Structural requirements for ochratoxin intoxication

Comprehensive study on the degradation of ochratoxin A in water by spectroscopic techniques and DFT calculations

The OTA degradation process becomes irreversible only if, following hydrolyzation of the lactone ring, the molecule fragments.

Ochratoxin A: Molecular Interactions, Mechanisms of Toxicity and Prevention at the Molecular Level

Ochratoxin A (OTA) is a widely-spread mycotoxin all over the world causing major health risks. The focus of the present review is on the molecular and cellular interactions of OTA. In order to get better insight into the mechanism of its toxicity and on the several attempts made for prevention or attenuation of its toxic action, a detailed description is given on chemistry and toxicokinetics of this mycotoxin. The mode of action of OTA is not clearly understood yet, and seems to be very complex. Inhibition of protein synthesis and energy production, induction of oxidative stress, DNA adduct formation, as well as apoptosis/necrosis and cell cycle arrest are possibly involved in its toxic action. Since OTA binds very strongly to human and animal albumin, a major emphasis is done regarding OTA-albumin interaction. Displacement of OTA from albumin by drugs and by natural flavonoids are discussed in detail, hypothesizing their potentially beneficial effect in order to prevent or attenuate the OTA-induced toxic consequences.

Evidence of ochratoxin A-detoxification activity of rumen fluid, intestinal fluid and soil samples as well as isolation of relevant microorganisms from these environments

Dietary ochratoxin A (OTA) has a negative impact on performance of chickens and pigs. To avoid losses in animal production through intake of this mycotoxin and to prevent carry over to humans, strategies for counteracting have to be developed. In contrast to physical and chemical detoxification methods inactivation of ochratoxins by enzymatic reactions represent a very specific and gentle process. For the development of a new feed additive various environments have been screened for microorganisms with the capability of degrading or of cleaving the phenylalanine-moiety of ochratoxin A. Two OTA-degrading bacterial strains were isolated from rumen fluid and four pure cultures capable of cleaving ochratoxin A were obtained from pig intestine. The highest number of ochratoxin A degrading strains were found amongst aerobic bacteria which have mainly been isolated from soil.

Toxicokinetics and toxicodynamics of ochratoxin A, an update

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic and immunotoxic to several species of animals and to cause kidney and liver tumours in mice and rats. Because of differences in the physiology of animal species, wide variations are seen in the toxicokinetic patterns of absorption, distribution and elimination of the toxin. Biotransformation of OTA has not been entirely elucidated. At present, data regarding OTA metabolism are controversial. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. Several major mechanisms have been shown as involved in the toxicity of OTA: inhibition of protein synthesis, promotion of membrane peroxidation, disruption of calcium homeostasis, inhibition of mitochondrial respiration and DNA damage. The contribution of metabolites in OTA genotoxicity and carcinogenicity is still unclear. The genotoxic status of OTA is still controversial because contradictory results were obtained in various microbial and mammalian tests, notably regarding the formation of DNA adducts. More recent studies are focused on the OTA ability to disturb cellular signalling and regulation, to modulate physiological signals and thereby to influence cells viability and proliferation. The present paper offers an update on these different issues. In addition since humans and animals are likely to be simultaneously exposed to several mycotoxins, especially through their diet, the little information available on the combined effects of OTA and other mycotoxins has also been reviewed.

Inhibitory effects of the fragrance inhalation of essential oil from Acorus gramineus on central nervous system

The present study was designed to evaluate central inhibitory effects of the essential oil from Acori graminei Rhizoma (AGR), the dry rhizomes of Acorus gramineus SOLANDER (Araceae) upon fragrance inhalation (aroma therapy). Preinhalation of the oil markedly delayed the appearance of pentylenetetrazole-induced convulsion. Furthermore, inhalation impressively inhibited the activity of gamma-aminobutyric acid (GABA) transaminase, a degrading enzyme for GABA as the inhalation period was lengthened. The GABA level was significantly increased and glutamate content was significantly decreased in mouse brain by preinhalation of the essential oil. The above results suggest that the anticonvulsive effect of this AGR oil is originated by the enhancement of GABA level in the mouse brain, because convulsion depends partially on GABA concentration which can be properly preserved by inhibiting GABA transaminase. Moreover, fragrance inhalation progressively prolonged the pentobarbital-induced sleeping time as inhalation time was lengthened. Ten hour inhalation corresponded almost to the effect (145% increase) of oral administration (60 mg/kg). This sedative effect after inhalation or oral administration of AGR essential oil suggests that this oil may act on the CNS via the GABAergic system. The inhibitory activity of preinhalation of the essential oil on lipid peroxidation, to which the anticonvulsive action is attributed, also supported the above results, confirming and amplifying our previous reports on the CNS inhibitory effects of AGR.

Toxicity of ochratoxin A in aBrevibacillus brevis - Growth inhibition assay

Ochratoxin A (OTA) is a nephrotoxic, carcinogenic and immunosuppressive mycotoxin. It can be detoxified by various microorganisms, e.g. different yeast strains, via metabolisation into ochratoxin α (OTα). Within this study a growth inhibition assay was developed to compare the toxicity of OTA and its degradation product OTα. As an indicator organismBrevibacillus brevis was used. The assay was performed in microtiterplates. Growth inhibition was determined by comparing the optical density values ofBrevibacillus brevis cultures grown in medium supplemented with OTA/OTα and OTA/OTα-free medium, respectively.It could be shown thatB. brevis is sensitive to OTA (EC100=0.5 mg/L±0.03 mg/L), which is not the case for its metabolite OTα. Therefore this bioassay is a useful tool to show the detoxification of OTA to OTα by microbial degradation.

Ochratoxin A from a toxicological perspective

Ochratoxin A (OTA) is a widespread mycotoxin which is produced mainly by the mould fungi Aspergillus ochraceus and Penicillum verrucosum during the storage of cereals, cereal products and other plant-derived products such as herbs, spices, grapes, etc. By carry over from mouldy fodder, ochratoxin A is also found in pork meat, offal and sausages containing pork blood. When ingested as a food contaminant, OTA is very persistent in human beings with a blood half-life of 35 days after a single oral dosage due to unfavourable elimination toxicokinetics. This renders the toxin among the most frequent mycotoxin contaminants in human blood in the EU, the US, Canada, and elsewhere, where it has been investigated. OTA is neither stored nor deposited in the body, but heterogeneous body distribution may impose serious damage to the kidneys. The toxin was classified a 2B cancer compound, being possibly carcinogenic for humans. It was among the strongest carcinogenic compounds in rats and mice. As the toxicological profile also includes teratogenesis, nephrotoxicity, and immunotoxicity, legislation authorities are currently discussing maximal residue levels (MRL) for OTA in various foodstuffs. In the present article arguments are presented which suggest an acceptable daily intake (ADI) of 1.5 ng OTA/kg body weight and a much lower MRL than 5 microgram OTA/kg cereals and cereal products as has been postulated by the EU commission.

Kinetic profiles of ochratoxin A and ochratoxin alpha during in vitro incubation in buffered forestomach and abomasal contents from cows

Pure ochratoxin A (OA) was added to buffered rumen fluid collected from fistulated cows and incubated under anaerobic conditions. The kinetic pattern of the disappearance of OA and the appearance of ochratoxin alpha (Oalpha) was principally the same with four diets fed to donor animals and with three donor animals. The concentration of OA declined to a very low or non-detectable level with half-lives at 0.17-1.84 h; its rate of disappearance was first order throughout. The concentration of Oalpha increased to a constant level under all conditions examined. The average amounts of Oalpha formed relative to the disappearance of OA were not different (p > 0.05) and ranged between 94 +/- 8 and 98 +/- 8 %. The rate of disappearance of OA differed (p < 0.001) between diets and animals. It was accelerated by increasing the content of concentrate in the diet. Ochratoxin alpha was not metabolized when added in pure form to buffered rumen fluid and incubated under the same conditions as OA. Pure OA was also added to buffered fluids from the forestomach and abomasum of a slaughtered cow. It disappeared exponentially in a mixture of fluids from the rumen and reticulum and in fluid from the omasum, with the average amounts of Oalpha formed relative to OA disappearance being 107 and 109 %, respectively. Ochratoxin A also was not metabolized in fluid from the abomasum. These studies demonstrate that OA is hydrolysed in the rumen via first order kinetics, diet and animal affect the rate of hydrolysis, OA is quantitatively converted to Oalpha and Oalpha is not degraded.

Free radical generation as induced by ochratoxin A and its analogs in bacteria (Bacillus brevis)

Lipid peroxidation is considered as one of the manifestations of cellular damage in the toxicity of ochratoxin A (OA). OA; its three natural analogs, OB, OC, and Oalpha; and four synthetic analogs, d-OA, the ethylamide of OA (OE-OA), O-methylated OA (OM-OA), and the lactone-opened OA (OP-OA) were used to study free radical generation in bacteria with Bacillus brevis as a model system. The uptake of the different ochratoxins by B. brevis varied substantially depending on the molecular structures. Electron paramagnetic resonance spectroscopy using alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone as a spin trapping agent showed an enhanced free radical generation due to the addition of OA and most of the analogs. The EPR signals could be further enhanced by the addition of Ca2+, a calcium ionophore and an ATPase uncoupler, whereas they were eliminated by incubating the growing cells with vitamin E. The spin adduct hyperfine splitting constants indicate the presence of alpha-hydroxyethyl radicals resulting from generated hydroxyl radicals, which are trapped by alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone. The results further suggest that OA induces free radical production in this model system by enhancing the permeability of the cellular membrane to Ca2+.

Metabolites of ochratoxins in rat urine and in a culture of Aspergillus ochraceus

We studied the metabolic profile of ochratoxin A (OA) in rats and in a culture of OA-producing Aspergillus ochraceus. Ochratoxin alpha (O alpha), ochratoxin beta (O beta), 4-R-hydroxyochratoxin A (4-R-OH OA), 4-R-hydroxyochratoxin B (4-R-OH OB), and 10-hydroxyochratoxin A (10-OH OA) were isolated from a culture of A. ochraceus and structurally characterized by 1H nuclear magnetic resonance spectroscopy, mass spectrometry and high-pressure liquid chromatography. 4-R-OH OA and O alpha were consistently produced and were the dominant biotransformed metabolites in the fungal culture and in rats treated with OA and ochratoxin C (OC), while the formation of 10-OH OA was conditional in the fungal system. Green fluorescent biomacromolecules were isolated by detergent extraction of the fungal culture followed by cold-acetone precipitation and gel filtration. Acid hydrolysis of the fluorescent macromolecules resulted in the release of several ochratoxins, including O alpha (80%), OA (2%), and OC (5%), and other unidentified fluorescent compounds but not OB and O beta. Cross-reactivity studies of the natural macromolecule conjugates of OA with anti-OA polyclonal antibodies indicated that they were covalently linked to the macromolecules via a group other than the carboxyl group. These studies demonstrated that a fungus can produce some of the same metabolites of OA as the rat and that O alpha, OA, and OC may be covalently linked to fungal macromolecules.

Synthesis of 14C-ochratoxin A and 14C-ochratoxin B and a comparative study of their distribution in rats using whole body autoradiography

Methods for preparation of labelled ochratoxin A and B are described. The method for preparation of labelled ochratoxin B involves the synthesis of the azide of ochratoxin beta via the mixed anhydride and subsequent conjugation to labelled phenylalanine to yield 14C-ochratoxin B. The labelled ochratoxins were injected into male Wistar rats and after different survival times they were sacrificed and subjected to whole body autoradiography. The distribution pattern of ochratoxin A in the rat did not differ from that earlier registered for mouse. The previously known, high susceptibility of rats (and not mice) to ochratoxin A-induced cancer could thus not be explained by an accumulation of the toxin in specific cells or organs. The distribution patterns of ochratoxin A and B were almost congruent--the only apparent difference being a much longer retention of the labelled ochratoxin A in the blood compared to ochratoxin B, which was much faster excreted. When analyzing tissue extracts for labelled metabolites only the extracts from the rats injected with ochratoxin B were found to contain easily detectable concentrations, while no metabolites of ochratoxin A were seen.

Mechanism of ochratoxin A stimulated lipid peroxidation

Lipid peroxidation, measured as malondialdehyde formation or by oxygen uptake, was stimulated markedly by the mycotoxin ochratoxin A (OTA) in a reconstituted system consisting of phospholipid vesicles, the flavoprotein NADPH-cytochrome P450 reductase, Fe3+, EDTA and NADPH. Deletion of EDTA lowered the extent of lipid peroxidation but did not eliminate it. Fluorometric and spectrophotometric studies demonstrated the formation of a 1:1 Fe3(+)-OTA complex. The rate of reduction of Fe3+ to Fe2+ was enhanced markedly in the presence of OTA, and there was a further increase in the rate when EDTA was also included. The data indicate that OTA stimulates lipid peroxidation by complexing Fe3+ and facilitating its reduction. Subsequent to oxygen binding, an iron-oxygen complex of undetermined nature initiates lipid peroxidation. Free hydroxyl radicals appear not to participate in lipid peroxidation stimulated by Fe3(+)-OTA.

Influence of ochratoxin B on the ochratoxin A inhibition of phenylalanyl-tRNA formation in vitro and protein synthesis in hepatoma tissue culture cells

Ochratoxin B (OTB), the dechloro-analogue of ochratoxin A (OTA), was studied separately and in combination with OTA on the aminoacylation of phenylalanine tRNA (tRNAPhe) catalysed by mice liver phenylalanyl-tRNA synthetase. OTB was neither a significant inhibitor of the reaction nor an antagonist of OTA. OTB was also assayed for its possible antagonistic effect on the in vivo protein synthesis inhibition caused by OTA in hepatoma tissue culture cells. No prevention of OTA inhibition could be found for OTB. It rather showed a slight additional inhibitory activity when mixed (100-180 microM) with low concentrations of OTA (40-60 microM). In conclusion, these results are not in favor of an antagonistic effect of OTB with respect to OTA action, at least on the level of cellular protein synthesis.

Lipid peroxidation as a possible cause of ochratoxin A toxicity

Addition of the mycotoxin ochratoxin A (OA), a nephrotoxic carcinogen, to rat liver microsomes greatly enhanced the rate of NADPH or ascorbate-dependent lipid peroxidation as measured by malondialdehyde formation. NADPH-dependent lipid peroxidation in kidney microsomes was similarly enhanced by OA. The process required the presence of trace amounts of iron but cytochrome P-450 and free active oxygen species appeared not to be involved. The efficiency of several ochratoxins (ochratoxins A, B, C, alpha and O-methyl-ochratoxin C) to enhance lipid peroxidation was related to the presence and reactivity of the phenolic hydroxyl group. Furthermore, the ability of these ochratoxins to enhance lipid peroxidation in microsomes correlated precisely with their known toxicities in chicks. Administration of ochratoxin A to rats also resulted in enhanced lipid peroxidation in vivo as evidenced by a seven-fold increase in the rate of ethane exhalation. These results suggest that lipid peroxidation may play a role in the observed toxicity of ochratoxin A in animals; a mechanism is proposed. (Formula: see text). Ochratoxin A: X = Cl; R1 = R2 = R3 = R4 = H Ochratoxin B: X = H; R1 = R2 = R3 = R4 = H Ochratoxin C: X = Cl; R1 = R2 = R3 = H; = R4 = CH3 O-Methyl-ochratoxin C: X = Cl; R2 = R3 = H; R1 = R4 = CH3 (4R)-4-hydroxyochratoxin A: X = Cl; R1 = R3 = R4 = H; R2 = OH (4S)-4-hydroxyochratoxin A: X = Cl; R1 = R2 = R4 = H; R3 = OH Fig. 1. Chemical structures of the various ochratoxins.

Evaluation of ochratoxin A for mutagenicity in a battery of bacterial and mammalian cell assays

Ochratoxin A (OA), a nephrotoxic mycotoxin, was evaluated for genotoxic potential in a battery of in vitro and in vivo assays. OA was not mutagenic to Salmonella typhimurium, either with or without metabolic activation, in the plate incorporation (Ames) test at concentrations of 50-600 micrograms OA/plate or in the gradient plate assay at concentrations of 0.1-1000 micrograms OA/ml. No induction of unscheduled DNA synthesis was evident in primary cultures of rat hepatocytes exposed to concentrations of OA ranging from 0.000025 to 500 micrograms/ml. In the mouse lymphoma forward mutation assay, exposure of L5178Y TK+/- mouse lymphoma cells to OA did not increase the numbers of L5178Y TK-/- mutants. There was no significant difference between the numbers of sister-chromatid exchanges in cells from OA-treated Chinese hamsters and those in cells from the negative-control animals.

Analysis of ochratoxin B alone and in the presence of ochratoxin A, using carboxypeptidase A

A method is described for ochratoxin B analysis, which is adapted to the earlier described method of ochratoxin A analysis, using carboxypeptidase A (K. Hult and S. Gatenbeck, J. Assoc. Off. Anal. Chem. 59:128-129, 1976). The fluorescence spectra of ochratoxins A and B coincide too much to allow direct discrimination of the two compounds. A method using the differences in kinetic parameters of the enzymatic hydrolysis of the two compounds is suggested for the analysis of mixtures of the ochratoxins.

Ochratoxin A and citrinin induced nephrosis in Beagle dogs. II. Pathology

Beagle dogs were given ochratoxin A (0.1 and 0.2 mg/kg) and citrinin (5 and 10 mg/kg) alone and in two dose combinations for 14 days. The gross lesions included focal peritonitis and intestinal intussusceptions in dogs given citrinin. Changes in the kidneys of dogs given ochratoxin A were degeneration and necrosis with desquamation of tubular epithelial cells, primarily in the straight segment of the proximal tubules. Dogs given 10 mg/kg citrin had similar changes in the distal tubules and collecting ducts. Dogs given combined doses of citrinin and ochratoxin A had degeneration and necrosis in proximal and distal tubules, and in thin segments and the collecting ducts; there were desquamated cells and granular casts in the lumina. Dogs given ochratoxin A had necrosis of lymphoid tissues in the spleen, tonsil, thymus, peripheral lymph nodes and lymph nodules of the ileum, colon and rectum. There was ulceration of the mucosa of the intestine in dogs given large combined doses of ochratoxin A and citrinin.

Degradation of ochratoxin A by a ruminant

The fate of ochratoxin A during incubation with contents from the four stomachs of the cow was studied. It was concluded that ochratoxin A was cleaved into the nontoxic ochratoxin alpha and phenylalanine by the contents from all but the abomasum.

Ochratoxicosis in Beagle dogs. II. Pathology

The gross lesions in 23 young male Beagle dogs given daily oral doses of 0.2–3.0 mg/kg body weight ochratoxin A consisted of moderate to severe mucohemorrhagic enteritis of the cecum, colon, and rectum; tonsillitis: and enlargement of lymph nodes, which were edematous, hyperemic and focally necrotic. Histopathologic alterations in the kidneys consisted of necrosis and desquamation of epithelial cells mainly in, but not limited to, proximal convoluted tubules. Many proximal and distal convoluted tubules contained eosinophilic, granular casts. Necrosis of lymphoid tissues was a prominent feature of the mycotoxicosis and focally occurred in, but was not limited to, germinal centers in the spleen, tonsils, thymus, and lymph nodes and the lymphoid nodules of the jejunum, ileum, cecum, colon, rectum and nictitating membrane. Necrosis did not occur in bone marrow. Hepatic alterations consisted of slight to moderate centrilobular necrosis and fatty change and occurred mainly in dogs receiving 0.2–0.3 mg/kg body weight ochratoxin A daily for 11–14 days.