Determination of the isoelectric point value of 3-mercaptopyruvate sulfurtransferase and its shift by treatment with oxidized glutathione.

The isoelectric point (pI) value of 3-mercaptopyruvate sulfurtransferase (MST) from human erythrocytes was determined to be 6.3 at 10 degrees C by isoelectric focusing in horizontal slab polyacrylamide gel containing 2% carrier ampholyte (pH 3-10). The value was determined by comparison with the electrofocused bands of bovine pancreatic ribonuclease A-glutathione mixed disulfides (RNase-SG), which were composed of 8 species containing 1 (RNase-SG1) through 8 (RNase-SG8) moles of glutathione per mole of ribonuclease A with different pI values ranging from 5.3 (RNase-SG8) to 8.8 (RNase-SG1). The pI value of the same enzyme in a 110,000 X g supernatant of rat liver was 5.9, which was the same as that of rat erythrocyte enzyme. Treatments of rat hemolysate with oxidized glutathione or diamide resulted in a shift of the pI of MST to a lower value, 5.7-5.5. This shift was inhibited when these treatments were performed in the presence of dithiothreitol. These results indicate that the treatment of the enzyme with oxidized glutathione results in the formation of enzyme-glutathione mixed disulfide.