Oxidative inactivation of the lymphoid tyrosine phosphatase mediated by both general and active site directed NO donors

Oxidative modification of protein tyrosine phosphatases (PTPs) has recently been recognized as an important regulatory mechanism in biological systems. Reported herein is the oxidative inactivation of the lymphoid tyrosine phosphatase (LYP) with both the general nitrosating reagent sodium nitroprusside (SNP) and also a novel peptide-based nitrosating reagent, Ac-ARLIEDNE(HcyNO)TAREG-NH₂, where HcyNO = S-nitrosohomocysteine. The SNP oxidatively inactivated LYP with a k_inact of 0.383 per min and a K_I of 27.4 μM and mixed-type inactivation kinetics. The peptide was a competitive LYP inactivator with a k_inact of 0.0472 per min and a K_I of 7.00 μM. LYP nitrosation by SNP was characterized by the addition of several NO moieties to the enzyme, while oxidation of LYP by the peptide did not result in the formation of a LYP-NO adduct. We propose that general NO donors promiscuously nitrosate any free cysteine residue while the active-site directed peptide selectively oxidizes the catalytic cysteine residue, resulting in the formation of a disulfide bond between the catalytic cysteine residue and a second cysteine in the active site.