Thioredoxin reductase : its properties and regeneration

Abstract

Thioredoxin reductase (TR) is a ubiquitous multifunctional enzyme that is widely distributed through out prokaryotic and eukaryotic organisms. It was fast discovered as the hydrogen donor for ribonucleotide reductase in E.coil. Since then, lots of studies have been carried out in characterizing the enzyme and evaluating its functions. However, information on TR from mammalian tissues, especially brain still remains largely unknown. In. this study, porcine brain TR was purified and characterized. Its regeneration and possible functional roles were also investigated. Porcine brain TR was purified to homogeneity by the method developed in our laboratory without using acid and heat treatment Purified TR was a functionally active monomeric enzyme having a molecular weight of 58kDa. This is different to those in previous studies in which the TR obtained were dimers and not active in the monomeric form. Functional studies of TR show that it is active in reducing disuifide bond of small and large protein substrates in the presence of NADPH and thioredoxin. The differences in molecular weight of TR to others previously described could be attributed to harsh acid and heat treatment employed in their purification procedures. The TR enzyme could regenerate hydrogen peroxide inactivated malate dehydrogenase (MDH). This shows that MDH may be a possible substrate for TR in the brain and one possible role of TR in brain is to alleviate the effect caused by oxidative stress through regeneration, of damaged enzymes. Comparative study of porcine brain TR with porcine red cell TR showed that they have similar physiological characteristic, but may differ structurally. We suggest that there is a possible existence of isozymes of TR which may play different roles in different tissues. In the regulation, study of TR, ribonucleoside 5-triphosphates were tested as TR inhibitors while protein. disulfide isomerase (PDI) was used as its regenerator. Adenosine 5-triphosphates (ATP) and cytidine 5r-triphosphate (CTP) showed inhibitory effect on TR activity within their physiological concentrations. PDI was able to regenerate denatured TR. The PDI regeneration property was independent of ATP presence. ATP, CTP and PDI thus may act as the regulators of TR through their inhibition and regeneration effect respectively.