Pyrroloquinoline quinone (coenzyme PQQ) and the oxidation of SH residues in proteins

PQQ catalyzes the oxidation of cysteamine, dithiothreitol(DTT), and reduced glutathione,respectively, at neutral pH. The time course of cysteamine oxidation was monitored by absorption spectroscopy using the 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) titrating reagent. Under pseudo first order condition, a second order rate constant for the conversion of cysteamine to cystamine was determined. PQQ facilitates the oxidation of SH residues critically connected with the catalytic activity of enzyme from mammal;i.e., 4-aminobutyrate aminotransferase. PQQ treated enzyme recover catalytic activity upon addition of thiol compounds; cysteamine, DTT, and reduced glutathione, as well as thioredoxin from bacteria. In addition to these experiments, 4-aminobutyrate aminotransferase was partially purified from bacteria ( E. coli ) , which was treated with PQQ by the same manner. Inactivated 4- aminobutyrate aminotransferase recovers catalytic activity upon addition of thiol compounds and the rate of recovery is accelerated by the addition of thioredoxin.