Activity of amino-laevulinic acid synthetase in normal and porphyric human livers

Alcohol and Indian porphyrics

The role of alcohol as the precipitating factor in the induction of acute attacks of acute intermittent porphyria was studied in an Indian population. Thirty-four teetotal patients with acute intermittent porphyria, in remission, were given 60 ml of 30% ethanol. Except for two patients, all had negative Watson-Schwartz tests prior to the alcohol. Within 24 hours, the Watson-Schwartz test became positive in 16 of these 32 patients (50%). In 8 out of the 34 patients (23.5%) a clinical attack was precipitated, including both patients who had a positive Watson-Schwartz test prior to the alcohol. It was concluded that alcohol does precipitate an acute attack in a significant percentage of patients of Indian origin with acute intermittent porphyria. Patients already excreting porphobilinogen are at a greater risk of developing an acute attack on alcohol ingestion. This study is the first from India and probably first of its kind to be reported from any country.

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.

The effect of dimethyl sulfoxide on heme synthesis and the acute phase reaction in human HepG2 hepatoma cells

Effects of DMSO on heme synthesis and enzymes of the heme biosynthetic pathway were examined in human HepG2 hepatoma cells. HepG2 cells contain measurable levels of ALA synthase and ALA dehydratase, and their levels are increased after treatment of cells with DMSO. DMSO treatment also led to increases in heme content and the synthesis of haptoglobin, while it decreased the synthesis of albumin and AFP. Changes in plasma protein synthesis after DMSO treatment are characteristic of those known to occur in the acute phase reaction. These findings suggest that profound changes in heme synthesis may occur during the acute phase reaction.

Excretion of porphyrins and porphyrin precursors during neuromuscular paralysis produced by dithiobiuret

Urinary excretion of porphyrin precursors delta-aminolevulinic acid (ALA) and porphobilinogen (PBG) and total porphyrins was measured during intoxication of rats with 2,4-dithiobiuret (DTB), a chemical which produces delayed-onset neuromuscular weakness, in an attempt to ascertain whether or not DTB poisoning in the rat would serve as an animal model of the neurologic symptoms of acute intermittent porphyria. Daily administration of DTB (1 mg/kg/day, i.p.) produced flaccid skeletal muscle weakness first detected after 4 to 5 days of treatment. Onset of skeletal muscle weakness was associated with a significant increase in urinary excretion of ALA. The excretion of PBG and total porphyrin was also increased; however, the increase was not significant. The increase in porphyrins and porphyrin precursors was due to increased urine output which coincided with the onset of neuromuscular weakness; urinary concentrations of ALA, PBG, and porphyrins were not increased by DTB. Measurements of free-erythrocyte protoporphyrin, taken after 7 days of DTB treatment, indicated a significant elevation of free erythrocyte protoporphyrin concentration. The pattern of alterations in the heme precursors associated with DTB-induced paralysis in rats is quite different from that observed in humans afflicted with acute intermittent porphyria. Therefore, we conclude that DTB-induced paralysis in the rat does not represent an accurate animal model of acute intermittent porphyria.

Treatment of the porphyrias: mechanisms of action

The porphyrias are diseases that result from inherited or acquired abnormalities of porphyrin-heme synthesis in the liver and the bone marrow. Only the hepatic porphyrias are known to be aggravated by exposure to a variety of exogenous drugs and chemicals. Simple avoidance of these agents will reduce the risk of developing hepatic porphyria and may lead to clinical improvement in patients with active disease. Some types of therapy of the hepatic porphyrias are effective because of their ability to modulate the activity of delta-aminolevulinic acid synthetase, the rate-limiting enzyme for heme synthesis. Most of the porphyrias are associated with cutaneous photosensitivity, the treatment of which centers about either reducing the excessive production of porphyrins or of inhibiting the photobiological response to these photosensitizing chemicals in the skin.

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?

Effects of antihypertensive drugs on hepatic heme biosynthesis, and evaluation of ferrochelatase inhibitors to simplify testing of drugs for heme pathway induction

Effects of a series of antihypertensive drugs on the activity of delta-aminolevulinate synthase and on the formation of porphyrins and cytochrome P-450 were examined in the 18-day-old chick embryo liver in ovo. Hydralazine, pargyline, phenoxybenzamine, clonidine, and spironolactone were found to induce delta-aminolevulinate synthase in this system. These drugs therefore have the potential to precipitate clinical expression in human hereditary hepatic porphyrias and should be avoided or used with caution in patients with these disorders. Differential effects of these and other drugs were observed in the avian liver, in that delta-aminolevulinate synthase was more commonly induced than were porphyrins and cytochrome P-450; the synthase was usually highest 6-12 h after injection, whereas porphyrins and cytochrome P-450 were highest at 24 h. Furthermore marked porphyrin accumulation was not seen with many drugs that induce delta-aminolevulinate synthase and cytochrome P-450 but was more characteristic of compounds that reduced the metabolism of protoporphyrin to heme, such as 1,4-dihydro-3,5-dicarbethoxycollidine (DDC) and high doese of hydralazine. A sensitive and convenient method to test for capacity to induce heme biosynthesis was adapted for use in the chick embryo liver. This employed a relatively small "priming" dose (0.25 mg) of DDC given with a drug being tested and a fluorometric assay of porphyrins in a liver homogenate obtained at 24 h. This simple method should facilitate screening for those drugs which induce the synthesis of delta-aminolevulinate synthase and/or cytochrome P-450 and are potentially dangerous to patients with hereditary hepatic porphyria.

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.

The spectrophotometric determination of uroporphyrinogen I synthetase activity

A simple spectrophotometric method for uroporphyrinogen I synthetase in erythrocytes is described. Results obtained on intermittent acute porphyria patients and carriers are similar to the results obtained with fluorimetric methods. Reproducibility, relationship between enzyme activity and enzyme concentration, and effect of time on enzymatic activity are described.

Comparative effects of glycerol and dextrose on porphyrin precursor excretion in acute intermittent porphyria

The effects of dietary manipulations on excretion of the porphyrin precursors, delta-aminolevulinic acid (ALA), and porphobilinogen (PBG) were studied in eight patients with acute intermittent porphyria. Three diet periods of 9-17 days comprised each study. In each patient, a "baseline" protein, fat, and carbohydrate intake was kept constant throughout. In addition, during the first diet period each patient received 150 g dextrose; during the second, this was replaced by an isocaloric amount of neutral fat; and during the third, the fat was replaced by 150 g glycerol. In each of the patients, three comparisons of the effect of diet on both ALA and PBG excretion were made: (1) 300 g carbohydrate versus 150 g carbohydrate (dextrose versus fat), (2) 150 g carbohydrate + 150 g glycerol versus 150 g carbohydrate (glycerol versus fat), and (3) 300 g carbohydrate versus 150 g carbohydrate + 150 g glycerol (dextrose versus glycerol). For each of these three diet comparisons, there are sixteen individual comparisons possible for the effect of diet on porphyrin precursor excretion, eight for ALA and eight for PBG. Thus, the mean values for ALA and PBG excretions during each of the diet periods are statistically compared internally within each individual patient. Increasing carbohydrate intake from 150 g to 300 g by isocaloric substitution of dextrose for fat produced a significant (p less than 0.05) decline in eight of the sixteen comparisons of ALA and PBG excretion. Addition of 150 g glycerol by isocaloric substitution for fat caused a significant (p less than 0.05) decline in nine of the sixteen possible comparisons. In the sixteen comparisons of isocaloric dextrose and isocaloric glycerol-substituted diets, dextrose produced significantly (p less than 0.05) lower porphyrin precursor excretion in four cases and glycerol produced significantly (p less than 0.05) lower values in five. One patient showed no significant change on any of the diets. Of the four patients having symptoms believed referrable to porphyria during the study, three reported an improvement in symptoms during the high glycerol intake. The effects of dietary perturbations on porphyrin precursor excretion in acute intermittent porphyria are variable, but glycerol appears to be capable of decreasing the excretions and may prove useful in treating some of these patients.

Studies in porphyria. IV. Expression of the gene defect of acute intermittent porphyria in cultured human skin fibroblasts and amniotic cells: prenatal diagnosis of the porphyric trait

The gene lesion of the porphyrin-heme synthetic pathway in acute intermittent porphyria (AIP) is reflected in a deficient level of activity of the cytosol enzyme uroporphyrinogen I synthetase (URO-S). A marked URO-S deficiency has been demonstrated in the liver and in circulating erythrocytes of individuals with both active and latent AIP. This enzymic abnormality accounts for the excessive production and excretion into urine of the porphyrin precursors, lamda-aminolevulinic acid (ALA) and porphobilinogen (PBG) in AIP subjects. In this study, utilizing cell culture techniques, a marked URO-S deficiency has also been demonstrated in skin fibroblasts from AIP patients and in cells derived through aminocentesis from an approximately 17-wk old fetus. The prenatal diagnosis of the AIP trait in this fetus was confirmed postnatally by the demonstration in the child of a deficient level of erythrocyte URO-S activity which was comparable to those found in her AIP mother and affected sibling and which was approximately one-half the levels characterizing her normal father and aunt and a second unaffected sibling. The identification of the URO-S deficiency in cultured human fibroblasts from AIP patients was facilitated by a newly developed, sensitive assay for the enzyme activity. In this assay, the ability of such cells to convert ALA to protoporphyrin was quantitated; in the sequence of reactions involved in this transformation, URO-S is limiting so that the gene defect of AIP could be simply and precisely determined by appropriate spectrofluorometry of cell extracts. The technique described has distinct advantages over the direct enzymatic assay for URO-S activity in cultured human skin fibroblasts and permits clear differentiation of AIP carrier from normal individuals.

Measurement of delta-aminolevulinic acid synthetase activity in human erythroblasts

A new, specific, and simple method for the determination of delta-aminolevulinic acid (ALA) synthetase activity in human bone marrow cells has been developed. ALA synthetase of erythroblasts was partially purified so as to permit the use of [(14)C]succinyl-CoA as a substrate for this enzyme. In this enzyme preparation there were negligible activities of succinyl-CoA hydrolase, alpha-ketoglutarate dehydrogenase, and succinyl-CoA synthetase and there was no activity of ALA dehydrase. The ALA formed from [(14)C]succinyl-CoA has been isolated by column chromatography. Radioactivity in the eluate from the column has been proved by paper chromatography to be exclusively that of [(14)C]ALA. The entire assay can be completed within 4 h, and [(14)C]succinyl-CoA was incorporated into [(14)C]ALA on the order of several percent. Moderate to marked decreases of ALA synthetase activity have been demonstrated in the erythroblasts of all cases of sideroblastic anemia. In the cases of iron deficiency anemia, on the other hand, normal or slightly elevated activity has been obtained.

A microassay for uroporphyrinogen I synthase, one of three abnormal enzyme activities in acute intermittent porphyria, and its application to the study of the genetics of this disease

A new spectrofluorometric assay is described for quantitating uroporphyrinogen I synthase (EC 4.3.1.8) activity in volumes of human blood as small as 2 mul. By this sensitive assay the inheritance of the enzyme's activity has been studied and the genetic defect for acute intermittent porphyria has been confirmed to be autosomal dominant in nature. There is a 3-fold range of uroporphyrinogen I synthase activity in erythrocytes in the normal population, with a mean V(max) +/- SD of 35.7 +/- 8.4 nmol of uroporphyrinogen I formed per ml of erythrocytes per hr, at 37 degrees . One-half this level of enzyme activity (18.0 +/- 5.0) is found in erythrocytes from patients with clinically manifest acute intermittent porphyria; and in erythrocytes from those of their relatives, including prepubertal children, who have the latent gene defect for the disease. The K(m) of erythrocyte enzyme of normal people is 12.3 +/- 3.9 muM, whereas the K(m) of the erythrocyte enzyme of patients with acute intermittent porphyria is 6.2 +/- 3.9 muM, as determined on whole blood lysates. Three enzymic changes have now been identified in patients with acute intermittent porphyria; a high level of delta-aminolevulinate synthase activity; a low level of uroporphyrinogen I synthase activity; and a deficiency of steroid Delta(4)-5alpha reductase activity.

Studies in porphyria. I. A defect in the reductive transformation of natural steroid hormones in the hereditary liver disease, acute intermittent porphyria

A variety of 5beta steroid metabolites derived from hormones natural to man are potent inducers experimentally of delta-aminolevulinate synthetase, the rate-limiting enzyme in porphyrin-heme formation. This mitochondrial enzyme is found at high levels of activity in the livers of patients with the genetic disease, acute intermittent porphyria (AIP). In this study the metabolism of (14)C-labeled testosterone was examined in AIP patients to determine whether there was a disproportionate conversion of the hormone to its 5beta, compared to its 5alpha metabolite. The results indicate that AIP subjects do generate a substantially greater than normal fraction of 5beta metabolite from this steroid; the excessive degree of ring A reduction of testosterone taking place via the 5beta pathway in the porphyric patients averages 350% greater than in the nonporphyric subjects. In one asymptomatic AIP patient the disproportionate generation of 5beta metabolite from the hormone reached a level 10 times the normal mean. Studies with a second (14)C-labeled hormone, dehydroisoandrosterone, whose metabolism in man resembles that of testosterone, confirmed the derangement in reductive transformation of steroids found in the individuals carrying the genetic lesion of AIP. These findings define a new endocrine abnormality in AIP patients and raise the possibility that endogenously derived 5beta steroids may contribute by an induction mechanism to the increased levels of hepatic delta-aminolevulinate synthetase activity found in AIP patients.

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.

Repression of the overproduction of porphyrin precursors in acute intermittent porphyria by intravenous infusions of hematin

In a patient with a severe attack of acute intermittent porphyria, hematin given intravenously caused marked diminution of serum delta-aminolevulinic acid and porphobilinogen. The decline of aminolevulinate was more rapid than that of porphobilinoge. After 2 days of hematin administration, about 5 days were required for delta-aminolevulinic acid, and 11 days for porphobilinogen to return to the concentrations that were detected before treatment. Urinary excretion of both compounds also decreased after hematin administration. Considerable amounts of porphobilinogen were also found in the cerebrospinal fluid of the patient.

Heme biosynthesis in intermittent acute prophyria: decreased hepatic conversion of porphobilinogen to porphyrins and increased delta aminolevulinic acid synthetase activity

Hepatic conversion of porphobilinogen to porphyrins was less than 50% of control levels in human subjects with the genetic disease, intermittent acute porphyria. This relative block in heme biosynthesis may be relevant to a concomitant 6- to 10-fold elevation in delta-aminolevulinic acid synthetase activity, since this first and rate-controlling enzyme in the biosynthetic pathway is subject to negative feedback regulation by the end product, heme. A micro-radio-chemical assay of delta-aminolevulinic acid synthetase, and some of its applications, are described.

The occurrence of substances in human plasma capable of inducing the enzyme delta-aminolevulinate synthetase in liver cells

We have demonstrated the presence, in the plasma of several patients with acute intermittent porphyria, of a substance which strongly induced the synthesis of porphyrins in chick embryo liver cells growing in primary culture. The induction response evoked by this humoral agent was in all respects similar to that elicited by drugs and hormones which are known to enhance porphyrin production by inducing the de novo formation of delta-aminolevulinate synthetase, the rate-limiting enzyme in this pathway. Inducing properties were not found in the plasma of normal individuals or in that from porphyric patients in remission. Significant inducing activity was, however, found in the plasma of some normal subjects ingesting drugs or contraceptive steroid mixtures. The occurrence of a potent inducer of delta-aminolevulinate synthetase in the plasma of certain porphyric patients may have clinical significance for these genetically susceptible individuals. It will also permit chemical characterization of those humoral agents which may be related to the episodic exacerbations of this hereditary liver disease.