GLYCATION REACTION OF ASPARTATE-AMINOTRANSFERASE BY VARIOUS CARBOHYDRATES IN AN IN-VITRO SYSTEM

Effects of various carbohydrates on cytosolic aspartate aminotransferase were studied in an in vitro system. When the purified aminotransferase was incubated with glucose or glucose-6-phosphate, the enzymatic activity and isoelectric points were essentially unaffected. On the other hand, fructose, fructose-6-phosphate, and ribose lowered both the enzymatic activities and isoelectric points in the absence of pyridoxal phosphate, and such alteration of the enzyme was partially prevented by the presence of pyridoxal phosphate. We suggest that these carbohydrates may bind to the Lys258 residue of the enzyme, which is the binding site of pyridoxal phosphate. Glyceraldehyde and dihydroxyacetone had powerful effects on the enzymatic activity and shifted the isoelectric points to the anodic side. In these cases, the addition of pyridoxal phosphate showed little effect on such alterations of the enzyme. Carbohydrates, except for glucose and glucose-6-phosphate, also resulted in fragmentation and polymerization of the enzyme. It was found that six and seven of the lysyl residues of the enzyme per subunit were modified with glyceraldehyde and dihydroxyacetone, respectively based on the analysis of amino acid composition.