Thiolation of Low-Mr Phosphotyrosine Protein Phosphatase by Thiol-Disulfides

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Abstract
Thiol‐disulfides cause a time‐ and a concentration‐dependent inactivation of the low‐Mr phosphotyrosine protein phosphatase (PTP). We demonstrated that six of eight enzyme cysteines have similar reactivity against 5,5‐dithiobis(nitrobenzoic acid): Their thiolation is accompanied by enzyme inactivation. The inactivation of the enzyme by glutathione disulfide also is accompanied by the thiolation of six cysteine residues. Inorganic phosphate, a competitive enzyme inhibitor, protects the enzyme from inactivation, indicating that the inactivation results from thiolation of the essential active‐site cysteine of the enzyme. The inactivation is reversed by dithiothreitol. Although all PTPs have three‐dimensional active‐site structures very similar to each other and also have identical reaction mechanisms, the thiol group contained in the active site of low‐Mr PTP seems to have lower reactivity than that of other PTPs in the protein thiolation reaction.

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