Impaired monoamine oxidase activity in dogs with portacaval shunt

Prevalence of low monoamine oxidase function in alcoholism

Several studies have found a trend for low platelet monoamine oxidase activity (MAO) in alcoholism but with a great deal of overlap in MAO activity of alcoholics versus controls. The main objective of this study was to carry out a detailed assessment of MAO function that included the measurement of key kinetic parameters (i.e., Km, Vmax) in three groups of male subjects: (a) 51 hospitalized chronic alcoholics, (b) 16 recovering alcoholics with 2-10 years of abstinence, and (c) 21 controls. MAO activity was assayed radiochemically with [14C]tyramine as substrate (43-729 microM). The present study demonstrated that alcoholics had low platelet MAO activity (p less than 0.05). Kinetic analysis revealed a substantial reduction (p less than 0.01) in enzyme Vmax values of chronic and recovering alcoholics. Greater than 95% of the alcoholics had Vmax values lower than the smallest value of control subjects. Moreover, 100% of the alcoholics in both groups exhibited exceedingly low Vmax values that were below the 25th percentile of controls. In summary, results of MAO Vmax determinations provided us with a better separation of the alcoholics from controls. Measurements of platelet MAO function that include enzyme Vmax may provide us a reliable biochemical marker for alcoholism.

Salicylate and mitochondrial monoamine oxidase function in Reye's syndrome

The main objective of this investigation was to study the effect of salicylate on platelet mitochondrial monoamine oxidase (MAO) activity isolated from blood of two patients with Reye's syndrome. Comparative studies were made with hospitalized children without Reye's syndrome (n = 27) and healthy children (n = 19) serving as controls. Platelet MAO was measured by a radioenzymatic technique with [14C]tyramine as a substrate. The results of this study showed that salicylate (1.0 mM) caused an appreciable inhibition of the platelet MAO activity of patients with Reye's syndrome at the onset of the illness. This was demonstrated by a greater than 50% reduction in enzyme maximum velocity (Vmax) value. The salicylate MAO-inhibitory effect was maintained throughout the duration of the illness. Salicylate had only a minimal MAO-inhibitory effect on platelets isolated from blood of recovered Reye's syndrome patients, healthy controls, and non-Reye's hospitalized children, and no apparent effect on enzyme Vmax values. These preliminary findings suggest that salicylate-induced mitochondrial injury may affect MAO function in children with Reye's syndrome.

Preparation and biodistribution of 99mTc-labeled tyramine iminodiacetic acid

The synthesis and biodistribution properties of 99mTc-labeled N-substituted tyramine, [N-(4-hydroxyphenethyl)iminodiacetic acid] are described. Tissue distribution studies in rats were indicative of high hepatic and kidney extraction, accompanied by rapid plasma and urinary clearance and minimal biliary excretion. These findings were substantiated by organ image analysis. The preliminary data indicate that this labeled material may represent a new class of radiopharmaceuticals for the evaluation of hepatic and renal functions.

Monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine in the dopaminergic synaptosomes from rat corpus striatum

This communication describes conditions under which the monoamine oxidase-catalyzed metabolism of 3,4-dihydroxyphenylethylamine can be assayed in dopaminergic synaptosomes from the rat striatum. In contrast to the activity of the isolated enzyme or that of free mitochondria, the synaptosomal reaction exhibits sigmoidal kinetics with respect to substrate concentration. This is consistent with a kinetic mechanism in which intrasynaptosomal substate partitions between reaction and saturable storage in synaptic vesicles. The reaction is inhibited at moderately decreased oxygen tension, where catecholamine uptake is unaffected. The specificity of this effect suggests that it reflects limited availability of oxygen as an enzyme substrate.

Evidence for central hypertyraminemia in hepatic encephalopathy

In mongrel dogs, the effect of end-to-side portacaval shunt on plasma, cerebrospinal fluid (CSF) and brain tyramine, tyrosine, dopamine, norepinephrine, and epinephrine were studied. It was found that the level of tyramine in plasma, CSF, and selected brain regions increased steadily after the construction of the shunts. These elevations became more pronounced when the dogs manifested symptoms of hepatic encephalopathy. In postshunted dogs with stage II and III hepatic encephalopathy, tyramine concentration in corpus striatum (1,312 +/- 371), hypothalamus (400 +/- 67.0), and midbrain (660 +/- 78.7 ng/g) was significantly (P less than 0.05) higher than the level in dogs with stage 0 and I hepatic encephalopathy and sham-operated dogs serving as controls (corpus striatum, 831 +/- 140; hypothalamus, 167 +/- 40.0; and midbrain, 132 +/- 37.4 ng/g). This was followed by a concomitant depletion of dopamine and norepinephrine in these brain regions (postshunt: dopamine 104 +/- 20.0, 3,697 +/- 977, and 105 +/- 14.1; norepinephrine 521 +/- 71.6, 81.6 +/- 13.7, and 218 +/- 31.7 ng/g; vs. sham group: dopamine 532 +/- 83.1, 8,210 +/- 1,126, and 192 +/- 35.0; norepinephrine 1,338 +/- 425, 124 +/- 21.3, and 449 +/- 89.7 ng/g) of encephalopathic dogs with portacaval shunt. Furthermore, tyramine, tyrosine, dopamine, and norepinephrine levels in plasma and CSF increased markedly as clinical features in the dogs' behavior characteristic of hepatic encephalopathy occurred, including hypersalivation, ataxia, flapping tremor, somnolence, and coma. Cerebral hypertyraminemia and a defect in sympathetic neurotransmission may contribute to the development of hepatic encephalopathy of liver disease.