Rat liver tryptophan pyrrolase activity and brain 5-hydroxytryptamine

Tryptophan tolerance and metabolism in endogenous depression

The concentration of free and total tryptophan and kynurenine in plasma from 49 female depressives and 26 female controls was measured following oral loading with L-tryptophan, 100 mg/kg body weight. There was no significant difference between five depressives and six controls in the area under curve for free or total tryptophan or kynurenine in plasma. The peak concentration of kynurenine occurred 4 h after loading and in correlated significantly with the area under curve for kynurenine. There was no significant correlation between the L-tryptophan dose (g) and the plasma concentration of kynurenine a 4 h in the 49 depressives or 26 controls. The mean plasma levels of tryptophan and kynurenine at 4 h in the depressives were not significantly different from control levels. There was no clear relationship between the plasma levels of tryptophan or kynurenine at 4 h and the therapeutic response in 13 depressives treated with L-tryptophan for 15 days. It is concluded that the absorption, the plasma clearance, and the degradation to kynurenine of loading doses of L-tryptophan are normal in depressed patients. Results furthermore suggest that the plasma levels of tryptophan and kynurenine at 4 h are poor predictors of the response to L-tryptophan treatment in depressives.

Mechanism of decline in rat brain 5-hydroxytryptamine after induction of liver tryptophan pyrrolase by hydrocortisone: roles of tryptophan catabolism and kynurenine synthesis

1 Two mechanisms have been proposed to explain the decline in brain tryptophan and 5-hydroxytryptamine (5-HT) after administration of hydrocortisone and the subsequent induction of liver pyrrolase. These are depletion of tryptophan by high rates of tryptophan catabolism and inhibition of tryptophan uptake by elevated levels of the tryptophan catabolite, kynurenine.2 The increase in plasma kynurenine after hydrocortisone injection (25 mg/kg) was small, and kynurenine, at a concentration ten fold greater, did not inhibit tryptophan uptake by brain as measured by the Oldendorf technique. Thus, inhibition of tryptophan uptake by kynurenine is not an important mechanism in the control of brain tryptophan and 5-HT.3 The decline in brain tryptophan after hydrocortisone was comparable to that seen in other tissues, which comprise more than half of the body weight of a rat.4 The total decline in free tryptophan stores in whole animals treated with hydrocortisone was estimated to be about 450 mug. This amount of tryptophan would be catabolized by tryptophan pyrrolase in about 20 min, when the enzyme is induced, according to an earlier estimate of the rate of tryptophan catabolism in vivo.5 Tryptophan pyrrolase activity remains high for much longer than 20 min, suggesting that there is net protein catabolism, which releases tryptophan and prevents non-protein tryptophan levels falling very far.6 These results demonstrate that the decline in brain tryptophan and 5-HT after hydrocortisone is caused by depletion of tryptophan stores due to the high activity of tryptophan pyrrolase. However, our data suggest that this effect is diminished by release of tryptophan from proteins. Thus, peripheral protein metabolism may be an important factor in the control of brain tryptophan levels and 5-HT synthesis.

The daily rhythm of plasma tryptophan and tyrosine in depression

The study dealt with the level of and diurnal alterations in the concentration of tryptophan, free tryptophan and tyrosine in the blood plasma of 20 inhibited depression patients and 10 healthy controls. The results suggested that there was no distinct relationship between either the total plasma tryptophan or plasma tyrosine level and depression. On the other hand, the free plasma tryptophan level was, at all the times of day at which measurements were made, either significantly or almost significantly higher in the patients than in the controls. It was further found that the results of measurement were related to the patients' clinical improvement, as measured by the Hamilton test, in such a way that after four weeks of treatment the free plasma tryptophan level in 'poorly improved' patients continued to be significantly higher in comparison with the controls, whereas the values for the 'well improved' patient group did not differ greatly from the corresponding values for the control group any longer. It may be hypothesized that the rise in the free plasma tryptophan in depressive patients might represent an effort made by the peripheral body to compensate for the slowed-up serotonin metabolism of the brain, whereby the tryptophan mobilized from the periphery would serve as a sort of 'endogenous antidepressant' provided by the organism itself.

Tryptophan pyrrolase--a biochemical factor in depressive illness?

Reserpine, which depletes brain amines, not infrequently causes depression (Bunney and Davis, 1965), whereas monoamine oxidase inhibitors (MAOI) which raise brain amine levels (Macleanet al.,1965) may alleviate depressive symptoms. The alleviation has been reported to be enhanced by tryptophan (Coppenet al.,1967), which is a precursor of the amines 5-hydroxytryptamine (5HT) and tryptamine. The apparent prophylaxis by tryptophan of an annual episode of depression has also been reported (Hertz and Sulman, 1968). These findings suggest that elevation of one or both of the above amines is responsible for the therapeutic effect of MAOI, and that their insufficiency may be responsible for symptoms of the disease. Defective 5HT synthesis in depressive illness is indicated by a number of findings. (1) Depressed patients responding to the MAOI drug iproniazid were found in two studies (Pare and Sandier, 1959; Praag and Leijnse, 1963), though not in a third (Burgermeisteret al.,1963), to have a lower initial urinary excretion of the 5HT metabolite 5-hydroxyindole acetic acid (5HIAA) than those who did not respond. (2) Depressed patients have a low content of 5HIAA in the lumbar cerebrospinal fluid (Ashcroftet al.,1966), which rises on recovery. Although work using animals (Guldberg and Yates, 1968; Ecclestonet al.,1968) shows that changes in brain 5HT are paralleled by changes of 5HIAA in the CSF, the significance of this result is not altogether clear, as lumbar 5HIAA depends not only on brain 5HT metabolism but also on 5HIAA transport within the CSF (Ashcroftet al.,1966). Furthermore, a small group of acute schizophrenics on phenothiazines also had low 5HIAA. (3) There is some direct evidence of low 5HT (Shawet al.,1967) or 5HIAA (Bourneet al.,1968) in the hindbrains of depressive suicides. (4) While single doses of 5-hydroxytryptophan, the immediate precursor of 5HT, do not alleviate depression, the remission of a prolonged episode of severe depression by administration of 5-hydroxytryptophan for five days has been reported (Persson and Roos, 1967).