Hepatic delta-aminolaevulinic acid synthetase in an attack of hereditary coproporphyria and during remission

Free radicals involvement in neurological porphyrias and lead poisoning

Porphyrias are inherited and acquired diseases of erythroid or hepatic origin, in which there are defects in specific enzymes of the heme biosynthetic pathway. In patients with intermittent acute porphyria and lead poisoning the erythrocytic activities of superoxide dismutase and glutathione peroxidase are reported to be increased. Our studies demonstrated that d-aminolevulinic acid, a heme precursor accumulated in both diseases, undergoes enolization at pH less than 7.0 before it autoxidizes. The autoxidation of d-aminolevulinic acid, in the presence or absence of oxyhemoglobin has been proposed as a source of oxy and carbon-centred radicals in the cells of intermittent acute porphyria and saturnism carriers. Thus, the increased levels of antioxidant enzymes can be viewed as an intracellular response against the deleterious effects of these extremely reactive species.

The porphyrias

The porphyrias are metabolic disorders in which there are excessive accumulation and excretion of porphyrins and porphyrin precursors. Each of the porphyrias has a specific enzyme defect in the pathway of heme biosynthesis that explains the pattern of biochemical abnormalities that occur. However, some patients have the enzyme defect but do not have clinical or biochemical manifestations, indicating that other factors (e.g., demand for increased heme biosynthesis) are also important in causing disease expression. The major clinical manifestations are neurologic dysfunction and photosensitivity. The precise cause of the neurologic dysfunction has not been defined, but the likely possibilities are overproduction of delta-aminolevulinic acid, which may act as a neurotoxin, or a deficiency of heme (or both). The cutaneous lesions in the porphyrias are due to the photo-sensitizing and other effects of porphyrins that are deposited in the skin or are circulating in dermal blood vessels. Therapy is directed to modify the biochemical abnormalities. Most importantly, intravenous administration of hematin is used in the treatment of acute attacks of neurologic dysfunction. Prevention remains a cornerstone in management of patients with porphyria, and those with gene defects should be counseled regarding factors that precipitate acute attacks.

Delta-Aminolaevulinic acid and amino acid neurotransmitters

The effects of the porphyrin precursor delta-aminolaevulinic acid (ALA) on gamma-aminobutyric acid (GABA) and L-glutamate transmitter systems was investigated in rat brain. It was found that ALA inhibited GABA and glutamate uptake and stimulated basal efflux of the amino acids in purified nerve endings. These effects were evident only at relatively high concentrations of ALA (at least 100 microM). Such concentrations probably do not occur in the nervous systems of patients suffering from acute porphyria. In addition, it was found that ALA inhibited the stimulated release of GABA from nerve endings probably by acting as an agonist at GABA autoreceptors. This effect was found at very low concentrations of ALA (1 microM). It is therefore likely that the neuropsychiatric manifestations of the acute porphyric attack are attributable, to some extent, to reduced GABA release at central synapses.

Enzyme defects in the porphyrias and their relevance to the biochemical abnormalities in these disorders

Defects in enzymes of the heme biosynthesis pathway underlie the biochemical abnormalities which occur in the porphyrias. Porphyrins and porphyrin precursors are accumulated and excreted in excessive amounts because of the enzyme defects. This is illustrated by studies in protoporphyria and variegate porphyria, disorders in which the biochemical abnormalities indicate a defect(s) in the terminal part of the heme biosynthesis pathway. The activity of heme synthease (ferrochelatase), which catalyzes the chelation of ferrous iron to protoporphyrin, is deficient in tissues of patients with protoporphyria. This causes protoporphyrin to be accumulated and excreted excessively. In variegate porphyria protoporphyrinogen oxidase, which catalyzes the oxidation of protoporphyrinogen to protoporphyrin, appears to be defective. As a result, protoporphyrinogen may be excreted in increased amounts in bile, where it is subsequently auto-oxidized to protoporphyrin. The following questions have arisen as a result of the demonstrations of enzyme defects in tissues of patients with porphyria: (1) Will different defects in the same enzyme be found among patients who fulfill the clinical and biochemical criteria for diagnosis of a specific porphyria? That is, does genetic heterogeneity exist in each of the porphyrias? (2) Why do some patients with an enzyme defect not have biochemical abnormalities? (3) Why is one type of tissue, usually the liver, the major site of expression of the biochemical abnormality, when the enzyme defect can be demonstrated in all tissues?

Enzyme abnormalities in the porphyrias

Evidence is presented that each of the porphyrias represents a different inborn error of metabolism in haem biosynthesis. Control of the pathway takes place by feedback repression and inhibition by haem of delta-aminolaevulinic-acid synthase. It is suggested that insituations where the activity of this enzyme is derepressed, prophobilinogen deaminase represents a secondary control step.

Hereditary coproporphyria and epilepsy

A 9-year-old boy with mental deterioration and epilepsy suffered an acute attack of hereditary coproporphyria associated with worsening of seizure control. Leucocyte coproporphyrinogen oxidase activity was undetectable in the patient during this attack, and was reduced in his mother, a latent case. The complex relationship between porphyria, epilepsy, and anticonvulsant drugs is discussed.

Reduced ferrochelatase activity: a defect common to porphyria variegata and protoporphyria

Erythroid ferrochelatase activity has been studied in the normoblasts of patients with porphyria variegata and protoporphyria. Two methods were used for the investigation: one using intact cells and the other lysed cells, each measuring the amount of haem synthesized by normoblasts. In patients with porphyria variegata, ferrochelatase activity estimated by both methods was approximately 50% of the normal, and in protoporphyria the ferrochelatase activity was normal in intact normoblasts but was 20% of the normal in sonicated normoblasts (marrow lysates). It is suggested therefore that in porphyria variegata a dominantly inherited structural gene mutation results in an active ferrochelatase whereas in protoporphyria the genetic mutation results in an unstable ferrochelatase. The mechanism of the enzyme instability is not known though a number of postulates are discussed.

Hereditary coproporphyria presenting with photosensitivity induced by the contraceptive pill

A 26-year-old woman developed photosensitivity and jaundice after starting on an oral contraceptive agent. She was found to have hereditary coproporphyria. Histological examination of two liver biopsies taken during and after the acute attack did not reveal any changes thought to be due to hereditary coproporphyria. Screening of the patient's family revealed a further six latent cases.

The primary enzyme defect in hereditary coproporphyria

The activity of coproporphyrinogen oxidase (E.C. 1.3.3.3) in cultured skin fibroblasts from three patients with hereditary coproporphyria (H.C.) was approximately half that in fibroblasts from normal subjects and patients with other types of porphyria. It is suggested that this is the primary defect in H.C., which is inherited as an autosomal dominant, and that the same abnormality is present in the liver. Consideration of the probable relative activities of the enzymes of haem biosynthesis in the liver in H.C. suggests that the acute attacks of porphyria which are its major clinical manifestation occur when the activity of uroporphyrinogen-I-synthase (E.C. 4.3.1.8) becomes rate-limiting for haem synthesis.

Heme synthetase deficiency in human protoporphyria. Demonstration of the defect in liver and cultured skin fibroblasts

The final step in heme biosynthesis is chelation of porphyrin with Fe++ catalyzed by the mitochondrial enzyme heme synthetase. We have employed a sensitive radiochemical assay for this enzyme, using 59Fe and deuteroporphyrin or protoporphyrin as substrates. In this method iron is maintained in the ferrous state, oxygen is excluded from the incubation system, and labeled heme product is extracted into ethyl acetate. This assay has been used to measure the activity of heme synthetase in homogenates of liver, obtained by needle biopsy, and in sonicates of human skin fibroblasts, cultured in vitro. In addition, activity of the first enzyme of the heme synthetic pathway, delta-aminolevulinic acid synthetase, has been measured in fibroblast lysates. Lysates of fibroblasts from eight patients with protoporphyria had activities of delta-aminolevulinic acid synthetase which did not differ significantly from those of eight normal fibroblast lines, whereas activity of heme synthetase, with either deuteroporphyrin or protoporphyrin as substrate, was markedly decreased in sonicates of skin fibroblasts from these patients, the mean being 8% of control with deuteroporphyrin and 14% with protoporphyrin as substrate. In homogenates of liver from five patients with protoporphyria, activity of heme synthetase was also significantly less than that found in six patients without prophyria, the mean being 13% of control with protoporphyrin as substrate. These results provide evidence that decreased activity of heme synthetase is the basic defect in the heme synthetic pathway in protoporphyria. This deficiency is probably responsible for protoporphyrin accumulation and hence the biochemical and clinical features observed in protoporphyria.

Decreased red cell uroporphyrinogen I synthetase activity in intermittent acute porphyria

Intermittent acute porphyria has recently been distinguished biochemically from other genetic hepatic porphyrias by the observation of diminished hepatic uroporphyrinogen I synthetase activity and increased delta-aminolevulinic acid synthetase activity. Since deficient uroporphyrinogen I synthetase may be reflected in nonhepatic tissues, we have assayed this enzyme in red cell hemolysates from nonporphyric subjects and from patients with genetic hepatic porphyria. Only patients with intermittent acute porphyria had decreased erythrocyte uroporphyrinogen I synthetase activity which was approximately 50% of normal. The apparent K(m) of partially purified uroporphyrinogen I synthetase was 6 x 10(-6)m in both nonporphyrics and patients with intermittent acute porphyria. These data provide further evidence for a primary mutation affecting uroporphyrinogen I synthetase in intermittent acute porphyria. Further-more, results of assay of red cell uroporphyrinogen I synthetase activity in a large family with intermittent acute porphyria suggest that this test may be a reliable indicator of the heterozygous state.