An essential tryptophan residue for rabbit muscle creatine kinase.

The tryptophan residues in rabbit muscle creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) have been modified by dimethyl(2-hydroxy-5-nitrobenzyl) sulfonium bromide after reversible protection of the reactive SH groups. The modification of two tryptophan residues as measured by spectrophotometric titration leads to complete loss of enzymatic activity. Control experiments show that reversible protection of the reactive SH groups as S-sulfonates followed by reduction results in nearly quantitative recovery of enzyme activity. The presence of a 410 nm absorption maximum and the decrease in fluorescence of the modified enzyme indicate the modification of tryptophan residues. At the same time, SH determinations after reduction of the modified enzyme show that the reagent has not affected the protected SH groups. Quantitative treatment of the data (Tsou, C.-L. (1962) Sci. Sin. 11, 1535 1558) shows that among the tryptophan residues modified, one is essential for its catalytic activity. The presence of substrates partially protects the modification of tryptophan residues as well as the inactivation, suggesting that the essential tryptophan residue is situated at the active site of this enzyme.