The relationship between delta-aminolaevulinate synthetase induction and the concentration of cytochrome P-450 and catalase in rat liver

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.

Effect of lead on hepatic delta-aminolaevulinic acid synthetase activity in the rat: a model for drug sensitivity in intermittent acute porphyria

The hereditary hepatic porphyrias are disorders of porphyrin and haem synthesis characterized by a marked idiosyncrasy towards a variety of lipid soluble drugs. Most of these agents are inducers of the haemoprotein cytochrome P450, the terminal oxidase in drug metabolism. The primary genetic defect in intermittent acute porphyria is a partial deficiency of uroporphyrinogen I synthetase, which may result in a secondary derepression of delta-aminoaevulinic acid synthetase, the rate-limiting enzyme in the haem pathway. Analogous defects at more distant sites may explain the other hereditary hepatic porphyrias. As drug sensitivity may be related to the defect in haem synthesis, we investigated the effects of experimental partial blocks in haem synthesis produced by lead in rats. Drug effects on delta-aminolaevulinic acid synthetase, cytochrome P450, And drug metabolism were studied. Our findings indicate: a) While partial impairment of haem biosynthesis has only minor effects on delta-aminolaevulinic acid synthetase activity, it greatly enhances the sensitivity of delta-aminolaevulinic acid synthetase to induction by drugs and steroids, which when given alone, have little or no inducing effect on the enzyme. b) The experimental partial block in haem synthesis delays and impairs drug-mediated induction cytochrome P450 and drug metabolism in vitro. The findings may explain why a large number of structurally unrelated compounds with little effect on normal liver can precipitate "aucte porphyria".

Experimental porphyria induced by 3-(2,4,6-trimethylphenyl)-thioethyl)-4 methylsydnone

Administration of 3-[2-(2,4,6-trimethylphenyl)-thioethyl]-4-methylsydnone (TTMS) induces hepatic porphyria in rats, mice and dogs. The protoporphyrin pigment in livers of rats and mice is found mainly in bile ducts and leads to bile duct proliferation and portal inflammation. Dog livers contain protoporphyrin predominantly in bile canaliculi. The birefringence of the pigment appears to be associated with bilamellar components within the pigment. The markedly depressed catalase activity in livers of rats does not increase after clofibrate administration. The catalase activity of mouse liver is depressed slightly and responds to clofibrate treatment.

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

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 effects of chronic phenobarbitone administration and subsequent withdrawal on the activity of rat liver tryptohan pyrrolase and their resemblance to those of ethanol

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.

Biochemical effects of the porphyrinogenic drug allylisopropylacetamide. A comparative study with phenobarbital

Successive administrations of allylisopropylacetamide, a potent porphyrinogenic drug, increase liver weight, microsomal protein and phospholipid contents. There is an increase in the rate of microsomal protein synthesis in vivo and in vitro. The drug decreases microsomal ribonuclease activity and increases NADPH-cytochrome c reductase activity. Phenobarbital, which has been reported to exhibit all these changes mentioned, is a weaker inducer of delta-aminolaevulinate synthetase and increases the rate of haem synthesis only after a considerable time-lag in fed female rats, when compared with the effects observed with allylisopropylacetamide. Again, phenobarbital does not share the property of allylisopropylacetamide in causing an initial decrease in cytochrome P-450 content. Haematin does not counteract most of the biochemical effects caused by allylisopropylacetamide, although it is quite effective in the case of phenobarbital. Haematin does not inhibit the uptake of [2-(14)C]allylisopropylacetamide by any of the liver subcellular fractions.

A model for the regulation of delta-aminolaevulinate synthetase induction in rat liver

A reciprocal relationship exists between the cytochrome P-450 content and delta-aminolaevulinate synthetase activity in adult rats. In young rats the basal delta-aminolaevulinate synthetase activity is higher and the cytochrome P-450 content is lower compared with the adult rat liver. Administration of allylisopropylacetamide neither induces the enzyme nor causes degradation of cytochrome P-450 in the young rat liver, unlike adult rat liver. Allylisopropylacetamide fails to induce delta-aminolaevulinate synthetase in adrenalectomized-ovariectomized animals or intact animals pretreated with successive doses of the drug, in the absence of cortisol. The cortisol-mediated induction of the enzyme is sensitive to actinomycin D. Allylisopropylacetamide administration degrades microsomal haem but not nuclear haem. Haem does not counteract the decrease in cytochrome P-450 content caused by allylisopropylacetamide administration, but there is evidence for the formation of drug-resistant protein-bound haem in liver microsomal material under these conditions. Phenobarbital induces delta-aminolaevulinate synthetase under conditions when there is no breakdown of cytochrome P-450. On the basis of these results and those already published, a model is proposed for the regulation of delta-aminolaevulinate synthetase induction in rat liver.