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.Recent work has suggested that the central nervous system plays a role in the regulation of rat hepatic tyrosine transaminase (EC 2.6.1.5)activity ( 14).A 90% depletion of the neurotransmitter, norepinephrine, by reserpine or cr-methyl-p-tyrosine is associated with a 2-to a-fold rise in enzyme activity in the fasted, adrenalectomized rat (1).Repletion of norepinephrine reverses this increase in enzyme activity, and puromycin prevents this rise (1).Norepinephrine may alter tyrosine transaminase activity by direct interaction with the enzyme system or may operate indirectly, through a number of mediators.Since certain amines and amino acids are known to bind pyridoxal 5'-phosphate (5), the cofactor required for tyrosine transaminase activity, the effect of catecholamines upon enzyme catalysis was examined.In the course of determining tyrosine transaminase substrate specificity, Canellakis and Cohen noted inhibition of enzyme activity by epinephrine (6) and subsequent studies showed reversal of this effect by ascorbic acid (7).This communication reports that norepinephrine, and related catecholamines, competitively inhibit rat hepatic tyrosine transaminase activity in vitro.Evidence supporting a specific mechanism of inhibition is presented, and relevant studies in wivo are reported. EXPERIMENTAL PROCEDUREChemicalso-Ketoglutaric acid (monopotassium salt) and pyridoxal 5'-phosphate were obtained from Sigma.L-Tyrosine (Grade A), ( -) -arterenol bitartrate, nn-arterenol bitartrate, 3hydroxytyramine hydrochloride, L-3,4-dihydroxyphenylethanolamine, nn-normetanephrine hydrochloride, nn-octopamine hydrochloride, histamine dihydrochloride, and 3,4-dihydroxy-nphenylalanine were obtained from Calbiochem.Serotonin creatinine sulfate, L-phenylalanine, and pyridoxine hydrochloride were purchased from Nutritional Biochemicals.3',5'-Ditritiotyrosine was obtained from ,Amersham-Searle (Des Plaines, Illinois).Tyramine hydrochloride was obtained from Eastman Kodak.Pentosol (pentobarbital sodium) was purchased from A. J. Buck and Son (Washington, D. C.). m-Tyramine hydrochloride and 2,3-dihydroxyphenylethylamine hydrochloride were generous gifts of Dr.