His64(E7)-->Tyr apomyoglobin as a reagent for measuring rates of hemin dissociation

To develop an assay for hemin dissociation, His64(E7) was replaced by Tyr in sperm whale myoglobin producing a holoprotein with a distinct green color due to an intense absorption band at 600 nm. Val68(E11) was replaced by Phe in the same protein to increase its stability. When excess Tyr64-Val68 apoglobin is mixed with either metmyoglobin or methemoglobin, the solution turns from brown to green, and the absorbance changes can be used to measure complete time courses for hemin dissociation from either holoprotein. This assay has been used to measure rates of hemin dissociation from native metmyoglobin, four myoglobin mutants (Ala64(E7), Ala68(E11), Phe68(E11), and Glu45(CD3)), native methemoglobin, valence hybrid hemoglobins, and two mutant hemoglobins ((alpha(Gly-E7)beta(native))2, and (alpha(native)beta(Gly-E7))2). Two kinetic phases were observed for hemin dissociation from native human hemoglobin at pH 7.0 and 37 degrees C. Valence and mutant hybrid hemoglobins were used to assign the faster phase (k = 7.8 +/- 2.0 h-1) to hemin dissociation from ferric beta subunits and the slower (k = 0.6 +/- 0.15 h-1) to dissociation from alpha subunits. The corresponding rate for wild-type metmyoglobin is 0.007 +/- 0.004 h-1.