DESTABILIZATION OF TYROSINE AMINOTRANSFERASE BY AMINO-ACIDS

Several L-amino acids (tyrosine, glutamate, methionine, tryptophan, an d phenylalanine) and penicillamine destabilized purified tyrosine amin otransferase by removing enzyme-bound pyridoxal 5'-phosphate. The dest abilization was measured as a progressive loss of enzyme activity in s amples taken at intervals from a primary mixture that was incubated at 37 degrees C. Each destabilizing amino acid either served as a substr ate for this enzyme or was a product of transamination. In contrast, L -cysteine destabilized the enzyme only if liver homogenate was added, which generated polysulfide by desulfuration. Cysteine complexed free pyridoxal-5'-phosphate but did not remove it from the enzyme. Other am ino acids did not destabilize tyrosine aminotransferase at the concent rations tested.