Inhibition of tyrosine transaminase activity by norepinephrine.

The neurotransmitter, norepinephrine, and structurally related primary catecholamines inhibited rat hepatic tyrosine transaminase activity (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) in vitro. The inhibition was competitive with pyridoxal 5’-phosphate cofactor and an “apparent” I& of 4.0 x 10-e M was determined. (The “apparent” K,,, was 6.6 X 10-T M.) Those amines which were associated with enzyme inhibition formed spectrophotometrically demonstrable complexes with pyridoxal 5’phosphate, and full enzyme inhibition was achieved by the preliminary incubation of norepinephrine and cofactor alone. The relevance of this mechanism of regulation was examined in vivo. Increasing doses of norepinephrine suppressed and ultimately abolished the pyridoxine-induced rise of tyrosine transaminase activity in the fasted, adrenalectomized rat.