Detection of S-Nitrosation and S-Glutathionylation of the Human Branched-Chain Aminotransferase Proteins.

The human branched-chain aminotransferase (hBCAT) enzymes play an integral role in brain glutamate and branched-chain amino acid (BCAA) metabolism. Optimal hBCAT activity is dependent on the oxidation state of their redox reactive thiols, where post-translational modification by nitric oxide (NO) and glutathione results in reversible inhibition. Incubation of the cytosolic isoform (hBCATc) with S-nitrosating agents was found to inhibit in both a time and dose dependent manner through formation of a mixture of products including cysteine-nitric oxide (SNO) and S-glutathionylation. Mechanistic details of these redox interactions were studied using labeling with fluorescein-5-maleimide and confirmed via mass spectrometry and Western blot analysis. Though the mitochondrial isoform (hBCATm) was inhibited by nitrosating agents adduct formation could only be observed by DTNB titration as neither SNO, S-glutathionylation or disulfide bond formation could be detected. These studies revealed that the two isoforms of hBCAT, namely hBCATc and hBCATm, were differently regulated by S-nitrosation or S-glutathionylation pointing to distinct functional/mechanistic responses to GSNO modification. Detection of these adducts is essential for studies into the effect of NO on cells and the redox proteome which can offer insight into several pathological states and normal functioning of the cell.