Aspartate aminotransferase (GOT) from young and old human erythrocytes.

Abstract

Abstract In the absence of added cofactor, the specific aspartate aminotransferase activity ratio, enzyme from young erythrocytes/enzyme from old erythrocytes, is 3.5. Addition of pyridoxal or pyridoxamine phosphate diminishes this ratio to 1.7. Aspartate aminotransferase associated with old erythrocytes thus falls into 3 groups: enzyme which is active, enzyme which is inactive but can be reactivated by addition of pyridoxal or pyridoxamine phosphate, and enzyme that is "lost" (permanently inactive). Comparative studies are presented with aspartate aminotransferase from young, unfractionated, and old erythrocytes. These include the resolution of holoenzyme into apo- and coenzyme and the kinetics of reactivation of apoenzyme with pyridoxal phosphate; the electrophoretic mobility of holo-, apo-, and reactivated enzyme, and the effect of storage, pH, and high salt concentration on the holoenzyme. All our results imply that those parts of aspartate aminotransferase in older erythrocytes which are still active, or can be reactivated by addition of coenzymes, are similar in their properties to the aspartate aminotransferase in young red cells. Possibilities for the fate of the "lost" enzyme are discussed. It should be noted that the differential activity of aspartate aminotransferase in young and old red cells, which has been used as an indicator of red cell age, can be the consequence of cofactor level and/or enzyme protein modification.