Thiolation of low-M-r phosphotyrosine protein phosphatase by thiol-disulfides

Thiol-disulfides cause a time- and a concentration-dependent inactivation o f the low-M-r phosphotyrosine protein phosphatase (PTP). We demonstrated th at six of eight enzyme cysteines have similar reactivity against 5,5'-dithi obis(nitrobenzoic acid): Their thiolation is accompanied by enzyme inactiva tion. The inactivation of the enzyme by glutathione disulfide also is accom panied by the thiolation of six cysteine residues. Inorganic phosphate, a c ompetitive enzyme inhibitor, protects the enzyme from inactivation, indicat ing that the inactivation results from thiolation of the essential active-s ite cysteine of the enzyme. The inactivation is reversed by dithiothreitol, Although all PTPs have three-dimensional active-site structures very simil ar to each other and also have identical reaction mechanisms, the thiol gro up contained in the active site of low-M-r PTP seems to have Lower reactivi ty than that of other PTPs in the protein thiolation reaction.