States of amino acid residues in proteins. XIX. Modification of arginine residues in myoglobin

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Abstract The treatment of horse-heart metmyoglobin with glyoxal caused an abrupt drop of absorbance of its Soret band, while a little effect was found on the Soret band of methemoglobin similarly treated with glyoxal. The spectral change of metmyoglobin proceeded by about 80% during 3 h of treatment at pH 8.9, and amino acid analysis indicated that 1.1 mole of arginine per mole of myoglobin was modified by the treatment and that the lysine content decreased by approx. 30%. Modification of only the lysine residues with iodoacetic acid, O- methylisourea or acetic anhydride caused a much smaller spectral change. The spectrum of glyoxal-treated myoglobin changed with varying the pH value of the solution ( p K = 6.5 ± 0.5 ), while the spectrum of nontreated myoglobin did not change. It was deduced from these results that the modification of one of the arginine residues affected the interaction between His 93 an the heme group, thus causing the lowering of the Soret band. The ethylisocyanide-binding affinity of glyoxal-treated myoglobin changed with pH and showed a maximum near pH 6. The Hill constant, n , estimated from the ethylisocyanide binding increased from 1.1 to 1.4 when the pH value was lowered from 7 to 6.2. Glyoxal-modified myoglobin was homogeneous at pH 7.0 as examined by gel filtration with Sephadex G-75, chromatography with carboxymethylcellulose, electrophoresis with polyacrylamide gel and ultracentrifugal analysis. The sedimentation coefficient of glyoxal-modified myoglobin was 1.6 S at pH 7 and slightly higher than this S value at pH 5.5 or 6.0.

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