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1. Chronic ethanol administration to rats was previously shown to enhance brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain secondarily to the NAD(P)H-mediated inhibition of liver tryptophan pyrrolase activity. 2. At 24h after ethanol withdrawal, all the above effects were observed because liver [NAD(P)H] was still increased. By contrast, all aspects of liver and brain tryptophan metabolism were normal at 12 days after withdrawal. 3. At 7–9 days after withdrawal, brain 5-hydroxytryptamine synthesis was decreased, as was tryptophan availability to the brain. Liver tryptophan pyrrolase activity at these time-intervals was maximally enhanced. 4. Administration of nicotinamide during the withdrawal phase not only abolished the withdrawal-induced enhancement of tryptophan pyrrolase activity on day 8, but also maintained the inhibition previously caused by ethanol. Under these conditions, the withdrawal-induced decreases in brain 5-hydroxytryptamine synthesis and tryptophan availability to the brain were abolished, and both functions were enhanced. Nicotinamide alone exerted similar effects in control rats. 5. It is suggested that ethanol withdrawal inhibits brain 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to the enhanced liver tryptophan pyrrolase activity. 6. The results are discussed in relation to the possible involvement of 5-hydroxytryptamine in dependence on ethanol and other drugs.

1. Liver tryptophan pyrrolase exists as holoenzyme and apoenzyme in rat, mouse, pig, turkey, chicken and possibly man. 2. The apoenzyme is absent from cat, frog, gerbil, guinea pig, hamster, ox, sheep and rabbit. 3. The hormonal mechanism of induction of the pyrrolase is absent from species lacking the apoenzyme. 4. The concentrations of tryptophan in livers and sera of these species are lower than in species possessing the apoenzyme. 5. Species lacking the apoenzyme or the hormonal induction mechanism have a deficient kynurenine pathway and are sensitive to the toxicity of tryptophan. 6. It is suggested that these species are not suitable as models for studying human tryptophan metabolism. 7. The possible significance of these findings in relation to veterinary and human neonatal care is discussed.

Chronic administration of pyrazole in the diet of rats does not cause toxicity and prevents the chronic effects of ethanol on: (1) the redox states of the hepatic NAD(P) couples; (2) liver tryptophan pyrrolase activity; (3) brain tryptophan and 5-hydroxytryptamine metabolism.

1. When assayed in fresh homogenates, guinea-pig liver tryptophan pyrrolase exists only as holoenzyme. It does not respond to agents that activate or inhibit the rat liver enzyme in vitro. Only by aging (for 30min at 5°C) does the guinea-pig enzyme develop a requirement for ascorbate. 2. The guinea-pig liver enzyme is activated by the administration of tryptophan but not cortisol, salicylate, ethanol or 5-aminolaevulinate. 3. The tryptophan enhancement of the guinea-pig liver pyrrolase activity is prevented by 0, 34 and 86% by pretreatment with actinomycin D, cycloheximide or allopurinol respectively. 4. The guinea-pig liver tryptophan pyrrolase is more sensitive to tryptophan administration than is the rat enzyme. On the other hand, the concentrations of tryptophan in sera and livers of guinea pigs are 45–52% less than those in rats. 5. It is suggested that tryptophan may regulate the activity of guinea-pig liver tryptophan pyrrolase by mobilizing a latent form of the enzyme whose primary function is the detoxication of its substrate.

1. Chronic ethanol administration enhances rat brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain. This increased availability is not insulin-mediated or lipolysis-dependent. 2. Under these conditions, tryptophan accumulates in the liver and apo-(tryptophan pyrrolase) activity is completely abolished, but could be restored by administration of regenerators of liver NAD+ and/or NADP+. 3. All four regenerators used (fructose, Methylene Blue, phenazine methosulphate and sodium pyruvate) prevented the ethanol-induced increase in liver tryptophan concentration and the increased availability of tryptophan to the brain. 4. It is suggested that the enhancement of brain tryptophan metabolism by chronic ethanol administration is caused by the decreased hepatic tryptophan pyrrolase activity. The results are briefly discussed in relation to previous work with ethanol. 5. Fructose enhances the conversion of tryptophan into 5-hydroxyindol-3-ylacetic acid in brains of ethanol-treated rats, whereas Methylene Blue inhibits this conversion in both control and ethanol-treated animals.

1. Acute ethanol administration causes a biphasic change in rat liver tyrosine aminotransferase activity. 2. The initial decrease is significant with a 200 mg/kg dose of ethanol, is prevented by adrenoceptor-blocking agnets and by reserpine, but not by inhibitors of ethanol metabolism, and exhibits many of the characteristics of the inhibition caused by noradrenaline. 3. The subsequent enhancement of the enzyme activity by ethanol is not associated with stabilization of the enzyme, but is sensitive to actinomycin D and cycloheximide. 4. It is suggested that the initial decrease in aminotransferase activity is caused by the release of catecholamines, whereas the subsequent enhancement may be related to the release of glucocorticoids.

1. Acute administration of ethanol exerts a biphasic effect on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid. Both effects are associated with corresponding changes in the availability of circulating free tryptophan. 2. The initial increases in the above concentrations are prevented by ergotamine, are unaltered by allopurinol and are potentiated by theophylline, whereas the later decreases are prevented by both ergotamine and allopurinol. 3. It is suggested that the initial enhancement by ethanol of brain tryptophan metabolism is caused by catecholamine-mediated lipolysis followed by displacement of protein-bound serum tryptophan, whereas the activation of liver tryptophaan pyrrolase, which is produced by the same mechanism, leads to the later decreases in the brain concentrations of tryptophan and its metabolites. 4. The initial effects of ethanol can be reproduced by an equicaloric dose of sucrose, and a comparison of the two treatments alone could therefore be misleading. 5. The effects of ethanol on liver and brain tryptophan metabolism have also been examined in mice, and a comparison of the results with those previously reported suggests that the ethanol effects are strain-dependent.

Administration of pyridoxine stabilizes rat liver tyrosine aminotransferase in vivo, whereas administration of cortisol, cyclic AMP, glucagon, insulin, tryptophan or tyrosine does not. The results of these and other experiments with pyridoxine are discussed in relation to the mechanisms of action of this vitamin on the activity of the enzyme.

The chronic morphine-induced inhibition of rat liver tryptophan pyrrolase activity and the resultant increases in tryptophan availability to the brain and brain 5-hydroxytryptamine (5-HT) synthesis are reversed within 10 min after naloxone administration. The possible involvement of hepatic tryptophan metabolism in morphine dependence is briefly discussed.

Chronic phenobarbitone administration inhibits the apo-(tryptophan pyrrolase) activity in homogenates of rat liver and subsequent withdrawal enhances the enzyme activity by 2.5-fold. Similar effects have been previously produced by chronic ethanol administration and withdrawal, but, whereas NADH may cause the ethanol inhibition, that by phenobarbitone may be mediated by NADPH.

Berry, M. N. & Friend, D. S. (1969) J. Cell B id . 43, 506-520 Berry, M. N. & Kun, E. (1972) Eur. J. Biochern. 27, 395-400 Hillbom, M. E. & Lindros, K. 0. (1971) Metub. Clin. Exp. 20, 843 Lee, Y. P. & Lardy, H. A. (1965) J. Biol. Chem. 240, 1427-1436 Marjanen, L. (1972) Biochem. J. 127, 633-639 Williamson, D. H., Lund, P. & Krebs, H. A. (1967) Bioehetn. J. 103, 514-527 Zebe, E., Delbruck, A. & BCicher, Th. (1959) Biochem. 2. 331,254-272

We have previously shown that chronic ethanol administration enhances brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain secondary to the decreased liver tryptophan pyrrolase activity. We now find that ethanol enhances the brain metabolism of a tryptophan load by the same mechanism. The results are discussed in relation to ethanol preference and the need for further clinical work on the effects of alcoholism on tryptophan metabolism.

Acute ethanol administration exerts multiple effects on rat brain catecholamine synthesis, associated with corresponding changes in cerebral tyrosine concentration. Catecholamine synthesis is enhanced at 1 hr by an increased availability of circulating tyrosine to the brain after inhibition of liver tyrosine aminotransferase activity. Tyrosine hydroxylation in vivo and tyrosine hydroxylase activity measured in vitro are also enhanced at 1 hr. Catecholamine synthesis is inhibited at 2–4 hr when tyrosine availability to the brain is decreased because of an enhancement of liver tyrosine aminotransferase activity. Serum neutral amino acid concentrations are decreased at 5 hr. This is followed 1 hr later by normalization of cerebral catecholamine synthesis. By 8 hr after ethanol administration, the latter becomes enhanced because of increased cerebral uptake of tyrosine. Catecholamine synthesis is inhibited at 12 hr because of enhanced transamination of brain tyrosine. Tyrosine metabolism finally returns to normal at 16 hr after ethanol administration. These results are discussed in relation to previous work with ethanol, and to central and peripheral mechanisms of regulation of brain catecholamine synthesis.

1. Drugs such as phenobarbitone and phenylbutazone, which increase the concentration of microsomal haem and cytochrome P-450, also increase the saturation of rat liver apo-(tryptophan pyrrolase) with its haem activator, as does the haem precursor 5-aminolaevulinate. 2. At 4h after the administration of the porphyrogens 2-allyl-2-isopropylacetamide, 3,5-diethoxycarbonyl-1,4-dihydrocollidine and griseofulvin, the total pyrrolase activity is increased whereas the haem saturation of the apoenzyme is decreased. This decreased saturation is prevented by pretreatment of the animals with the inhibitor of drug-metabolizing enzymes, SKF 525-A. 3. Pretreatment of rats with the above porphyrogens inhibits the rise in holo-(tryptophan pyrrolase) activity produced by subsequent administration of cortisol, tryptophan and 5-aminolaevulinate with two single exceptions, the possible reasons for which are discussed. 4. At 24h after the administration, in starved rats, of a single daily injection of the above porphyrogens for 1 or 2 days, the holoenzyme activity is significantly increased. 5. It is suggested that the saturation of rat liver apo-(tryptophan pyrrolase) with its haem activator can be modified by treatment known to cause destruction, inhibition of synthesis, increased utilization and enhanced synthesis of liver haem. The possible involvement of the latter phenomenon in the aetiology of mental disorders in some patients with porphyria is discussed.

The apoenzyme activity of rat liver tryptophan pyrrolase is decreased in vitro by 16–100% by concentrations of many antidepressants of 0.01–1 mM. Apo-(tryptophan pyrrolase) activity is also decreased by 37–86% at 2hr after administration of a 10 mg/kg dose of many antidepressants. This inhibition appears to be due to the prevention of the conjugation of the apoenzyme with its cofactor haem. Brain tryptophan concentration is elevated by 19–39% at 3.5 hr after administration of the above dose of antidepressants. Isocarboxazid is the only antidepressant tested that affects neither liver pyrrolase activity nor brain tryptophan concentration. The non-antidepressants chlorpromazine, β-flupenthixol, mefenamic acid and pargyline are also ineffective in both respects. The increase in brain tryptophan concentration caused by administration of mianserin, viloxazine, desipramine or tranylcypromine is associated with an accumulation of tryptophan in the liver and an increased availability of the circulating amino acid to the brain. It is suggested that antidepressants increase brain tryptophan concentration by inhibiting liver tryptophan pyrrolase activity. The results are briefly discussed in relation to the therapeutic effects of the drugs.

1 Administration to rats of a 0.5 mg/kg dose of any of 19 antidepressants, but not that of many other drugs, causes a significant inhibition of the total enzyme and apoenzyme activities of liver tryptophan pyrrolase (of 24-48% and 37-65% respectively) and elevates brain tryptophan concentration by 13-66%. 2 When liver tryptophan pyrrolase activity is enhanced by pretreatment with cortisol or haematin, subsequent administration of a 0.5 mg/kg dose of some, but not other, antidepressants causes inhibition, which is weak (up to 38%). 3 This weak inhibition of the enhanced pyrrolase activity together with other pharmacological and physiological factors could explain the time lag between the start of antidepressant medication and the occurrence of a therapeutic response. 4 The cortisol-induced and haematin-activated pyrrolases respond differentially to inhibition by imipramine and amitriptyline, and this may explain the differential response to these two drugs of depressed patients in relation to urinary excretion of the noradrenaline metabolite 3-methoxy-4-hydroxyphenylglycol. 5 The results are discussed in relation to the mechanism of action of antidepressants and the possible involvement of disturbed hepatic tryptophan metabolism in depressive illness.

1. Chronic administration of ethanol, morphine, nicotine or phenobarbitone has previously been shown to enhance rat brain 5-hydroxytryptamine (5-HT) synthesis by increasing the availability of circulating tryptophan to the brain secondarily to the NADPH-mediated inhibition of liver tryptophan pyrrolase activity. 2. Naloxone reverses the above enhancement of 5-HT synthesis and the accompanying increase in tryptophan availability to the brain and the inhibition of liver tryptophan pyrrolase activity. 3. It is suggested that naloxone exerts these effects by antagonizing the chronic drug-induced increase in liver [NADPH]. 4. Naloxone increases serum corticosterone concentration in rats chronically treated with the above four drugs of dependence. Possible explanations of this effect are discussed.

The activity of rat liver tryptophan pyrrolase (EC 1.13.11.11) is inhibited by chronic ethanol administration by a mechanism not involving a defective apoenzyme synthesis nor a decreased cofactor availability, but associated with an increased liver [NADH] and [NADPHJ (Badawy & Evans, 1973a,b, 1975). These two reduced nucleotides are allosteric inhibitors of the activity of the purified enzyme (Cho-Chung & Pitot, 1967), and their possible involvement in the inhibition by ethanol is suggested by the finding (Badawy & Evans, 1975) that the latter can be reversed by regeneration of liver NAD+ or NADP+ in vivo or by the addition of these oxidized nucleotides in vitro. Mnrrland (1974) has, however, suggested that ethanol acts chronically by inhibiting apo-(tryptophan pyrrolase) synthesis because the drug treatment inhibits the dexamethasone induction of the enzyme activity in the isolated perfused rat liver. However, we have found (Badawy & Evans, 19736) that the cortisol induction of the pyrrolase activity in intact rats is not impaired by chronic ethanol administration. A possible explanation of the finding by Mnrrland (1974) is that his ethanol-treated rats have also been given sucrose, which inhibits the pyrrolase activity (Badawy & Evans, 1976~). We have also shown in this latter study that sucrose acts via glucose, and that the latter sugar inhibits the pyrrolase activity by an NADHand NADPH-mediated mechanism. Moreover, we have shown that the decreased cortisol induction of the enzyme activity in glucose-pretreated rats is due to inhibition of activity and not synthesis of the apoenzyme. Since the results of experiments by Mnrrland (1974) do not permit an interpretation of a possible ethanol effect in the absence of sucrose, we have compared the cortisol induction of the pyrrolase activity in rats chronically treated with water, ethanol or sucrose. Locally bred male Wistar rats (body wt. was lOOg at the start of the experiments) were allowed free access to cube diet 41B and one of the foliowing three drinking fluids for 28-31 days: water, sucrose (9%, w/v) or ethanol [ 5 % (v/v) for 2 days, 7.5% for 2 further days and finally 10x1. Some of these rats also received, 4h before death, an