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.

Decreased liver activity of porphyrin-metal chelatase in hepatic porphyria caused by 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Studies in rats and mice

1. A difference has been found between rats and mice in their sensitivity to the porphyrogenic effect of drugs. Mice are more sensitive than rats to 3,5-diethoxycarbonyl-1,4-dihydrocollidine, but less sensitive than rats to 2-allyl-2-isopropylacetamide. 2. Use has been made of this difference in sensitivity to ascertain the importance of the decrease of liver porphyrin-metal chelatase activity in porphyria caused by 3,5-diethoxycarbonyl-1,4-dihydrocollidine. Mice, which are more sensitive than rats to the stimulation of 5-aminolaevulinate caused by this drug, are also more sensitive with respect to the decrease of chelatase activity. 3. In both species, after treatment with 3,5-diethoxycarbonyl-1,4-dihydrocollidine, the ratio between chelatase activity and 5-aminolaevulinate activity is linear with respect to the reciprocal of the liver porphyrin concentration. This suggests that under these conditions the degree of porphyrin accumulation depends on the balance between rate of porphyrin formation and rate of porphyrin utilization. 4. Compound SKF 525-A (2-diethylaminoethyl 3,3-diphenylpropylacetate) when given before 3,5-diethoxycarbonyl-1,4-dihydrocollidine prevents the appearance of porphyria in the rat and also largely prevents the decrease of chelatase activity. In the mouse it is much less effective in preventing porphyria and it is almost completely inactive in protecting the chelatase from a decrease in activity. 5. Cycloheximide, when given before 3,5-diethoxycarbonyl-1,4-dihydrocollidine also inhibits the induction of 5-aminolaevulinate synthetase and the appearance of porphyria in the rat, but does not prevent the decrease of chelatase activity. These results suggest that two successive stages can be distinguished in the induction process: a first stage leading to inhibition of haem synthesis and a second stage requiring synthesis of protein in the liver and leading to stimulation of 5-aminolaevulinate synthetase.

Selective inhibition of hemoglobin formation in chick embryo blood islands by chloramphenicol

Chloramphenicol markedly inhibited the formation of hemoglobin by chick blastodiscs explanted before the 8-somite stage of development onto minimal or egg homogenate medium. Sensitivity to inhibition of hemoglobin formation was reduced at the 8-somite stage. Chloramphenicol had no effect upon the incorporation of leucine into total protein, and only slightly inhibited the cycloheximide-resistant incorporation of leucine.

Regulation of onset of rapid hemoglobin formation in chick blastodisc blood islands at the transcription and translation levels. A quantitative analysis of effects of actinomycin, ethanol, and heme precursors

Amounts of hemoglobin formed by blastodiscs explanted for 24 h on minimal medium containing 2 μg/ml of actinomycin were independent of initial stage of development between the 2- and 6-somite stages. No additional hemoglobin was formed by 2- to 4-somite blastodiscs in response to further supplements of aminolevulinate. In contrast, levels of hemoglobin formed in the presence of actinomycin by both younger and older blastodiscs were dependent on initial stage of development, and also were enhanced by aminolevulinate. Protoporphyrin both stimulated formation of hemoglobin on minimal medium to levels comparable with those formed on egg homogenate medium, and prevented inhibition by actinomycin of the synthesis of macromolecules. Ethanol in concentrations up to 1% had no effect on levels of hemoglobin formed or incorporation of precursors into macromolecules.On egg homogenate medium low concentrations of ethanol markedly stimulated hemoglobin formation, probably in part through induced synthesis of aminolevulinate synthetase enzyme. High concentrations of ethanol also inhibited incorporation of leucine into polypeptide. Incorporation of uridine into polynucleotide was only partially inhibited on egg homogenate medium containing 2 μg/ml of actinomycin. Hemoglobin formation was markedly less inhibited than on minimal medium and was stimulated by both aminolevulinate and protoporphyrin.

The effect of exogenous -aminolaevulinate on rat liver haem and cytochromes

The activity of delta-aminolaevulinate synthetase is generally regarded as rate-limiting for hepatic haem biosynthesis. It has been suggested that cytochrome synthesis may also be regulated by changes in delta-aminolaevulinate synthetase activity. This hypothesis was studied by injecting product, delta-aminolaevulinate, into adult rats over a 4-240h period. The concentrations of hepatic mitochondrial cytochromes a, b, c and c(1) were unchanged by treatment with delta-aminolaevulinate, allylisopropylacetamide or phenobarbital. In control animals, total microsomal haem content equalled the sum of cytochromes b(5) plus P-450. After delta-aminolaevulinate administration the total amount of microsomal haem, measured as the pyridine haemochromogen, exceeded these components, indicating the formation of a ;free' haem pool. Haem synthesis does not appear rate-limiting for hepatic cytochrome synthesis in the adult rat.