The effects of chemical porphyrogens and drugs on the activity of rat liver tryptophan pyrrolase

Multiple roles of haem in cystathionine β-synthase activity: implications for hemin and other therapies of acute hepatic porphyria

The role of haem in the activity of cystathionine β-synthase (CBS) is reviewed and a hypothesis postulating multiple effects of haem on enzyme activity under conditions of haem excess or deficiency is proposed, with implications for some therapies of acute hepatic porphyrias. CBS utilises both haem and pyridoxal 5′-phosphate (PLP) as cofactors. Although haem does not participate directly in the catalytic process, it is vital for PLP binding to the enzyme and potentially also for CBS stability. Haem deficiency can therefore undermine CBS activity by impairing PLP binding and facilitating CBS degradation. Excess haem can also impair CBS activity by inhibiting it via CO resulting from haem induction of haem oxygenase 1 (HO 1), and by induction of a functional vitamin B₆ deficiency following activation of hepatic tryptophan 2,3-dioxygenase (TDO) and subsequent utilisation of PLP by enhanced kynurenine aminotransferase (KAT) and kynureninase (Kynase) activities. CBS inhibition results in accumulation of the cardiovascular risk factor homocysteine (Hcy) and evidence is emerging for plasma Hcy elevation in patients with acute hepatic porphyrias. Decreased CBS activity may also induce a proinflammatory state, inhibit expression of haem oxygenase and activate the extrahepatic kynurenine pathway (KP) thereby further contributing to the Hcy elevation. The hypothesis predicts likely changes in CBS activity and plasma Hcy levels in untreated hepatic porphyria patients and in those receiving hemin or certain gene-based therapies. In the present review, these aspects are discussed, means of testing the hypothesis in preclinical experimental settings and porphyric patients are suggested and potential nutritional and other therapies are proposed.

Hypothesis: Metabolic targeting of 5-aminolevulinate synthase by tryptophan and inhibitors of heme utilisation by tryptophan 2,3-dioxygenase as potential therapies of acute hepatic porphyrias

Metabolic targeting of liver 5-aminolevulinate synthase (5-ALAS) by inhibition of heme utilisation by tryptophan (Trp) 2,3-dioxygenase (TDO) or the use of tryptophan is proposed as a therapy of acute hepatic porphyrias. 5-ALAS, the rate-limiting enzyme of heme biosynthesis, is under negative feedback control by a small regulatory heme pool in the hepatic cytosol. Acute porphyric attacks, precipitated by fasting, certain hormones and some drugs, involve induction of 5-ALAS secondarily to depletion of the above pool, and the resultant elevation of 5-ALA levels initiates the abdominal and neurological symptoms of attacks. By utilising the regulatory heme, cytosolic TDO undermines the feedback control, thus allowing 5-ALAS induction to occur, e.g. upon glucocorticoid induction of TDO during fasting (starvation) and exogenous glucocorticoid administration. Currently, glucose therapy is the preferred strategy for reversing moderate attacks induced by fasting (calorie restriction), with more severe attacks being treated by intravenous heme preparations. Reversal of fasting-induced attacks by glucose is explained by the previously demonstrated reversal of increased heme utilisation by TDO. Inhibitors of this utilisation are therefore potential therapeutic targets in acute attacks and also for maintenance of a symptomless state. Existing TDO inhibitors other than glucose include allopurinol, nicotinamide and recently developed potent inhibitors such as LM10 used in cancer therapy. Based on studies in rats, the hypothesis predicts that the safety or otherwise of drugs in the hepatic porphyrias is determined by their ability to inhibit TDO utilisation of heme under basal conditions or after glucocorticoid induction or heme activation of TDO, in parallel with reciprocal changes in 5-ALAS induction. Tryptophan is also proposed as a potential therapy of acute attacks either alone or as an adjunct to the recently proposed 5-ALAS1 gene silencing. Trp increases heme biosynthesis by enhancing 5-ALA dehydratase activity and, based on a Trp-5-ALA model presented herein, Trp offers several advantages over heme therapy, namely rapid conversion of 5-ALA into heme, a greatly enhanced heme availability, a near complete inhibition of 5-ALAS induction, assumed rapid clearance of 5-ALA and hence accelerated resolution of symptoms of attacks, and finally provision of the neuroprotective metabolite kynurenic acid to neutralise the neurological symptoms. The hypothesis also addresses heme regulation in species lacking the TDO free apoenzyme and its glucocorticoid induction mechanism and proposes detailed assessment of heme biosynthesis in these species. Detailed proposals for testing the hypothesis are presented.

Substrate-dependent irreversible inactivation of cytochrome P-450: conversion of its haem moiety into modified porphyrins

2-Allyl-2-isopropylacetamide and other drugs containing either an allyl, a vinyl or an ethynyl unsaturated side chain are metabolized by liver cytochrome P-450 to reactive derivatives that irreversibly inhibit the haemoprotein by a suicidal type of inactivation. The main target is the haem moiety of cytochrome P-450 which is converted into abnormal porphyrins. These have been isolated from the liver of treated rats, extensively purified and compared with model porphyrins. The abnormal porphyrins incorporate metal ions in vitro much more readily than does their parent porphyrin, protoporphyrin. They are also much more basic than protoporphyrin, and on titration with a strong acid they readily give rise to a porphyrin monocation which then requires relatively large amounts of acid for conversion to the porphyrin dication. In all these respects and also in the intensity of their bathochromic shifts these abnormal porphyrins closely resemble N-alkylated porphyrins and they markedly differ from porphyrins that are substituted at one of their meso-carbon positions or which bear electron-withdrawing substituents at the beta-positions of the pyrrole rings. This suggests strongly that reactive derivatives of the unsaturated drugs act as electrophilic reagents and alkylate one of the pyrrole nitrogen atoms of cytochrome P-450. A model centred on the apoprotein of cytochrome P-450 is considered for the degradation of liver haem caused by unsaturated drugs. The apocytochrome may accept exchangeable pools of liver haem for degradation, leading to a state of haem depletion and to activation of delta-aminolaevulinate synthase.

The effect of age on the enzyme activities of tryptophan metabolism along the kynurenine pathway in rats

BACKGROUND

Quinolinic acid and other kynurenine metabolites of the oxidative metabolism of tryptophan play an important role in several pathophysiological conditions. We aimed to study the effect of age on the enzyme activities of tryptophan metabolism along the kynurenine pathway.

METHODS

Enzyme activity was investigated in liver, kidneys and small intestine obtained from Sprague-Dawley rats of various ages (1 week, 2-3, 12 and 18 months).

RESULTS

We found age-related differences in the liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase activities, which decreased significantly with age. Also liver kynureninase activity declined with age, while the activity in kidneys did not show an evident age-related pattern from 2-3 months to 18 months of age. Liver kynurenine oxoglutarate transaminase was quite similar through all considered age groups, while the activity in kidneys was significantly lower in newborn rats and progressively increased up to 12 months, then significantly decreased at 18 months of age. Liver and kidney 3-hydroxyanthranilate 3,4-dioxygenase progressively and significantly increased from newborns to 12 months of age; in the group of rats aged 18 months, the enzyme activity tended to diminish, although not significantly. The liver aminocarboxymuconate-semialdehyde decarboxylase activity increased up to 12 months of age, then tended to decrease at 18 months, while in the kidneys, in which the activity was higher than in the liver at all the considered ages, the activity remained constantly elevated from 2-3 months to 18 months of age.

CONCLUSIONS

A progressive decline in the enzyme activities involved in tryptophan metabolism along the kynurenine pathway in rat tissues was found with age, except for aminocarboxymuconate-semialdehyde decarboxylase, which, on the contrary, was increased after 2-3 months to the other older groups of age. The altered metabolism of tryptophan with ageing can lead to a decreased biosynthesis of nicotinic acid, tryptophan being the major source of body stores of NAD coenzymes, which are involved in almost all biogenetic and biosynthetic pathways of the organism.

Tryptophan Metabolism Along the Kynurenine Pathway in Rats

Enzyme activities along the kynurenine pathway, liver tryptophan 2,3-dioxygenase, small intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase, involved in the catabolism of tryptophan, were studied in male adult Wistar albino rats. Intestine superoxide dismutase and serum tryptophan were also determined. Hepatic tryptophan 2,3-dioxygenase is present both as holoenzyme and apoenzyme, but the total activity is inferior to that of intestine indole 2,3-dioxygenase which, therefore, actively oxidizes tryptophan in rats. However, this activity is inhibited by scavengers for the superoxide anion, such as superoxide dismutase, which also shows to be active in small intestine of rat. However, the more active enzymes appeared to be kynurenine 3-monooxygenase and 3-hydroxyanthranilate 3,4-dioxygenase. The former is equally active in both liver and kidney, the latter is more active in liver. Kynurenine-oxoglutarate transaminase is much more active in kidney than in liver, and much more active than kynureninase, which shows similar activities in both tissues. In contrast to the high activity of 3-hydroxyanthranilate 3,4-dioxygenase, aminocarboxymuconate-semialdehyde decarboxylase is 30-35 times less active, showing the efficiency of conversion of tryptophan to NAD. These data demonstrate that rat is a useful animal model for studying tryptophan metabolism along the kynurenine pathway. Serum tryptophan appeared to be 90% bound to proteins. Results demonstrate that, in rat, tryptophan is mainly metabolised along the kynurenine pathway. Therefore, rat is a suitable animal model for studying tryptophan metabolism in the pathological field.

Enzyme activities involved in tryptophan metabolism along the kynurenine pathway in rabbits

The following enzyme activities of the tryptophan-nicotinic acid pathway were studied in male New Zealand rabbits: liver tryptophan 2,3-dioxygenase, intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase. Intestine superoxide dismutase and serum tryptophan were also determined. Liver tryptophan 2,3-dioxygenase exists only as holoenzyme, but intestine indole 2,3-dioxygenase is very active and can be considered the key enzyme which determines how much tryptophan enters the kynurenine pathway also under physiological conditions. The elevated activity of indole 2,3-dioxygenase in the rabbit intestine could be related to the low activity of superoxide dismutase found in intestine. Kynurenine 3-monooxygenase appeared more active than kynurenine-oxoglutarate transaminase and kynureninase, suggesting that perhaps a major portion of kynurenine available from tryptophan may be metabolized to give 3-hydroxyanthranilic acid, the precursor of nicotinic acid. In fact, 3-hydroxyanthranilate 3,4-dioxygenase is much more active than the other previous enzymes of the kynurenine pathway. In the rabbit liver 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase show similar activities, but in the kidney 3-hydroxyanthranilate 3,4-dioxygenase activity is almost double. These data suggest that in rabbit tryptophan is mainly metabolized along the kynurenine pathway. Therefore, the rabbit can also be a suitable model for studying tryptophan metabolism in pathological conditions.

Effects of the haem precursor 5-aminolaevulinate on tryptophan metabolism and disposition in the rat

5-Aminolaevulinate administration to rats inhibits cerebral 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to activation of hepatic tryptophan pyrrolase. The results show that tryptophan metabolism and disposition can be influenced by changes in liver haem concentration, and are discussed briefly in relation to mood disorders in the hepatic porphyrias.

Hemin-mediated oxidation of dithiothreitol reduces oxygen to H2O

Hemin catalyses the oxidation of dithiothreitol. One mole of oxygen is consumed for every 2 moles of dithiothreitol oxidized and the product is shown by spectral studies to be the intramolecular disulphide. The reaction shows a specificity for dithiol and for free heme moieties. Hemin molecules exhibit cooperativity in oxygen reduction. Oxygen radicals do not seem to be involved. H2O2 is not required for this oxidation of dithiothreitol and does not appear to be an intermediate in the reduction of O2 to H2O. However, an independent minor reaction involving a 2-electron transfer with the formation of H2O2 also occurs. These studies on the hemin-catalyzed oxidation of dithiothreitol provide a chemical model for a direct 4-electron reduction of O2 to H2O.

Stimulation of liver tryptophan pyrrolase during heat exposure

Exposure of rats to heat (39 +/- 1 degree C) stimulated liver tryptophan pyrrolase 2-fold between 3 and 48 h. Plasma corticosterone increased 2-fold after 1 h of heat exposure and decreased to a low value of 50% by 16 h. The effect of heat exposure on the enzyme was obtained in adrenalectomized animals. Stimulation by cortisol and tryptophan of the enzyme was also obtained in heat exposure, and the effects seemed to be additive. The concentration of tryptophan in the liver remained unchanged, and that in the plasma decreased to about 50% at 8 h exposure to heat and reverted to normal by 46 h. Simultaneous administration of noradrenaline to heat-exposed rats had no effect, whereas that of thyroxine partly prevented the stimulation of the enzyme activity. Hypothyroid conditions obtained by thyroidectomy or treatment with propylthiouracil significantly stimulated the enzyme activity. Cycloheximide treatment of heat-exposed rats did not prevent the stimulation of the enzyme activity. The results indicate that the effect of heat exposure on liver tryptophan pyrrolase is obtained, due to the accompanying hypothyroid condition, by increasing the activity of the existing protein by a mechanism possibly different from those known at present.

Inhibition of delta-aminolevulinate dehydratase in trichloroethylene-exposed rats, and the effects on heme regulation

A pronounced and irreversible depression of the erythroid and liver delta-aminolevulinate dehydratase (porphobilinogen synthase; 5-aminolevulinate hydro-lyase, EC 4.2.1.24) activity was observed in rats exposed to trichloroethylene, a widely used solvent. The depression could not be restored after the treatment with dithiothreitol and zinc; however, radioimmunoassay of delta-aminolevulinate dehydratase indicated that trichloroethylene exposure did not essentially decrease the amount of enzyme. The depression of the enzyme activity thus proved to be due not to a reduction in the enzyme amount but to enzyme inhibition. The purified holoenzyme (fully activated delta-aminolevulinate dehydratase with 1 atom zinc per subunit) and apoenzyme (fully activated enzyme with the remaining zinc less than 0.1 atom per subunit) were prepared to investigate the in vitro inhibition of the enzyme by trichloroethylene. Incubation with trichloroethylene did not inhibit the holoenzyme, but inhibited the apoenzyme dose-dependently. Trichloroethylene inhibited the holoenzyme when incubated with the mixed function oxidase system. The in vitro experiments reported here indicate two mechanisms of the enzyme inhibition by trichloroethylene. In the liver of rats exposed to trichloroethylene, cytochrome P-450 concentration and heme saturation of tryptophan pyrrolase (EC 1.13.11.11) are reduced; in addition, the activity of delta-aminolevulinate synthase (EC 2.3.1.37) increased. After exposure to trichloroethylene at 2.14 g/m3, urinary delta-aminolevulinic acid increased to 142% of the control, while the excretion of coproporphyrin was reduced to 19.6% of the control.

Activation of liver tryptophan oxygenase by hydrocortisone, hematin and tryptophan in streptozotocin-diabetic rats

This study compared changes in liver tryptophan oxygenase (TPO) activity in response to hydrocortisone, hematin and tryptophan administration to non-diabetic and diabetic (streptozotocin) rats. Hydrocortisone caused similar increases in apoenzyme (inactive), holoenzyme (heme-saturated) and total (holoenzyme + apoenzyme) TPO activities in non-diabetic and diabetic rats. The ability of hematin to increase total TPO activity was significantly less in diabetic rats. The largest differences between diabetic and non-diabetic rats were found with tryptophan which increased total TPO and holoenzyme activities 300% and 650% respectively in non-diabetic rats. However, tryptophan increased both apoenzyme (unchanged in non-diabetic rats) and holoenzyme activities by 300% in diabetic rats. These results indicate that in the diabetic state, the TPO-heme conjugation process is impaired, especially substrate mediated TPO-heme saturation.

Tryptophan and tryptophan pyrrolase in haem regulation. The role of lipolysis and direct displacement of serum-protein-bound tryptophan in the opposite effects of administration of endotoxin, morphine, palmitate, salicylate and theophylline on rat liver 5-aminolaevulinate synthase activity and the haem saturation of tryptophan pyrrolase

1. The increase in the haem saturation of rat liver tryptophan pyrrolase caused by tryptophan administration was previously shown to be associated with a decrease in 5-aminolaevulinate synthase activity. 2. It is now shown that similar reciprocal effects are caused by palmitate and salicylate, both of which increase tryptophan availability to the liver by direct displacement of the serum-protein-bound amino acid. 3. The reciprocal effects on the former two parameters caused by endotoxin and morphine are associated with an increase in liver tryptophan concentration produced by a lipolysis-dependent, non-esterified fatty acid-mediated, displacement of the serum-protein-bound amino acid. 4. All these changes and those caused by another lipolytic agent, theophylline, are prevented by the beta-adrenoceptor-blocking agent propranolol and by the opiate-receptor antagonist naloxone, whose anti-lipolytic nature is demonstrated. 5. High correlation coefficients have been obtained for one or more pairs of the following parameters: serum non-esterified fatty acid concentration, free serum tryptophan concentration, liver tryptophan concentration, liver 5-aminolaevulinate synthase activity, liver holo-(tryptophan pyrrolase) activity and the haem saturation of liver tryptophan pyrrolase. 6. It is suggested that liver tryptophan concentration may play an important role in the regulation of 5-aminolaevulinate synthase synthesis, and that the latter may be subject to control by changes in lipid metabolism and may be influenced by pharmacological agents that affect tryptophan disposition. 7. Preliminary evidence suggests that tryptophan may be bound in the liver and that such a possible binding may control its availability for its hepatic functions.

Tryptophan pyrrolase in haem regulation. The mechanisms of enhancement of rat liver 5-aminolaevulinate synthase activity by starvation and of the glucose effect on induction of the enzyme by 2-allyl-2-isopropylacetamide

1. Rat liver tryptophan pyrrolase activity is enhanced by a hormonal-type mechanism during the first 2 days of starvation and by a substrate-type mechanism during the subsequent 2 days. 5-Aminolaevulinate synthase activity is also enhanced during the first 2 days of starvation, but returns thereafter to values resembling those observed in the fed rat. Treatments that prevent or reversé the enhancement of tryptophan pyrrolase activity in 24-48h-starved rats also abolish that of 5-aminolaevulinate synthase activity. Starvation of guinea pigs, which does not enhance the pyrrolase activity, also fails to alter that of the synthase. It is suggested that the decrease in 5-aminolaevulinate synthase activity in 72-96h-starved rats represents negative-feedback repression of synthesis, possibly involving tryptophan participation, whereas the enhancement observed in 24-48h-starved animals is caused by positive-feedback induction secondarily to increased utilization of the regulatory-haem pool by the newly synthesized apo-(tryptophan pyrrolase). 2. Glucose, fructose and sucrose abolish the 24h-starvation-induced increases in rat liver tryptophan pyrrolase and 5-aminolaevulinate synthase activities. Cortisol reverses the glucose effect on 5-aminolaevulinate synthase activity, presumably by enabling pyrrolase to re-utilize the regulatory-haem pool after induction of synthesis of this latter enzyme. 3. The impaired ability of 2-allyl-2-isopropylacetamide to enhance markedly 5-aminolaevulinate synthase activity in 24h-starved rats treated with glucose is associated with a failure of the porphyrogen to cause loss of tryptophan pyrrolase haem. Cortisol restores the ability of the porphyrogen to destroy tryptophan pyrrolase haem and to enhance markedly 5-aminolaevulinate synthase activity, presumably by enhancing tryptophan pyrrolase synthesis and, thereby, its re-utilization of the regulatory-haem pool. It is tentatively suggested that 2-allyl-2-isopropylacetamide destroys the above pool only after it has become bound to (or utilized by) apo-(tryptophan pyrrolase).

Possible involvement of the enhanced tryptophan pyrrolase activity in the corticosterone- and starvation-induced increases in concentrations of nicotinamide-adenine dinucleotides (phosphates) in rat liver

1. Deoxycorticosterone, which does not enhance tryptophan pyrrolase activity, also fails to alter the concentrations of the NAD(P) couples in livers of fed rats, whereas corticosterone increases both pyrrolase activity and dinucleotide concentrations. 2. Starvation of rats increases serum corticosterone concentration, lipolysis, tryptophan availability to the liver, tryptophan pyrrolase activity and liver [NADP(H)]. Glucose prevents all these changes. 3. The beta-adrenoceptor-blocking agent propranolol prevents the starvation-induced lipolysis and the consequent increase in tryptophan availability to the liver, but does not influence the increase in serum corticosterone concentration, liver pyrrolase activity and [NADP(H)]. 4. Actinomycin D, which prevents the starvation-induced increases in liver pyrrolase activity and [NADP(H)], does not affect those in serum corticosterone concentration and tryptophan availability to the liver. 5. Allopurinol, which blocks the starvation-induced enhancement of pyrrolase activity, also abolishes the increases in liver [NADP(H)], but not those in serum corticosterone concentration or tryptophan availability to the liver. 6. It is suggested that liver tryptophan pyrrolase activity plays an important role in NAD+ synthesis from tryptophan in the rat.

Tryptophan pyrrolase in haem regulation. The mechanism of the permissive effect of cortisol on the enhancement of 5-aminolaevulinate synthase activity by 2-allyl-2-isopropylacetamide in the adrenalectomized-rat liver

The decreased ability of 2-allyl-2-isopropropylacetamide to enhance liver 5-amino-laevulinate synthase activity in the adrenalectomized rat is not associated with a marked depletion of the already low amount of tryptophan pyrrolase haem. Cortisol permits the porphyrogen markedly to enhance synthase activity by rendering it capable of causing a stronger depletion of pyrrolase haem, presumably as result of hormonal induction of pyrrolase synthesis.

Tryptophan pyrrolase in haem regulation. The relationship between the depletion of rat liver tryptophan pyrrolase haem and the enhancement of 5-aminolaevulinate synthase activity by 2-allyl-2-isopropylacetamide

Rat liver tryptophan pyrrolase haem is maximally depleted at 30 min after administration of a 400 mg/kg dose of 2-allyl-2-isopropylacetamide. This depletion lasts for 24 h, by which time 5-aminoleevulinate synthase activity becomes maximally enhanced. 2. though the above maximum depletion of pyrrolase haem (at 0.5h) is also produced by a 100 mg/kg dose of the porphyrogen, this does not enhance synthase activity at 24 h. It and smaller doses, however, cause a smaller but earlier enhancement of synthase activity (maximum at 2 h) and produce a similarly short-lived deplation of pyrrolase haem. 3. The depletion of pyrrolase haem and the enhancement of synthase activity by the porphyrogen are inhibited by compound SKF 525-A and phenazine methosulphate, and are potentiated by nicotinamide but not by phenobarbitone. Phenazine methosulphate and nicotinamide also exert opposite effects on hexobarbital sleeping-time. 4. 2-Allyl-2-isopropylacetamde also the depletes pyrrolase haem in vitro. It does so in liver homogenates of control rats in the presence, and in those of phenobarbitone-treated rats in the absence of added NADPH. 5. A discussion of the present results in relation to previous work with other haemoproteins suggests that, whereas cytochrome P-450 (haem) is primarily involved in the production of the active (porphyrogenic) metabolite(s) of 2-allyl-2-isopropylacetamide, the haem pool used by tryptophan pyrrolase may play an important role in the effects of this compound on haem biosynthesis.

Tryptophan pyrrolase, the regulatory free haem and hepatic porphyrias. Early depletion of haem by clinical and experimental exacerbators of porphyria

1. The importance of the early depletion of liver haem in the production of porphyria is discussed and further supporting evidence is presented from experiments with tryptophan pyrrolase, under conditions of exacerbation of experimental porphyria by therapeutic and other agents. 2. In addition to the early depletion of pyrrolase haem by porphyrogens, a further depletion is produced when rats are given a porphyrogen plus an analogue or one of 19 drugs known to exacerbate the human disease. 3. Non-exacerbators of human porphyrias do not cause a further early depletion of pyrrolase haem and it is suggested that this system may be used as a screening test for possible exacerbation of the disease by new and existing drugs. 4. A similar further early depletion of haem is produced by combined administration of lead acetate plus phenobarbitone, thus suggesting that the depletion is a more general phenomenon in experimental porphyria. 5. The relationship between tryptophan pyrrolase and the regulatory free haem is discussed. It is suggested that pyrrolase may play an important role in the regulation of haem biosynthesis.

The effects of acetate, metal cations, phenobarbitone, porphyrogens and substrates of glycine acyltransferase on the utilization of haem by rat liver apo-(tryptophan pyrrolase)

1. The utilization of haem by rat liver apo-(tryptophan pyrrolase) under basal conditions and after enhancement of the enzyme activity by various mechanisms was studied under the influence of treatments affecting various aspects of liver haem metabolism. 2. These treatments were: benzoate and p-aminobenzoate as substrates of glycine acyltransferase, acetate as an inhibitor of 5-aminolaevulinate synthase activity, enhancement of 5-aminolaevulinate dehydratase by aluminium, destruction of haem and inhibition of ferrochelatase by porphyrogens, increased haem utilization by phenobarbitone and enhancement of haem oxygenase activity by metal cations. 3. The results show that the haem saturation of the apoenzyme is sensitive to all these treatments. 4. The possible usefulness of tryptophan pyrrolase in studying the regulation of liver haem is suggested.

The regulation of rat liver tryptophan pyrrolase activity by reduced nicotinamide-adenine dinucleotide (phosphate). Experiments with glucose and nicotinamide

1. Chronic administration of glucose or nicotinamide in drinking water inhibits the activity of rat liver tryptophan pyrrolase, and subsequent withdrawal causes an enhancement. The enzyme activity is also inhibited by administration in drinking water of sucrose, but not fructose, which is capable of preventing the glucose effect. 2. The inhibition by glucose or nictinamide is not due to a defective apoenzyme synthesis nor a decreased cofactor availability. 3. The inhibition by nicotinamide is reversed by regeneration of liver NAD+ and NADP+ in vivo by administration of fructose, pyruvate or phenazine methosulphate. Inhibition by glucose is also reversed by the above agents and by NH4Cl. Reversal of inhibition by glucose or nicotinamide is also achieved in vitro by addition of NAD+ or NADP+. 4. Glucose or nicotinamide increases liver [NADPH]. [NADP+] is also increased by nicotinamide. [NADPH] is also increased by sucrose, but not by fructose, which prevents the glucose effect. Phenazine methosulphate prevents the increase in [NADPH] caused by both glucose and nicotinamide. 5. It is suggested that the inhibition of tryptophan pyrrolase activity by glucose or nicotinamide is mediated by both NADPH and NADH.

Regulation of rat liver tryptophan pyrrolase by its cofactor haem: Experiments with haematin and 5-aminolaevulinate and comparison with the substrate and hormonal mechanisms

1. The administration of haematin or 5-aminolaevulinate to rat enhances the activity of liver tryptophan pyrrolase; both endogenous and newly formed apoenzymes become strongly haem-saturated. Haem activation does not stabilize tryptophan pyrrolase. 2. Actinomycin D, puromycin or cycloheximide prevent the activation of the enzyme by 5-aminolaevulinate but not that by haematin. The latter is inhibited by haem-destroying porphyrogens. 3. The combined injection of either haematin or 5-aminolaevulinate with cortisol does not produce an additive effect, whereas potentation is observed when tryptophan is jointly given with either the cofactor or the haem precursor. 4. Further experiments on the substrate (tryptophan) mechanism of pyrrolase regulation are reported, and a comparison between this and the cofactor and hormonal mechanisms is made. 5. It is suggested that the substrate mechanism may also involve increased haem synthesis. 6. The role of tryptophan pyrrolase in the utilization of liver haem, and as a possible model for the exacerbation by drugs of human hepatic porphyrias, is discussed.

Stimulation of the pathway of porphyrin synthesis in the liver of rats and mice by griseofulvin, 3,5-Diethoxycarbonyl-1,4-dihydrocollidine and related drugs: evidence for two basically different mechanisms

Griseofulvin and isogriseofulvin cause, like 3,5-diethoxycarbonyl-1,4-dihydrocollidine, a fall in the activity of the hepatic enzyme porphyrin-metal chelatase and accumulation of protoporphyrin in the liver. Analogues of either griseofulvin or 3,5-diethoxycarbonyl-1,4-dihydrocollidine which do not decrease the chelatase activity are not porphyrogenic on their own, but can potentiate the porphyria caused by 3,5-diethoxycarbonyl-1,4-dihydrocollidine. This suggests the existence of two basically different mechanisms by which drugs stimulate the pathway of porphyrin synthesis in the liver.