Riboflavin photosensitized oxidation of myoglobin.

The reaction of the fresh meat pigment oxymyoglobin, MbFe(II)O-2, and its oxidized form metmyoglobin, MbFe(III), with triplet-state riboflavin involves the pigment protein, which is oxidatively cleaved or dimerized as shown by SDS-PAGE and Western blotting. The overall rate constant for oxidation of MbFe(II)O-2 by (3)Rib is (3.0 +/- 0.5) x 10(9) L.mol(-1).s(-1) and (3.1 +/- 0.4) x 10(9) L.mol(-1).s(-1) for MbFe(III) in phosphate buffer of pH 7.4 at 25 degrees C as determined by laser flash photolysis. The high rates are rationalized by ground state hydrophobic interactions as detected as static quenching of fluorescence from singlet-excited state riboflavin by myoglobins using time-resolved fluorescence spectroscopy and a Stern-Volmer approach. Binding of riboflavin to MbFe(III) has K-a = (1.2 +/- 0.2) x 10(4) mol.L-1 with Delta H degrees = -112 +/- 22 kJ.mol(-1) and Delta S-circle = -296 +/- 75 J.mol(-1).K-1. For meat, riboflavin is concluded to be a photosensitizer for protein oxidation but not for discoloration.