Serum aspartate aminotransferase storage and the effect of pyridoxal phosphate

The storage stability of the catalytic activity of aspartate aminotransferase (AST) remains the subject of conflicting reports. We reevaluated this issue for total AST activity and for the cytosolic and mitochondrial isoenzymes from human sera stored at room, refrigerator, and freezer temperatures for up to 28 days. We found that these enzymes were stable for catalytic activity and immunologic (mass) measurements up to 24 hours at ambient temperatures and for at least 28 days at 4 degrees, -20 degrees, and -80 degrees C. The addition of exogenous pyridoxal phosphate (0.1 mmol/L) to serum improved stability of AST storage at ambient temperature (22 degrees C) from 1 day to 7 days.