme or deuteroheme but not deuteroporphyrin. These maxima are attributed to alterations in the local environment of tryptophan and tyrosine residues. Spectrophotometric titrations of the tyrosine residues of hemopexin, heme-hemopexin and hemopexin in 8 M urea showed apparent p K values at 11.4, 11.7 and 10.9, respectively. Perturbation difference spectra produced by 20% v/v ethylene glycol are consistent with the exposure of 6–8 of the 14 tyrosine residues and 6–8 of the 15 tryptophan residues of rabbit hemopexin to this perturbant. Only small differences were found between the perturbation spectra of apo- and heme-hemopexin near 290 nm, suggesting that slight or compensating changes in the exposure to solvent of tryptophan chromophores occur. In the Soret spectral region, the exposure of heme in the heme-hemopexin complex to ethylene glycol was 0.7, relative to the fully exposed heme peptide of cytochrome c. The fluorescence quantum yields of rabbit apo- and heme-hemopexin were estimated to be 0.06 and 0.03, respectively, compared to a yield of 0.13 for l-tryptophan. Iodide quenched 50% of the fluorescence of tryptophan residues in the apo-protein and 20% of the residual fluorescence of the deuteroheme-hemopexin complex. Cesium was not an effective quencher. Modification of approximately 4 tryptophan residues with N-bromosuccinimide also decreased the relative fluorescence of apo-hemopexin by 50% and concomitantly reduced the heme-binding ability of the protein by 70%. The existence of sterically unhindered tryptophan residues in either apo- or heme-hemopexin is unlikely since no charge transfer complexes between these proteins and N-methylnicotinamide were detected.