The lack of L-thyroxine inhibition of human aspartate aminotransferases

1. L-Thyroxine, which inhibits several transaminases, was shown to have no such effect on human aspartate aminotransferases (AspAT). 2. No inhibitory effect was observed at optimal or suboptimal aspartate concentrations with the enzyme in human serum or isoenzymes purified from human tissues. 3. Thyroxine was also shown not to interfere with the stimulation of AspAT by pyridoxal phosphate. Human cytoplasmic and mitochondrial AspAT differ from some other pyridoxal-requiring enzymes in showing this lack of thyroxine inhibition.