Inhibition of Na+, K+-ATPase activity by delta-aminolevulinic acid

Neurological Manifestations of Acute Porphyrias

PURPOSE OF REVIEW

Porphyrias constitute a group of rare metabolic disorders that result in a deficiency of the heme biosynthetic pathway and lead to the accumulation of metabolic intermediaries. Patients with porphyria can experience recurrent neurovisceral attacks which are characterized by neuropathic abdominal pain and acute gastrointestinal symptoms, including nausea, vomiting, and constipation. Depending on the type of porphyria, patients can present with cutaneous manifestations, such as severe skin photosensitivity, chronic hemolysis, or evidence of neurologic dysfunction, including alterations in consciousness, neurovascular involvement, seizures, transient sensor-motor symptoms, polyneuropathy, and behavioral abnormalities.

RECENT FINDINGS

More recently, cases of posterior reversible encephalopathy syndrome, cerebral vasoconstriction, and acute flaccid paralysis have also been described. While the exact pathogenic mechanisms linking the accumulation of abnormal heme biosynthetic intermediaries to neurologic manifestations have not been completely elucidated, it has been proposed that these manifestations are more common than previously thought and can result in permanent neurologic injury. This article reviews the basic principles of heme synthesis as well as the pathogenic mechanism of disease, presentation, and treatment of acute hepatic porphyrias with emphasis on those with neurologic manifestations.

Mechanisms of Neuronal Damage in Acute Hepatic Porphyrias

Porphyrias are a group of congenital and acquired diseases caused by an enzymatic impairment in the biosynthesis of heme. Depending on the specific enzyme involved, different types of porphyrias (i.e., chronic vs. acute, cutaneous vs. neurovisceral, hepatic vs. erythropoietic) are described, with different clinical presentations. Acute hepatic porphyrias (AHPs) are characterized by life-threatening acute neuro-visceral crises (acute porphyric attacks, APAs), featuring a wide range of neuropathic (central, peripheral, autonomic) manifestations. APAs are usually unleashed by external "porphyrinogenic" triggers, which are thought to cause an increased metabolic demand for heme. During APAs, the heme precursors δ-aminolevulinic acid (ALA) and porphobilinogen (PBG) accumulate in the bloodstream and urine. Even though several hypotheses have been developed to explain the protean clinical picture of APAs, the exact mechanism of neuronal damage in AHPs is still a matter of debate. In recent decades, a role has been proposed for oxidative damage caused by ALA, mitochondrial and synaptic ALA toxicity, dysfunction induced by relative heme deficiency on cytochromes and other hemeproteins (i.e., nitric oxide synthases), pyridoxal phosphate functional deficiency, derangements in the metabolic pathways of tryptophan, and other factors. Since the pathway leading to the biosynthesis of heme is inscribed into a complex network of interactions, which also includes some fundamental processes of basal metabolism, a disruption in any of the steps of this pathway is likely to have multiple pathogenic effects. Here, we aim to provide a comprehensive review of the current evidence regarding the mechanisms of neuronal damage in AHPs.

Sex differences in vascular reactivity in mesenteric arteries from a mouse model of acute intermittent porphyria

BACKGROUND AND AIMS

Acute intermittent porphyria (AIP) results from a partial deficiency of porphobilinogen deaminase (PBGD). Symptomatic AIP patients, most of whom are women, experience acute attacks characterized by severe abdominal pain and abrupt increases in blood pressure. Here, we characterized the reactivity of mesenteric arteries from male and female AIP mice with ~30% of normal PBGD activity and wild type C57BL/6 mice.

METHODS

An acute porphyric attack was induced in AIP mice by treatment with phenobarbital. Vascular responses to K⁺, phenylephrine (PE), acetylcholine (ACh), and hemin were determined (Wire Multi Myograph).

RESULTS

Maximal contraction to PE was increased in arteries from male and female AIP mice (p < .05) during an induced attack of acute porphyria. Female AIP arteries had increased sensitivity to PE (p < .05) even after nitric oxide (NO) blockade with N_ω-nitro-L-arginine methyl ester (L-NAME) (p < .05). Maximal relaxation to ACh was similar in males and females with lower sensitivity in female AIP arteries (p < .05). Hemin induced greater relaxation in AIP arteries in both males and females (p < .05).

SUMMARY/CONCLUSIONS

Sex differences in this AIP mouse model include a pro-contractile response in females. These alterations may contribute to the increased blood pressure during an acute attack and provide a novel mechanism of action whereby heme ameliorates the attacks.

Porphyria-induced posterior reversible encephalopathy syndrome and central nervous system dysfunction

BACKGROUND AND AIM

An association between neuropsychiatric manifestations and neuroimaging suggestive of posterior reversible encephalopathy syndrome (PRES) during porphyric attacks has been described in numerous case reports. We aimed to systematically review clinical-radiological features and likely pathogenic mechanisms of PRES in patients with acute hepatic porphyrias (AHP) and porphyric attacks.

METHODS

PubMed, Scopus, Ovid MEDLINE, and Google Scholar were searched (July 30, 2019). We included articles describing patients with convincing evidence of an AHP, confirmed porphyric attacks, and PRES in neuroimaging.

RESULTS

Forty-three out of 269 articles were included, which reported on 46 patients. Thirty-nine (84.8%) patients were women. The median age was 24 ± 13.8 years. 52.2% had unspecified AHP, 41.3% acute intermittent porphyria, 4.3% hereditary coproporphyria, and 2.2% variegate porphyria. 70.2% had systemic arterial hypertension. Seizures, mental changes, arterial hypertension, and hyponatremia occurred more frequently than expected for porphyric attacks (p < .001). Seizures and hyponatremia were also more frequent than expected for PRES. The most common distributions of brain lesions were occipital (81.4%), parietal (65.1%), frontal (60.5%), subcortical (40%), and cortical (32.5%). Cerebral vasoconstriction was demonstrated in 41.7% of the patients who underwent angiography. 19.6% of the patients had ischemic lesions, and 4.3% developed long-term sequelae (cognitive decline and focal neurological deficits).

CONCLUSIONS

Brain edema, vasoconstriction, and ischemia in the context of PRES likely account for central nervous symptoms in some porphyric attacks.

Porphyric neuropathy

Porphyric neuropathy often poses a diagnostic dilemma; it is typically associated with the hepatic porphyrias, characterized by acute life-threatening attacks of neurovisceral symptoms that mimic a range of acute medical and psychiatric conditions. The development of acute neurovisceral attacks is responsive to environmental factors, including drugs, hormones, and diet. This chapter reviews the clinical manifestations, genetics, pathophysiology, and mechanisms of neurotoxicity of the acute hepatic porphyrias. While the etiology of the neurological manifestations in the acute porphyrias remains undefined, the main hypotheses include toxicity of porphyrin precursors and deficiency of heme synthesis. These hypotheses will be discussed with reference to novel experimental models of porphyric neuropathy.

Purple pigments: the pathophysiology of acute porphyric neuropathy

The porphyrias are inherited metabolic disorders arising from disturbance in the haem biosynthesis pathway. The neuropathy associated with acute intermittent porphyria (AIP) occurs due to mutation involving the enzyme porphobilinogen deaminase (PBGD) and is characterised by motor-predominant features. Definitive diagnosis often encompasses a combination of biochemical, enzyme analysis and genetic testing, with clinical neurophysiological findings of a predominantly motor axonal neuropathy. Symptomatic and supportive treatment are the mainstays during an acute attack. If administered early, intravenous haemin may prevent progression of neuropathy. While the pathophysiology of AIP neuropathy remains unclear, axonal dysfunction appears intrinsically linked to the effects of neural energy deficits acquired through haem deficiency coupled to the neurotoxic effects of porphyrin precursors. The present review will provide an overview of AIP neuropathy, including discussion of recent advances in understanding developed through neurophysiological approaches that have further delineated the pathophysiology of axonal degeneration.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Decrease of hepatic δ-aminolevulinate dehydratase activity in an animal model of fatigue

Fatigue can be defined physiologically as inability to maintain the expected power output. At present, no standard of fatigue are yet available. In order to find biomarkers of fatigue, we investigated the level of delta-aminolevulinic acid (ALA), the first intermediate metabolite in the heme biosynthetic pathway, in the plasma and urine of an animal model of fatigue. To prepare fatigued animals, we kept rats for 5 days in a cage filled with water to a height of 1.5 cm. As a result, the plasma and urinary ALA levels were increased in the fatigued animals as compared with those in the control animals. One day after the rats had been returned to their normal cages, these increased levels were restored to the control ones. We also examined the activity of the enzyme ALA dehydratase (ALAD), which is the second enzyme in the heme biosynthetic pathway, and ALAD gene expression during the fatigue and its recovery sessions. The ALAD activity, as well as its gene expression, in the liver of the fatigued animals was decreased as compared with those of the control animals. Both activity and gene expression of ALAD were recovered to their respective control levels after the rats had been allowed to rest in their normal cages for 1 day. Furthermore, the activity of ALA synthase (ALAS), the rate-limiting enzyme in the heme biosynthesis, in the liver was increased after the fatigue session for 5 days. Although this level of increase in the plasma concentration of ALA may not induce fatigue, increase in plasma and urinary ALA levels can be biomarkers of fatigue.

Transport of 5-aminolevulinic acid between blood and brain

Little is known about the movement of 5-aminolevulinic acid (delta-aminolevulinic acid; ALA) between blood and brain. This is despite the fact that increases in brain ALA may be involved in generating the neuropsychiatric symptoms in porphyrias and that systemic administration of ALA is currently being used to delineate the borders of malignant gliomas. The current study examines the mechanisms involved in the movement of [(14)C]ALA across the blood-brain and blood-CSF barriers in the rat. In the adult rat, the influx rate constant (K(i)) for [(14)C]ALA movement into brain was low ( approximately 0.2 microl/g per min), was unaffected by increasing plasma concentrations of non-radioactive ALA or probenecid (an organic anion transport inhibitor) and, therefore, appears to be a diffusional process. The K(i) for [(14)C]ALA was 3-fold less than that for [(14)C]mannitol, a molecule of similar size. This difference appears to result from a lower lipid solubility rather than saturable [(14)C]ALA transport from brain to blood. The K(i) for [(14)C]ALA for uptake into the neonatal brain was 7-fold higher than in the adult. However, again, this was unaffected by increasing plasma ALA concentrations suggesting a diffusional process. In contrast, at the blood-CSF barrier, there was evidence of carrier-mediated [(14)C]ALA transport from blood to choroid plexus and blood to CSF. Both processes were inhibited by administration of non-radioactive ALA and probenecid. However, experiments in choroid plexus epithelial cell primary cultures indicated that transport in these cells was polarized with [(14)C]ALA uptake from the apical (CSF) side being about 7-fold greater than uptake from the basolateral (blood) side. In total, these results suggest that the brain is normally fairly well protected from changes in plasma ALA concentration by the very low blood-brain barrier permeability of this compound and by a saturable efflux mechanism present at the choroid plexus.

Inhibition of adenylate cyclase activity by 5-aminolevulinic acid in rat and human brain

The effect of the haem precursor 5-aminolevulinic acid (ALA) on the production of cyclic adenosine-monophosphate (cAMP) by rat cerebellar membranes was investigated. It was found that ALA dose-dependently decreased cAMP levels (maximal inhibition of 38%, at 1 mM), due to an inhibition of basal adenylate cyclase activity. ALA also inhibited fluoride- and Gpp(NH)p-stimulated, but not the forskolin-stimulated adenylate cyclase activity. 5-Aminovaleric acid (an inhibitor of GABA(B) receptors) did not prevent the inhibition, indicating that it was not mediated by the activation of the G(i)-protein coupled GABA(B) receptor. In addition, the nucleotide binding site of G-protein appeared not to be affected by ALA since it did not inhibit [3H]Gpp(NH)p binding to our membrane preparation. Antioxidants (glutathione, ascorbate and trolox) completely prevented the inhibition indicating that ALA effect was mediated by an oxidative damage of adenylate cyclase. ALA also inhibited the activity of adenylate cyclase in membranes isolated from rat cortex and striatum and from human cortex. These results may be of value in understanding the neurochemical mechanisms underlying the neurotoxic effects of ALA.

Mechanisms of 5-aminolevulinic acid uptake at the choroid plexus

5-Aminolevulinic acid (5-ALA) is a precursor of porphyrins and heme that has been implicated in the neuropsychiatric symptoms associated with porphyrias. It is also being used clinically to delineate malignant gliomas. The blood-CSF barrier may be an important interface for 5-ALA transport between blood and brain as in vivo studies have indicated 5-ALA is taken up by the choroid plexuses whereas the normal blood-brain barrier appears to be relatively impermeable. This study examines the mechanisms of 5-[(3)H]ALA uptake into isolated rat lateral ventricle choroid plexuses. Results suggest that there are two uptake mechanisms. The first was a Na(+)-independent uptake system that was pH dependent (being stimulated at low pH). Uptake was inhibited by the dipeptide Gly-Gly and by cefadroxil, an alpha-amino-containing cephalosporin. These properties are the same as the proton-dependent peptide transporters PEPT1 and PEPT2, which have recently been shown to transport 5-ALA in frog oocyte expression experiments. Choroid plexus uptake was not inhibited by captopril, a PEPT1 inhibitor, suggesting PEPT2-mediated uptake. The presence of PEPT2 and absence of PEPT1 in the choroid plexus were confirmed by western blotting. The second potential mechanism was both Na(+) and HCO(3)(-) dependent and appears to be an organic anion transporter, although it is possible that removal of Na(+) and HCO(3)(-) may indirectly affect PEPT2 by affecting intracellular pH. The presence of PEPT2 and a putative Na(+)/HCO(3)(-)-dependent organic anion transporter is important not only for an understanding of 5-ALA movement between blood and brain but also because these transporters may affect the distribution of a number of drugs between blood and CSF.

Spontaneous porphyria of the Long-evans cinnamon rat: an animal model of Wilson's disease

To confirm and extend our previous microspectrophotometric observations of 30-week-old male Long-Evans Cinnamon (LEC) rats, an animal model of human Wilson's disease, we analyzed the porphyrin patterns of the organs, urine, and plasma of LEC rats. Abnormal accumulation of porphyrins, especially highly carboxylated porphyrins (uro- and heptaporphyrin), in the kidneys and liver was seen in male and female LEC rats aged 30 weeks and also in 10-week-old rats, before the onset of spontaneous hepatic dysfunction. Accumulation of copper and iron in the kidneys was not observed in the 10-week-old rats. Massive accumulation of porphyrins was observed only in the kidneys of the 30-week-old male LEC rat, indicating that this symptom is related to sex and age. Renal accumulation of porphyrins was reflected in the rate of urinary porphyrin excretion. Hepatic accumulation of porphyrins appeared to be independent of sex and age. These results indicate that neither renal nor hepatic porphyrin accumulation is the result of renal deposition of metals or of spontaneous hepatic dysfunction and that porphyrinuria in the LEC rat is closely related to the renal accumulation of porphyrins. In contrast to these organs, a reduction in the porphyrin levels was observed in the brain of the LEC rat. This was independent of sex and age. The present work stresses the existence of an abnormal heme metabolism in the LEC rat, and thus, the necessity to study the heme metabolism in human Wilson's disease is strongly suggested.

Environmental lead exposure and activity of delta-aminolevulinic acid dehydratase (ALA-D) in maternal and cord blood

The hypothesis that environmental lead exposure measured from blood (Pb-B) inhibits delta-aminolevulinic acid dehydratase activity (ALA-D) from whole blood was tested in 241 urban mothers and their newborns. Geometric means and (5th and 95th Percentiles) for maternal and cord Pb-B were 6.4 microg dl(-1) (3.4-11.9) and 4.6 microg dl(-1) (2.8-9.2). Spearman correlations between mother and cord Pb-B and ALA-D were all negative but statistically significant only for cord Pb-B and mother ALA-D. A potential lead threshold, was identified between 3.2 and 4.8 microg dl(-1), above which ALA-D may be inhibited by lead, and below which ALA-D may be insensitive or even activated. In conclusion, low environmental exposure to lead is responsible for a demonstrable biochemical effect. This potential ALA-D inhibition may lead to neurotoxic effects, especially in newborns who have high level of neurogenesis.

Effects of delta-aminolaevulinic acid upon electrical activity of rat spinal cord in vivo

Bath applied delta-aminolaevulinic acid (ALA, 1-1000 microM), significantly depressed the frequency of spontaneous antidromic activity in the lumbar dorsal roots, and the amplitude of the monosynaptic component of the dorsal root-evoked ventral root reflex, in isolated rat spinal cord in a concentration-related manner. In contrast bath concentrations up to 1 mM ALA were found to produce no significant change in either the conducted afferent volley, nor field potentials recorded in the lumbar dorsal horn. These results indicate that the raised levels of ALA found in cerebrospinal fluid during porphyrias and lead poisoning may contribute to the neurological symptoms of these diseases.

Effect of reactive oxygen species promoted by delta-aminolevulinic acid on porphyrin biosynthesis and glucose uptake in rat cerebellum

1. delta-Aminolevulinic acid (ALA) has been reported to promote reactive oxygen species (ROS). Overproduction and accumulation of ALA, as it occurs in acute intermittent porphyria (AIP), can be the origin of an endogenous source of ROS, which can then exert their oxidative damage to cell structures. 2. To investigate the induction of lipid peroxidation by ALA, thiobarbituric acid reactive substances and conjugated diene formation were measured by using minimal tissue units (MTUs) obtained from rat cerebellum. Malondialdehyde levels increased with ALA concentration and incubation time (72% at 1.0 mM ALA and 127% at 4.0 mM ALA for 4 hr), and conjugated diene formation was enhanced 50% in incubations with 1.0 mM ALA for 4 hr. 3. ALA-promoted ROS by exposure of cerebellum MTUs to 1.0 mM ALA during different intervals (1-4 hr) was partly reduced by the addition of antioxidants such as superoxide dismutase (SOD; 50 U/ml), catalase (4.5 microM) and dimethylsulfoxide (150 mM), demonstrating the involvement of O2-., H2O2 and OH. in ALA autooxidation. 4. Porphobilinogen biosynthesis was 170% increased when cerebellum MTUs were incubated with 1.0 mM ALA for 4 hr in the presence of SOD, suggesting that protein damage was promoted by ALA autooxidation. 5. These findings provide the first experimental evidence of the involvement of ALA-promoted ROS in the damage of proteins related to porphyrin biosynthesis, specially ALA-D. Oxidation of this enzyme would lead to further accumulation of ALA in AIP patients, which may be the origin of the well-known neuropsychiatric manifestations.

Melatonin protects against lipid peroxidation induced by delta-aminolevulinic acid in rat cerebellum, cortex and hippocampus

The in vitro and in vivo effect of melatonin on delta-aminolevulinic acid-induced lipid peroxidation in rat cerebellum, cortex and hippocampus was determined. The concentration of malonaldehyde and 4-hydroxyalkenals was assayed as an index of induced membrane oxidative damage. The rise in malonaldehyde+4-hydroxyalkenals concentrations induced by delta-aminolevulinic acid in cerebellar homogenates was concentration-dependent (P < 0.001) and also time-dependent in cerebellar, cortical and hippocampal homogenates (P < 0.01). In vitro melatonin and vitamin E protected, in a concentration-dependent manner, against delta-aminolevulinic acid-induced lipid peroxidation in cortical, cerebellar and hippocampal homogenates. In in vivo experiments it was demonstrated that delta-aminolevulinic acid-induced lipid peroxidation (40 mg/kg) in cerebellum and hippocampus was reduced by acute melatonin (10 mg/kg) treatment (P < 0.05). The results show that both in vitro and in vivo melatonin confers protection against delta-aminolevulinic acid-induced oxidative toxicity in brain regions. The findings suggest that melatonin may be useful in reducing neural damage in individuals suffering from acute intermittent porphyria.

Increased delta aminolevulinic acid and decreased pineal melatonin production. A common event in acute porphyria studies in the rat

Tryptophan (TRP) is the precursor of melatonin, the primary secretory product of the pineal gland. Hepatic heme deficiency decreases the activity of liver tryptophan pyrrolase, leading to increased plasma TRP and serotonin. As a paradox, patients with attacks of acute intermittent porphyria (AIP), exhibit low nocturnal plasma melatonin levels. This study using a rat experimental model was designed to produce a pattern of TRP and melatonin production similar to that in AIP patients. Pineal melatonin production was measured in response to: (a) a heme synthesis inhibitor, succinylacetone, (b) a heme precursor, delta-aminolevulinic acid (Ala), (c) a structural analogue of Ala, gamma-aminobutyric acid. Studies were performed in intact rats, perifused pineal glands, and pinealocyte cultures. Ala, succinylacetone, and gamma-aminobutyric acid significantly decreased plasma melatonin levels independently of blood TRP concentration. In the pineal gland, the key enzyme activities of melatonin synthesis were unchanged for hydroxyindole-O-methyltransferase and decreased for N-acetyltransferase. Our results strongly suggest that Ala overproduced by the liver acts by mimicking the effect of gamma-aminobutyric acid on pineal melatonin in AIP. They also support the view that Ala acts as a toxic element in the pathophysiology of AIP.

High delta-aminolevulinic acid uptake in rat cerebral cortex: effect on porphyrin biosynthesis

The response of nerve cells to high exogenous aminolevulinic acid (ALA) concentrations was studied by examining the changes in its uptake and in porphyrin biosynthesis. ALA was shown to be taken up by cerebral cortex particles by a non-saturable process. As opposed to other previously described experimental systems, it was also observed that 84-87% of porphyrins formed was found within the cells. Exposure of cerebral cortex particles to high exogenous ALA (0.8-4.0 mM) showed that ALA can be accumulated in relatively high concentrations in brain cells (21.04 +/- 1.05 nmol/mg protein). Under these experimental conditions, porphyrin biosynthesis was found to be markedly inhibited (52%). 2.4 mM ALA caused an initial stimulation of glucose uptake after 1 hr incubation and a later fall to below control values, being consistent with the fact that acute porphyric crisis could be precipitated by the action of ALA on energy metabolism. ALA toxicity could be due both to its accumulation in the cells and to deficient heme concentrations, with an additional effect on glucose metabolism. These findings provide the basis for a useful brain tissue model to investigate the nature of the metabolic mechanisms occurring in acute intermittent porphyria (AIP) patients.

Porphyrinogenesis in rat cerebellum. Effect of high delta-aminolevulinic acid concentration

1. delta-Aminolevulinic acid (ALA) uptake as well as precursor accumulation and porphyrin biosynthesis were investigated in rat cerebellum, using as experimental approach minimal tissue units called particles. 2. ALA was shown to be taken up into cerebellum particles by a non saturable process up to 4.0 mM ALA whereas PBG and porphyrin formation exhibited a hyperbolic response reaching the plateau at about 1.0 and 1.5 mM ALA respectively. 3. Exposure of cerebellum particles to high exogenous ALA amounts (0.01-4.0 mM) indicated that ALA can be accumulated in relatively high concentrations in the cells (40 nmol/mg protein). Under these experimental conditions, PBG-D presented a low activity (3.25 pmol/mg protein/4 hr) showing to be a secondary control step in heme biosynthesis. 4. Incubation of cerebellum particles in the presence of a physiological concentration of glucose revealed that 1.0 mM ALA decreased glucose uptake by the cells (87% during 1 hr incubation), being consistent with the fact that acute attacks are precipitated by fasting and that sugar administration appeared to be an efficient treatment of AIP crisis. 5. These findings provide the basis for a useful model to study the nature of the metabolic mechanism underlying the acute attack.

delta-Aminolevulinic acid effects on neuronal and glial tumor cell lines

Acute intermittent porphyria (AIP) or precursor syndrome is a well described neuropathic clinical entity with incompletely known etiology. The most prominent biological abnormalities associated with this syndrome are elevations in serum and hepatic delta-aminolevulinic acid (ALA) and porphobilinogen (PBG). We determined the impact of ALA and PBG on human neuroblastoma and glioblastoma tumor cell survival as measured by the MTT assay. ALA proved to be cytotoxic in neuroblastoma cells, while PBG lacked cytotoxic effects. This cytotoxic effect of ALA could be enhanced by deferoxamine and diminished by heme, presumably through modulation of ALA synthesis. In conclusion, ALA excess may prove to be associated with the development of neuropathy in AIP.

A comparative study of the effects of delta-aminolaevulinic acid and the GABAA agonist, muscimol, in rat jejunal preparations

Preparations of rat jejunum were tested for their responsiveness to the GABAA receptor agonist, muscimol, and to the haem precursor, delta-aminolaevulinic acid (ALA). Both muscimol (1.0-30 microM) and ALA (1.0 microM-3.0 mM) elicited a concentration-dependent increase in tone. Pretreatment with the GABAA antagonist, bicuculline (10(-5) M), blocked effects of muscimol at all concentrations tested and attenuated effects of 0.3 mM ALA. However, bicuculline enhanced responsiveness of the preparations to ALA at low concentrations (0.01-0.05 microM), as also did picrotoxin (10(-5) M), eliciting a significant increase of tone. The significance of these findings is discussed. This finding of pharmacological activity by ALA at concentrations comparable with its blood levels during acute attacks of intermittent porphyria provides support for the proposal that is may play a role in the aetiology of the gastrointestinal manifestations of this disease.

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.

Pharmacological effects of haem biosynthetic intermediates in isolated intestinal preparations

Spontaneous contractile activity of isolated rabbit jejunal preparations bathed in oxygenated Ringer-Locke buffered to pH 7.0 was inhibited by haemin and by all haem biosynthetic intermediates tested, there being decreases in tone or amplitude of contractions. Effects were concentration-dependent. The minimal effective concentrations were 0.4 mM for porphobilinogen, 0.6 mM for protoporphyrin IX, 0.8 mM for haemin, 3.0 mM for delta-aminolaevulinic acid (ALA), 4.5 mM for coproporphyrin I, 3.0 mM for glycine and 6.0 mM for succinate. Inhibition was short-lasting, other than for protoporphyrin (2.3 mM) and haemin (3.0 mM). Leucine at concentrations up to 12 mM had no significant effect. Inhibitory effects of ALA were followed by contractions of increased amplitude; pretreatment of preparations with indomethacin (56 microM) prevented this enhancement of contractile activity. Tone in isolated preparations of human taenia coli bathed in oxygenated Ringer-Locke was decreased by ALA (1.5-6.0 mM). A significant increase of tone followed the initial inhibitory effect. The relevance of these findings to acute porphyria is discussed.

Effects of delta-aminolaevulinic acid on contractile activity in the isolated small intestine of the rabbit. Role of adrenergic receptors

The porphyrin precursor, delta-aminolaevulinic acid (ALA) at concentrations of 0.23-7.6 mM caused dose-dependent inhibition of spontaneous contractions in isolated preparations of rabbit small intestine, suspended in Ringer-Locke solution. Contractions which returned after inhibition from 3.8 and 7.6mM ALA showed increased amplitude and from 7.6 mM ALA a reduction in rate. Pretreatment with prazosin (10(-7) M) significantly reduced the duration of inhibition exerted by 3.8 and 7.6 mM ALA and also the amplitude of contractions returning after this inhibition. Pretreatment with yohimbine (10(-6) M) and propranolol (7 X 10(-6) M) had no significant effects. Blocking of the release of noradrenaline in preparations by incubation with 6-hydroxydopamine (10(-3) M) or guanethidine (1.5 X 10(-5) M) did not prevent the inhibitory effects of ALA. The alpha 1 agonist, cirazoline (10(-5) M), inhibited contractile activity. This effect was blocked by prazosin (10(-7) M). The alpha 2 agonist, guanoxabenz (10(-5) M) had no detectable effect, in the presence of prazosin, to block any residual alpha 1 actions. It is concluded that alpha 1 receptors mediate inhibitory effects in this preparation and that ALA appears to have direct effects upon these receptors.