Interaction of thioredoxin with oxidized aminobutyrate aminotransferase. Evidence for the formation of a covalent intermediate.

Pig brain 4-aminobutyrate aminotransferase is inactivated by pre-incubation with pyrroloquinoline quinone (2,7,9-tricarboxy-1H-pyrrolo[2,3,f]quinoline- 4,5-dione; PQQ) at pH 7. The reaction of approximately 2 SH residues/dimer is sufficient to inactivate the enzyme. Reoxidized aminotransferase is reactivated by E. coli thioredoxin. Similar results were obtained with E. coli 4-aminobutyrate aminotransferase. The spectroscopic properties of thioredoxin, tagged with the fluorescence probe, anthraniloyl, were used to monitor its interaction with re-oxidized 4-aminobutyrate aminotransferase. During the regeneration of native aminotransferase by thioredoxin, the substrate forms a covalent intermediate with the oxidoreductase, as revealed by gel filtration chromatography. It is postulated that the substrate (oxidized aminotransferase) forms a covalent intermediate with thioredoxin through disulfide linkages.