The nonpolar peptide segment of cytochrome b5. Binding to phospholipid vesicles and identification of the fluorescent tryptophanyl residue.

The nonpolar peptide segment of cytochrome b5 consisting of the COOH-terminal 43 amino acid residues binds rapidly to dimyristyl lecithin vesicles above the transition temperature of the phospholipid. The binding is complete as indicated by density gradient centrifugation and is accompanied by approximately a 2-fold increase in the fluorescence emission of the protein, and insertion in the bilayer is apparently in an orientation indistinguishable from that of the whole cytochrome b5 molecule. Quantitative reaction of the protein with N-bromosuccinimide destroys the fluorescence of the protein with a stoichiometry which indicates that a single tryptophanyl residue is responsbile for the fluorescence. The fluorescent tryptophanyl residue is one of 2 adjacent residues, Trp-108 or Trp-109, as shown by carboxypeptidase digestion of N-bromosuccinimide-reacted nonpolar peptide.