Specific inhibition of protein disulphide reductase activity of a 60kda rat liver protein by adenine nucleotides

Abstract

A 60 kda protein which catalysed the reduction of DTN'B, 5.5'-dithio-bis(2 nitrobenzoic acid), in an NADPH-dependent manner was purified to single-band homogeneity from rat liver. Amino-terminal sequencing of the first twentyfive amino acids of the protein showed an overall sequence homology of over eighty percent with thioredoxin reductase purified from human placenta. In the presence of thioredoxin obtained either from plant or mammalian origin, the purified protein catalysed the reduction of the interchain disulphides of mouse immunoglobulin G. This protein disulphide reductase activity of the protein was inhibited by millimolar levels of adenosine tri- and di-phosphate in a metallic cation-independent manner. Adenosine monophosphate did not inhibit the reaction. Other nucleotides such as Uridine tri-, di- and monophosphates, Guanosine tri -, di- and monophosphates, Cytosine tri- and monophosphates also did not have any inhibitory effects. None of the above nucleotides affect the enzyme activity of the protein when DTNB was used as the substrate, indicating that both ATP or ADP did not cause an inactivation of catalytic site. Our results suggest that the purified 60kda protien is thioredoxin reductase. This is the first demonstration that both ATP and ADP, which occur in cells at millimolar quantities, can inhibit the protein disulphide reductase activity of the thioredoxin reductase/thioredoxin couple. This may represent a potential regulatory mechanism for the reduction of intracellular protein disulphides.