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The essential thiol group of arginine kinase from Homarus vulgaris muscle studied by difference spectrophotometry and N-ethyl-[1-14C] maleimide labelling.

Interaction of N-ethylmaleimide with the essential cysteinyl residue of ATP:l-arginine phosphotransferase from Homarus vulgaris muscle has been studied by means of difference spectrophotometry and labelling with radioactive N-ethyl-[1-14C]maleimide. 
 
 
 
Cysteine and glutathione were used as model compounds reacting with the inhibitor. The difference spectrum at 238 nm observed with the l-arginine-arginine kinase complex might be ascribed to the modification of ionized – SH groups. The absorption change observed near 287 nm. produced by tyrosyl perturbation, is the same when the enzyme reacts with the inhibitor or with l-arginine. 
 
 
 
Labelling data give evidence of a significant protection of the essential thiol group by l-arginine against N-ethyl-[1-14C]maleimide. This cysteinyl residue is included in the tryptic peptide T1A the amino acid sequence of which, recently determined, is very similar to that to the homologous peptides of muscle and brain rabbit creatine kinases and of earth-worm muscle lombricine kinase.