Actions of guanidinoethane sulfonate on taurine concentration, retinal morphology and seizure threshold in the neonatal rat

Administration of the taurine transport inhibitor, guanidinoethane sulfonate (GES) to pregnant rats depleted taurine concentrations to approximately one-half of normal values in the newborn progeny. By 5 days of age taurine concentrations had returned to normal in all organs tested with the exception of the lungs. Longer postnatal exposure to GES significantly depressed tissue taurine levels. Prenatal exposure to GES had no effect on fetal development or the capability of the newborn rat to biosynthesize or transport taurine. Pre- and postnatal exposure to GES produced a degeneration of the photoreceptor layer of the retina similar to that observed in cats fed a taurine deficient diet. The pentylene tetrazole chemoshock threshold in GES-treated pups was greater than that in control pups. These results indicate that prenatal exposure to GES deplete taurine concentrations in the newborn rat. Morphological changes are thereby produced in the retina of rat that are similar to those observed in animals having limited ability to synthesize taurine which are maintained on a taurine-free diet.

Reduction of brain taurine: Effects on neurotoxic and metabolic actions of kainate

The effects of chronic administration of 2-guanidinoethane sulfonic acid on the levels of intra- and extracellular amino acids in the rat hippocampus were studied. The tissue content of taurine was selectively reduced by almost one third after 9 days of peroral administration of 1% 2-guanidinoethane sulfonate. Extracellular levels of amino acids were monitored with the brain microdialysis method. The taurine concentration in the extracellular fluid was depressed in relation to the decrease in intracellular taurine. Unexpectedly, extracellular (but not intracellular) glutamate was doubled in 2-guanidinoethane sulfonate treated animals. The kainic acid evoked release of taurine was suppressed in the 2-guanidinoethane sulfonate group, whereas the kainate stimulated efflux of glutamate was elevated after 2-guanidinoethane sulfonate administration. The acute metabolic effects of kainate were studied by measuring the efflux of the adenosine triphosphate breakdown products hypoxanthine, xanthine, inosine and adenosine. No differences were found between control and 2-guanidinoethane sulfonate treated rats with respect to basal or kainic acid evoked release of purine catabolites. Also, the neuronal loss caused by kainate injection into the hippocampus was not modified by 2-guanidinoethane sulfonate treatment, suggesting that endogenous taurine does not affect these responses. We conclude that chronic administration of 2-guanidinoethane sulfonate does not sensitize central neurons to the metabolic and toxic actions of kainate.

The relationship between reactivity of metabolites of pyrrolizidine alkaloids and extrahepatic toxicity

Pyrrolizidine alkaloids (PAs) are a large group of structurally similar toxins. In animals, including man, they are hepatotoxic and in some cases pneumo- and neurotoxic. PAs are metabolized by the liver P450 system to reactive dehydroalkaloid (DHA) intermediates. PA toxicity is a result of alkylation of macromolecules by DHAs. We have measured the relative reactivity of a series of semi-synthetic DHAs by recording the rate at which they alkylate a model nucleophile, 4-(p-nitrobenzyl)pyridine. Rate data fit mono- or biexponential equations. Rank order of reactivity for the macrocyclic and open ester DHAs was the same as those measured for DHA hydrolysis. The reaction with 4-(p-nitrobenzyl)pyridine was easier to follow, however, as rates of reaction can easily be controlled by temperature or level of acid catalysis, and the final product can be measured colorimetrically. DHAs of the primarily hepatotoxic alkaloids, retrorsine and seneciphylline, were more reactive than DHAs of monocrotaline and trichodesmine, which additionally produce pneumo- and neurotoxicity, respectively. This suggests that DHAs with greater stability (longer half-lives) are able to survive long enough to reach target organs downstream form the liver. We believe that differences in PA metabolism and the nature of toxicity ultimately produced are in part related to differences in reactivity of the primary toxic intermediate, the DHA.

Disturbances of amino acids from temporal lobe synaptosomes in human complex partial epilepsy

We have studied the levels of neuroactive amino acids in synaptosomes (P2 fraction) isolated from brain tissue of ten patients with medically intractable epilepsy who were undergoing temporal lobectomy. First, lateral temporal tissue (nonfocal) was removed followed by medial temporal tissue (focal). A synaptosomal fraction (P2) was immediately prepared from each tissue and analyzed for free amino acid concentrations. Statistically significant reductions were seen in glutamine and GABA concentrations in focal tissue compared to nonfocal tissue. The ratio of excitatory amino acids (aspartate and glutamate) to inhibitory amino acids (taurine and GABA) was significantly higher in focal tissue compared to nonfocal. The glutamine/glutamate ratio was significantly reduced. These data support the hypothesis that alterations in the balance between excitatory and inhibitory amino acids may be involved in the expression of epilepsy.

Chronic administration of taurine to aged rats improves the electrical and contractile properties of skeletal muscle fibers

A reduction of resting chloride conductance (GCl) and a decrease of the voltage threshold for contraction are observed during aging in rat skeletal muscle. The above alterations are also observed in muscle of adult rat after taurine depletion. As lower levels of taurine were found by others in aged rats compared to young rats, we tested the hypothesis that a depletion of taurine may contribute to the alteration of the electrical and contractile properties we found in skeletal muscle during aging. This was accomplished by evaluating the potential benefit of a pharmacological treatment with the amino acid. To this aim 25-mo-old Wistar rats were chronically treated (2-3 mo) with taurine (1 g/kg p.o. daily) and the effects of such a treatment were evaluated in vitro on the passive and active membrane electrical properties of extensor digitorum longus muscle fibers by means of current-clamp intracellular microelectrode technique. Excitation-contraction coupling was also evaluated by measuring the voltage threshold for contraction with the intracellular microelectrode "point" voltage clamp method. In parallel muscle and blood taurine contents were determined by high-performance liquid chromatography. Taurine supplementation significantly raised taurine content in muscle toward that found in adult rats. Supplementation also significantly increased GCl vs. the adult value, in parallel the excitability characteristics (threshold current and latency) related to this parameter were ameliorated. The increase of GCl induced by taurine was accompanied by a restoration of the pharmacological sensitivity to the R(+) enantiomer of 2-(p-chlorophenoxy) propionic acid, a specific chloride channel ligand. In parallel also the protein kinase C-mediated modulation of the channel was restored; in fact the potency of 4-beta-phorbol 12, 13-dibutyrate in reducing GCl was lower in taurine-treated muscles vs. untreated aged, being rather similar to that observed in adult. The treatment also improved the mechanical threshold for contraction of striated fibers which in aged rats is shifted toward more negative potentials, moving it toward the adult values. Our results suggest that the reduction of taurine content could play a role in the alteration of electrical and contractile properties observed during aging. These findings may indicate a potential application of taurine in ensuring normal muscle function in the elderly.

A simple procedure for determining the aqueous half-lives of pyrrolic metabolites of pyrrolizidine alkaloids

We report a simple and rapid procedure for estimating the aqueous half-lives of the reactive metabolites of pyrrolizidine alkaloids that are responsible for toxicity. The metabolites (dehydroalkaloids; DHAs) were rapidly added to a 0.5 mM HEPES solution, pH 8.0. The subsequent fall in pH, due to ester hydrolysis, was followed potentiometrically. The change in pH was well described by single-component exponential decay, allowing the derivation of rate constants and half-lives of hydrolysis. Half-lives varied from 0.31 sec for dehydro-7-acetyllycopsamine to 5.36 sec for dehydrotrichodesmine. The results support the view that alkaloids whose DHA metabolites have longer half-lives produce greater extrahepatic toxicity.

Effects of monocrotaline, a pyrrolizidine alkaloid, on glutathione metabolism in the rat

Monocrotaline (MONO), a pyrrolizidine alkaloid, causes veno-occlusive disease of the liver, pulmonary arterial hypertension, and right ventricular hypertrophy. Toxicity is due to the hepatic formation of a pyrolic metabolite that can be detoxified by conjugation with glutathione (GSH). We have shown that the GSH content of the liver affects the quantity of the pyrrolic metabolite that is released from the liver. We have now examined whether MONO, in turn, affects GSH metabolism. Twenty-four hours after administration of MONO to rats (65 mg/kg, i.p.), the highest concentration of bound pyrrolic metabolites was found in the liver, followed by the lung and kidney. Heart and brain contained lower concentrations of these metabolites. Significantly higher levels of GSH were found in liver and lungs of MONO-treated rats than in saline-injected control animals. In the liver, activities of the following enzymes were elevated: gamma-glutamylcysteine synthetase, GSH synthetase, gamma-glutamyl transpeptidase, dipeptidase, and microsomal GSH transferase. The same changes were seen in the lung. In the heart, gamma-glutamyl transpeptidase activity was decreased markedly, and cytosolic GSH transferase activity was elevated. In the kidney, the activities of GSH synthetase, gamma-glutamyl transpeptidase, and cytosolic GSH transferase were increased. Our results establish a mutual interaction of MONO and sulfur metabolism. It appears that an early metabolic action of MONO is to modify sulfur amino acid metabolism, diverting cysteine metabolism from oxidation to taurine towards synthesis of GSH.

Effects of taurine and guanidinoethane sulfonate on toxicity of the pyrrolizidine alkaloid monocrotaline

Monocrotaline (MONO), a pyrrolizidine alkaloid, causes pulmonary arterial hypertension and right ventricular hypertrophy due to hepatic metabolism to the alkylating pyrrole dehydromonocrotaline. Taurine a sulfonic amino acid, is hepato- and cardioprotective in a variety of conditions. We have examined the effects of taurine and its amidino analog, guanidinoethane sulfonate (GES), in rats injected i.p. with MONO (65 mg/kg). Taurine and GES were given as 1% solutions in drinking water beginning 14 days before administration of MONO and continuing for 14 days therafter, when the rats were killed. The MONO group had right ventricular hypertrophy and pulmonary hyperplasia. Compared with control, no significant changes in the right ventricle/left ventricle weight ratio, or the right ventricle/body weight ratio occurred in rats also given taurine of GES. Lung weights in these two groups were higher than in the control group, but below that of the MONO-alone group. The lethality of MONO over 14 days was decreased by taurine (LD50 for MONO alone 80 mg/kg; for MONO + taurine 121 mg/kg). Rats given only MONO had lower hepatic concentrations of GSH and cysteine (Cys), and higher activities of microsomal GSH transferase activity were no different from control. Gamma-Glutamylcysteine (Glu-Cys) synthetase and gamma-glutamyl transpeptidase activities were elevated. In MONO-injected rats given GES, hepatic GSH levels were higher and Cys levels were lower than in either the MONO alone or MONO + taurine groups. Gamma-Glu-Cys synthetase activity was depressed. Microsomal GSH transferase, GSH peroxidase and gamma-glutamyl transpeptidase activities were elevated. Livers of MONO-injected animals showed higher levels of serine (reversed by both taurine and GES) and glycine (Gly; reversed by GES) and lower levels of glutamine. Compared with control rats, the following changes occurred in serum amino acids: MONO alone: increased aspartate, taurine and lysine; taurine-supplemented: increased taurine, methionine (Met) and lysine, and decreased Gly; GES-supplemented: decreased asparagine, serine, Gly, arginine, taurine, and valine. Compared with the MONO-alone group, the taurine-supplemented group had higher glutamate (Glu), Met and alanine, and the GES-supplemented group higher alanine and lower serine, Gly, arginine and valine. We conclude that taurine protects against MONO-induced lethality and right ventricular hypertrophy. GES also protects against right ventricular hypertrophy. However, these agents act by different mechanisms, taurine preventing many of the biochemical changes induced by MONO, with GES inducing additional changes.

Physicochemical and metabolic basis for the differing neurotoxicity of the pyrrolizidine alkaloids, trichodesmine and monocrotaline

Monocrotaline and trichodesmine are structurally closely related pyrrolizidine alkaloids (PAs) exhibiting different extrahepatic toxicities, trichodesmine being neurotoxic (LD(50) 57 mu mol/kg) and monocrotaline pneumotoxic (LD(50) 335 mu mol/kg). We have compared certain physicochemical properties and metabolic activities of these two PAs in order to understand the quantitative and qualitative differences in toxicity. Both PAs were metabolized in the isolated, perfused rat liver to highly reactive pyrrolic dehydroalkaloids that appear to be responsible for the toxicity of PAs. More dehydrotrichodesmine (468 nmol/g liver) than dehydromonocrotaline (116 nmol/g liver) was released from liver into perfusate on perfusion for 1 hr with 0.5 mM of the parent PA. Dehydrotrichodesmine had a significantly longer aqueous half-life (5.4 sec) than that of dehydromonocrotaline (3.4 sec). In vivo, significantly higher levels of bound pyrroles were found in the brain 18 hr after injection of trichodesmine (25 mg/kg; i.p.) than were seen following either an equal dose (25 mg/kg; i.p.) or an equitoxic dose (90 mg/kg; i.p.) of monocrotaline. Trichodesmine had a higher partition coefficient than monocrotaline for both chloroform and heptane, indicating its greater lipophilicity. The pK(a) of trichodesmine (7.07) was only slightly higher than that of monocrotaline (pK(a¿ 6.83), suggesting that a difference in degree of ionization was not a major factor affecting the relative ability of the dehydroalkaloids to cross the blood-brain barrier. We conclude that the greater lethality and neurotoxicity of trichodesmine compared to monocrotaline is due to two structural characteristics: (i) steric hindrance at position 14 of dehydrotrichodesmine results in greater resistance to hydrolysis, allowing more to be released from the liver and to be delivered to the brain; (ii) the larger isopropyl substituent at position 14 of dehydrotrichodesmine renders the molecule more lipophilic, leading to greater penetration of the brain.

Fluorimetric determination of monobromobimane and o-phthalaldehyde adducts of gamma-glutamylcysteine and glutathione: application to assay of gamma-glutamylcysteinyl synthetase activity and glutathione concentration in liver

The reversed-phase HPLC separation of fluorescent o-phthalaldehyde (OPA) derivatives has been applied to the assay of hepatic gamma-glutamylcysteine and glutathione (GSH) levels and the enzymes producing these peptides. The method has been compared to the assay using monobromobimane (MB) as the derivatizing agent. The OPA method has the advantage of faster derivatization, the lack of need to adjust the pH, isocratic separation and selectivity for GSH and gamma-glutamylcysteine. The MB method requires pH adjustment following derivatization and gradient elution chromatography. MB is also non-selective, yielding fluorescent derivatives of all biological thiols and more interfering peaks on the chromatogram. MB-based analyses are also approximately sixty times more expensive per sample. MB yields fluorescent degradation products on exposure to light. OPA adducts are stable for up to ten days when stored at -20 degrees C. OPA detection is sensitive to 12.5 pmol in the sample, at a signal-to-noise ratio of 2.5. The two methods correlate well. Hepatic gamma-glutamylcysteine synthetase in the same liver preparation was found to be 4.85 +/- 0.47 nmol min-1 mg-1 protein by the OPA method and 4.42 +/- 0.52 nmol min-1 mg-1 protein by the MB method. GSH concentrations were found to be 90.4 +/- 6.5 nmol/mg protein for the OPA method and 92.5 +/- 3.4 for the MB method.

The comparative metabolism of the four pyrrolizidine alkaloids, seneciphylline, retrorsine, monocrotaline, and trichodesmine in the isolated, perfused rat liver

Despite their similarity in structure, pyrrolizidine alkaloids (PAs) vary in their LD50s and in the organs in which toxicity is expressed. We have examined whether there are differences in the metabolism of certain PAs that are associated with these quantitative and qualitative differences in toxicity. Isolated rat livers were perfused with one of four PAs (seneciphylline, retrorsine, monocrotaline, and trichodesmine) at 0.5 mM for 1 hr, and the pyrrolic metabolites determined that were released into perfusate and bile or bound in the liver. The proportion of the PA removed by the liver varied from 93% for retrorsine to 55% for trichodesmine. However, trichodesmine-perfused livers released the greatest amount of the dehydroalkaloid into the perfusate. These reactive pyrrolic metabolites appear to be largely responsible for the toxicity of PAs. Over the course of a 1-hr perfusion, dehydroalkaloid release varied fourfold among the PAs examined. Seneciphylline and retrorsine significantly increased bile flow. Highest concentrations of PAs in bile were achieved at 30-40 min perfusion. Conversion of dehydroalkaloid to the conjugate 7-glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (GSDHP) is a detoxification reaction. GSDHP release into bile varied from 80 nmol/g liver for trichodesmine to 880 nmol/g for retrorsine. Release of the less toxic hydrolytic product of dehydroalkaloids, 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine, was also determined. Bound pyrroles in liver are probably an indication of heptatoxicity. At the end of perfusion these varied from 55 nmol/g for monocrotaline to 195 nmol/g for retrorsine. The chemical form of the bound pyrroles is a 7-thioether conjugate of 6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine. No 7,9-dithio conjugate was detected, indicating that only monoalkylation has been found. These differences in metabolic pattern reflect differences in reactivity of the initially formed dehydroalkaloid and can account for the toxicological differences between the parent PAs.

The sea anemone purine, caissarone: adenosine receptor antagonism

Caissarone, a sea anemone iminopurine, produced an increase in the twitch response of the electrically stimulated guinea-pig ileum-myenteric plexus. In the same assay, caissarone reduced the inhibitory response to the endogenous neuromodulator, adenosine, the A1 adenosine receptor agonist, R-phenylisopropyladenosine (R-PIA), and the A2 agonist, 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA) in a dose-dependent manner. Schild plot analysis of antagonism by caissarone yielded slopes of near unity, indicating that caissarone acts as a simple competitive antagonist at the adenosine receptor. The dissociation constants (KB) for caissarone ranged from 0.53 mM to 0.78 mM. In functional nicotinic receptor assays in two human cell lines, caissarone failed either to potentiate or to reduce carbamylcholine-mediated 86Rb+ efflux. Thus, the enhancing activity of caissarone on the gut could not be attributed to activity at the ganglionic nicotinic receptor. Based on structure and pharmacological activity, caissarone appears to be the first marine product described as an adenosine receptor antagonist.

Effect of the pyrrolizidine alkaloid, monocrotaline, on bile composition of the isolated, perfused rat liver

Monocrotaline is a hepatotoxic pyrrolizidine alkaloid, releasing high levels of metabolites into bile of isolated, perfused liver. Although perfusion of rat liver with 0.5 mM monocrotaline does not affect bile flow over a 1 hr study period, it markedly affects bile composition. Biliary release of conjugated and free GSH increases 30-fold. Marked increases are also observed in the biliary concentration of the related sulfur-containing substances, cysteine and cysteinylglycine. However, biliary release of the sulfur amino acids, taurine and methionine, is unaffected. Only two amino acids show mildly increased releases, 23% for glycine and 46% for aspartate. Release of bile acids, cholesterol and phospholipids also decrease, both in terms of mM concentration in bile and in terms of nmol secreted per g liver. Thus, exposure to monocrotaline causes disturbances in sulfur metabolism in the liver and in the composition of bile. The consequences of the digestive properties of bile and gastrointestinal toxicity remain to be established. As sulfhydryl compounds are involved in detoxification of monocrotaline metabolites, these findings indicate a mutual interaction of pyrrolizidine toxicity and sulfur metabolism. This suggests that dietary sulfur amino acid intake may influence susceptibility to pyrrolizidine poisoning.

The effect of the pyrrolizidine alkaloids, monocrotaline and trichodesmine, on tissue pyrrole binding and glutathione metabolism in the rat

One day after in vivo administration of equitoxic doses of the hepatotoxic and pneumotoxic pyrrolizidine alkaloid, monocrotaline (65 mg/kg, i. p.) or the related hepatotoxic and neurotoxic alkaloid trichodesmine (15 mg/kg, i. p.) hepatic GSH levels are increased by more than 50%. These doses of alkaloids represent 60% of the LD50 values. Accompanying these changes in GSH levels is an increase in the overall rate of GSH synthesis in supernatants of alkaloid-exposed livers. The ability of the rat to metabolize the two alkaloids was shown by the appearance of tissuebound pyrrolic metabolites of pyrrolizidines in various organs. The levels of these metabolites appear to correlate with organ toxicity. For the hepatic and pneumotoxic alkaloid, monocrotaline, higher levels are found in liver (17 nmoles/g tissue) and lung (10 nmoles/g) than for trichodesmine (7 nmoles/g and 8 nmoles/g, respectively). For the neurotoxic alkaloid, trichodesmine, higher levels are found in brain (3.8 nmoles/g tissue) than for monocrotaline (1.7 nmoles/g tissue).

The relationship between the concentration of the pyrrolizidine alkaloid monocrotaline and the pattern of metabolites released from the isolated liver

Hepatic metabolism of the pyrrolizidine alkaloid monocrotaline results in extrahepatic toxicity caused by the release of metabolites from the liver. We have quantified the release of pyrrolic metabolites into the perfusate and bile of isolated rat livers perfused with monocrotaline over the concentration range of 0.125-1.5 mM. Over a 1-hr perfusion period, the amount of dehydromonocrotaline released from the liver varied from 60 nmol/g liver at 0.125 mM monocrotaline to 460 nmol/g liver at 1.5 mM monocrotaline. As a percentage of total pyrrole release, this is a monotonic increase from 30 to 41%. The percentage of pyrroles released into the bile, representing mainly 7-glutathionyl-6,7-dihydro- 1-hydroxymethyl-5H-pyrrolizine (GSDHP), increased over the monocrotaline concentration range 0.125-1.0 mM, but fell sharply from 38% of total at the latter concentration to 21% of total at 1.5 mM monocrotaline. This is probably a reflection of glutathione depletion. Nonalkylating pyrrole released into the perfusate, represents largely 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP). Pyrrole released into perfusate showed an opposite pattern. The percentage of pyrroles released as DHP into the perfusate fell from 38% at 125 microM monocrotaline to 27% at 1.0 mM monocrotaline, but increased sharply to 38% at 1.5 mM monocrotaline. When calculated on a body weight basis, concentrations of monocrotaline of 500 microM result in the release from the liver of 5.3 mumol/kg of dehydromonocrotaline. This is comparable to the amount of dehydromonocrotaline, given in vivo, required for pneumotoxicity. The amounts of other pyrrolic metabolites released over a 1-hr period of perfusion are insufficient to produce pneumotoxicity in vivo. Based on the body weight of the donor rat, pyrrole release on perfusion of the isolated liver with 1,500 microM monocrotaline can be calculated as mumol/kg body weight. These amounts can then be compared to acute doses producing pneumotoxicity in vivo (given in parentheses): DHP, 13 mumol/kg body weight released (350 mumol/kg); GSDHP, 8 mumol/kg (300 mumol/kg); and dehydromonocrotaline, 14 mumol/kg (15 mumol/kg). This suggests, therefore, that dehydromonocrotaline is the pyrrolic metabolite contributing the most to the extrahepatic toxicity of monocrotaline.

Relationship between glutathione concentration and metabolism of the pyrrolizidine alkaloid, monocrotaline, in the isolated, perfused liver

The influence of GSH concentration on metabolism of monocrotaline was examined in the isolated, perfused rat liver. Chloroethanol (0.37 mmol/kg), diethyl maleate (5.6 mmol/kg), and buthionine sulfoximine (72.9 mmol/kg) given in vivo reduced hepatic GSH from 3.7 mumol/g wet weight to 1.5, 0.6 and 0.9 mumol/g, respectively. Livers were then perfused in vitro for 1 hr with monocrotaline (0.5 mM). GSH depletion had no effect on the total release of pyrrolic metabolites of monocrotaline. Depletion, however, markedly affected the pattern of pyrrole release. Biliary release of 7-glutathionyl-6,7-dihydro-1-hydroxy-methyl-5H-pyrrolizine (GSDHP) was reduced by up to 72%. Pretreatment with diethyl maleate or buthionine sulfoximine increased the level of protein-bound pyrroles in the liver by 107 and 84%, respectively. Such pyrroles are probably responsible for liver toxicity. GSH depletion also led to a doubling of dehydromonocrotaline release into the perfusate. This metabolite is probably responsible for the extrahepatic toxicity of monocrotaline. Release into perfusate of the relatively nontoxic metabolite, 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP) was correspondingly decreased. Hepatic GSH content was increased to 4.4 mumol/g by pretreatment with oxo-4-thiazolidine carboxylate (4.76 mmol/kg). This agent increased total pyrrolic metabolites by 54%. Biliary release of GSDHP and perfusate release of dehydromonocrotaline and DHP were all increased. Thus, hepatic GSH levels regulate the metabolism of monocrotaline and dehydromonocrotaline and, consequently, the hepatic and extrahepatic toxicity of monocrotaline. GSH depletion leads to a switch from the biliary release of the midly toxic GSDHP to the perfusate release of the highly toxic dehydromonocrotaline. GSH depletion also permits more dehydromonocrotaline in the liver to become available for macromolecular alkylation. These findings suggest that nutritional intake of sulfur-containing amino acids can influence the severity of pyrrolizidine poisoning.

Quantitation of the hepatic release of metabolites of the pyrrolizidine alkaloid, monocrotaline

Pyrrolizidine alkaloids such as monocrotaline are bioactivated in the liver to pneumotoxins that cause pulmonary arterial hypertension and right ventricular hypertrophy. The release of the highly reactive, alkylating pyrrole, dehydromonocrotaline, from the isolated rat liver perfused with monocrotaline has now been demonstrated and quantified, using thiopropyl Sepharose resin as a trapping agent. The isolated liver extracted 55% of the alkaloid over the course of a 1-hr perfusion with 0.5 mM monocrotaline. Of the total monocrotaline perfused, 0.4% was excreted into bile and 7.6% was detectable as pyrrolic metabolites. Of these metabolites, 156 nmol/g liver appeared in the bile as glutathionyldehydroretronecine, with the average concentration in bile being 3.53 mM. The perfusion medium at the end of the perfusion contained 113 nmol/g liver of the two pyrroles, dehydroretronecine and glutathionyldehydroretronecine. Remaining in the liver was 56 nmol/g of tissue-bound pyrroles. Over the course of a 1-hr perfusion, 88 nmol/g liver of dehydromonocrotaline was released into the perfusate, as determined by trapping with thiopropyl Sepharose, a resin that reacts only with alkylating pyrroles. This establishes that dehydromonocrotaline is released on perfusing the isolated liver with monocrotaline. The amount released under these conditions is equivalent to 1.08 +/- 0.06 mg/kg body weight, which can be compared to the intravenous dose of 4.85 mg/kg body weight of dehydromonocrotaline found by others to be a pneumotoxic dose.

Hepatic glutathione concentrations and the release of pyrrolic metabolites of the pyrrolizidine alkaloid, monocrotaline, from the isolated perfused liver

We have examined the relationship between the metabolism of the pyrrolizidine alkaloid, monocrotaline, and glutathione concentration in the isolated, perfused rat liver. On perfusion of monocrotaline (300 microM) through the isolated liver, high concentrations (1.1 mM) of its metabolite glutathionyldehydroretronecine are released into bile, while much lower amounts (4.86 microM; 0.05 mumol/g liver) accumulate in the perfusate over a 1 hr perfusion period. Metabolite concentration in both the bile and perfusate increase when the level of monocrotaline perfused is increased to 900 microM. Metabolite release is also elevated in livers pretreated with phenobarbital. Monocrotaline perfusion lowered glutathione concentrations in the liver from 30 min onwards. Livers from animals treated with buthionine sulfoximine or chloroethanol showed much lower glutathione levels after 60 min perfusion. Livers from chloroethanol-treated (but not buthionine sulfoximine-treated) animals showed significantly lower release of pyrroles into the bile on perfusion with monocrotaline, but there is no effect on the rate of build-up of pyrrolic metabolites in the perfusate. We conclude that hepatic glutathione concentrations and the release of pyrrolic metabolites of monocrotaline mutually interact. Exposure of the liver to monocrotaline reduces glutathione concentrations, while marked depletion of liver glutathione concentration leads to a decrease in the release of monocrotaline metabolites.

Detection of a reactive pyrrole in the hepatic metabolism of the pyrrolizidine alkaloid, monocrotaline

Pyrrolizidine alkaloids such as monocrotaline are bioactivated in the liver, resulting in veno-occlusive disease of the liver, pulmonary arterial hypertension, and right ventricular hypertrophy. We have searched for the formation of a reactive, alkylating pyrrole intermediate in the metabolism of monocrotaline by isolated rat liver microsomes, using the sulfhydryl-containing resin, thiopropyl sepharose 6B, as a trapping agent. Control experiments show that a toxic, chemically reactive, alkylating pyrrole such as dehydromonocrotaline binds covalently to the resin via a thioether bond, but that a less toxic, poorly alkylating pyrrole, such as dehydroretronecine, does not. Isolated hepatic microsomes metabolize monocrotaline to produce a pyrrole that binds to the resin, and that can be detected by means of the Ehrlich color reagent (p-dimethylaminobenzaldehyde). The pyrrole is releasable by silver nitrate treatment, thereby establishing it to be bound via a thioether linkage. In buffered ethanolic silver nitrate the major product is 7-ethoxy-1-hydroxymethyl-6,7-dihydro-5H-pyrrolizine (O7-ethyldehydroretronecine). This establishes that the thioether linkage is at the 7-position. The same product is obtained on release of the resin-bound pyrrole formed from the reaction of dehydromonocrotaline with the resin, thereby establishing the intermediacy of dehydromonocrotaline in the metabolism of monocrotaline.

Phospholipid methylation and taurine content of synaptosomes from cerebral cortex of developing rat

Changes in taurine concentration and rate of methylation of phosphatidylethanolamine have been examined in rat brain synaptosomes over the course of development. At 7, 14, 21, 28 and 56 days of age, rats were injected i.p. with 300 microCi/kg [3H-methyl]methionine. Synaptosomes (P2B fraction) were isolated from the cerebral cortex 9 h later and incorporation of the methionine methyl group into phospholipid and protein was investigated. Synaptosomal taurine and methionine concentrations were determined at the same ages, as were the concentrations of the major classes of phospholipids (phosphatidylethanolamine, phosphatidylinositol, phosphatidylcholine and phosphatidylserine). Methionine concentration increased between day 7 and 14 and fell thereafter. Phospholipid methylation rates calculated from the specific activity of synaptosomal methionine were high from days 7 and 14 and then fell, whereas protein methionylation increased between day 7 and 28 and then decreased. A strong correlation was found between the taurine concentration of the synaptosome and phospholipid methylation rates during brain development. Protein methionylation rates, however, showed no correlation with taurine concentration.