REGULATION OF HEPATIC TYROSINE TRANSAMINASE IN VIVO THROUGH INTERACTION OF NOREPINEPHRINE AND THE PYRIDOXAL PHOSPHATE CO-FACTOR

¹ Laboratory of Clinical Science, National Institute of Mental Health, Bethesda, Maryland

The interaction of norepinephrine and the pyridoxal-5'-phosphate co-factor in the regulation of hepatic tyrosine transaminase activity was investigated in the adrenalectomized rat. Kinetic data suggested that enzyme induction by co-factor was dependent on an increased rate of enzyme synthesis. This induction was blocked by norepinephrine and dopamine, primary catecholamines which form complexes with the co-factor, and by l-penicillainine, a known binding agent of pyridoxal-5'-phosphate. The circadian rhythm in tyrosine transaminase activity was abolished by the protein synthesis inhibitor cycloheximide and by norepinephrine. These results suggest that norepinephrine, by competing with apoenzyme for co-factor, lowers tyrosine transaminase synthesis and may thereby play a role in the generation of the circadian rhythm in enzyme activity.