Glutathione Dependent Reduction of Alloxan to Dialuric Acid Catalyzed by Thioltransferase (Glutaredoxin): A Possible Role for Thioltransferase in Alloxan Toxicity

Recombinant pig liver thioltransferase (rPLTT) catalyzes the reduction of alloxan to dialuric acid by glutathione (GSH). This is the second non-disulfide substrate, after dehydroascorbic acid, described for thioltransferase. The reaction kinetics, measured by a coupled assay including glutathione disulfide reductase and NADPH yielded a Km = 82 μM for alloxan, a kcat = 37 s−1, and a kcat/Km = 4.5 × 105 M−1 s−1. The presence of rPLTT suppressed the competitive formation of compound 305, an alloxan-GSH conjugate of unknown structure, and at GSH concentrations between 0.05 mM and 1.5 mM, oxygen consumption was greater than that recorded in the uncatalyzed reaction. Both superoxide dismutase and catalase inhibited oxygen consumption in 1.0 mM GSH and 0.2 mM alloxan in the presence of rPLTT. This study suggests that thioltransferase (glutaredoxin) plays a significant role in the cytotoxicity of alloxan in vulnerable tissues.