The fate of ochratoxin A in rats

Health Effects of Mycotoxins: A Toxicological Overview

Diseases caused by fungi are spread by direct implantation or inhalation of spores. Fungi can cause adverse human health effects to many organ systems. In addition to infection and allergy, fungi can produce mycotoxins and organic chemicals that are responsible for various toxicologic effects. We reviewed the published literature on important mycotoxins and systemic effects of mycotoxins. Scientific literature revealed a linkage between ingesting mycotoxin contaminated food and illness, especially hepatic, gastrointestinal, and carcinogenic diseases. Issues related to mycotoxin exposure, specific diseases, and management are discussed. Although there is agreement that diet is the main source of mycotoxin exposure, specific health effects and risk assessment from indoor nonagricultural exposure are limited by the paucity of scientific evidence currently available. Further research on the health effects of inhaling mycotoxins in indoor settings is needed.

Pharmacokinetics of ochratoxin A and its metabolites in rats

Ochratoxin A (OA) is a mycotoxin that is produced on moist grain. It is commonly found in the blood of swine in western Canada and is a potent nephrotoxic, carcinogen, and immunosuppressive agent. The pharmacokinetic characteristics of six analogs of OA including OA, OB (OA without chloride), OC (OA ethyl ester), and some metabolites, such as O alpha (OA without phenylalanine), OA-OH (hydroxylated OA), and a newly discovered form of OA, OP-OA (lactone opened ring of OA), were investigated in rats after a single intravenous administration of the compounds. All of the ochratoxin analogs were distributed following a two compartment open model. The elimination half-lives of OA, OP-OA, O alpha, OA-OH, OB, and OC were 103+/-16, 50.5+/-2.8, 9.6+/-2.3, 6+/-0.9, 4.2+/-1.2, and 0.6+/-0.2 hr, respectively. Total body clearance of OA, OP-OA, O alpha, OA-OH, and OB via the bile, urine, and metabolic routes were 3.1, 3.6, 40, 65, and 43 ml/hr kg, respectively. OA, OB, and O alpha were mainly cleared in the urine (> or = 48%), OA-OH in the bile (41%), and OP-OA as metabolites (43%). Metabolism accounted for 43, 44, 33, and 29% of the total clearance of OA, O alpha, OA-OH, and OB, respectively. It is concluded that OA has a long half-life and is very slowly cleared from the body and that its metabolites are cleared at a much faster rate with much shorter half-lives. Procedures should be devised to enhance the conversion in the body of OA to O alpha, OA-OH, or other metabolites as this would shorten its half-life and therefore its toxicity.

Ochratoxin A in blood and its pharmacokinetic properties

Since there are pathomorphological similarities between porcine mycotoxic nephropathy caused by ochratoxin A and Balkan endemic nephropathy (BEN), it has been suggested that the same aetiological agent has a role in BEN. Based on the results from several field and experimental studies carried out on pigs, an appropriate analytical method of monitoring possible human exposure to ochratoxin A was developed. The toxicokinetic properties of the toxin were species specific, although in all the animal species studied (with the exception of fish), as well as in humans, two binding proteins were found in the plasma. The monkey had the longest elimination half-life of the toxin, 510 hr, in contrast to the fish whose elimination half-life was only 0.68 hr. The fish kidney displayed a specific pattern of distribution. In the laying quail the most prominent observation was the accumulation of labelled ochratoxin A in egg yolk. Generally, [14C]ochratoxin A was eliminated rapidly from the quail body, but had a long retention time in the circulating blood in the mouse. Although the elimination of ochratoxin A from the body depending on its binding to plasma constituents, the existence of enterohepatic circulation might have been partially responsible for its prolonged retention and elimination from the body of mammals. The toxicokinetic profile of ochratoxin A did not contradict the mycotoxic hypothesis in the aetiology of BEN.

Renal lesions induced by ochratoxin A exposure in the F344 rat

Groups of 80 male and female F344 rats were exposed by gavage to ochratoxin A, a naturally occurring mycotoxin, at levels of 21, 70, and 210 micrograms/kg body weight for up to 2 years. Ochratoxin A induced non-neoplastic renal tubular epithelial changes consisting of cytoplasmic alteration, karyomegaly, degeneration, and cysts. Exposure-related renal tubular proliferative lesions included focal hyperplasia, tubular cell adenoma, and tubular cell carcinoma. Renal tubular cell adenoma occurred as early as 9 months in 1 high-dose male rat, and both adenomas and carcinomas were seen in males by 15 months. At the terminal sacrifice, renal tubular cell tumors were found in both male and female rats, but the response was more pronounced in the males. The incidence of renal tumors in the high-dose rats was the highest of any National Toxicology Program (NTP) study completed to date. In the high-dose males approximately one-third of the renal carcinomas developed metastases. This study demonstrates that ochratoxin A is a potent renal carcinogen in the F344 rat and suggests that contamination of feedstuff by this mycotoxin may pose a potential hazard to domestic animals and man.

Toxicokinetics of ochratoxin A in several species and its plasma-binding properties

The toxicokinetic profile of ochratoxin A was studied after the oral or intravenous administration of 50 ng/g b.w. to fish, quail, mouse, rat and monkey. The elimination half-life varied from 0.68 h after oral administration to fish, up to 840 h after intravenous administration to monkey. The distribution volume ranged from 57 ml/kg in fish to 1500 ml/kg in quail. The plasma clearance was most rapid in quail and fish, 72 and 58 ml/kg.h, respectively, while it was only 0.17 ml/kg.h in monkey. The bioavailability was as low as 1.6% in fish but as high as 97% in mouse. The binding abilities of ochratoxin A to plasma proteins were also studied. From these data we calculated the free fraction of toxin in plasma, which we found to be less than 0.2% in all species investigated (including man) except fish. A similar but smaller investigation on the toxicokinetics and binding properties of ochratoxin B was also performed. Ochratoxin B was more readily eliminated and had a lower affinity for plasma proteins, which partly may explain its lower toxicity.

Uptake of ochratoxin A by slices of pig kidney cortex

Ochratoxin A (OCT A) is a nephrotoxin causing selective necrosis of the proximal tubule. Being an organic anion OCT A might be expected to enter the tubule cells by the organic anion transport system. Pig renal cortical slices were used to characterize the OCT A transport. OCT A (5 X 10(-3) mM) was accumulated against a concentration gradient with a slice to medium ratio of 8.9 +/- 2.9 in the presence of oxygen. This accumulation was markedly reduced in a nitrogen atmosphere (S/M ratio = 2.9 +/- 0.5). OCT A accumulation was dependent on medium concentration. With increasing concentration (5 X 10(-4)-5 X 10(-1) mM), slice to medium ratio initially rose from 6.9 +/- 2.0 to 11.7 +/- 1.2 whereupon it declined to 5.4 +/- 1.1. This pattern indicates that both carrier mediated transport and intracellular metabolism may contribute to the net accumulation of the toxin. OCT A (10(-4) to 1 mM) inhibited p-aminohippurate (PAH) and phenolsulphophthalein (PSP) uptake in a dose-dependent manner. Up to 10(-1) mM, OCT A did not inhibit acetylation of PAH suggesting that aerobic metabolism and the energy supply for the transport process were unaffected. Kinetic studies revealed a competitive inhibition of the PSP transport. It is concluded that OCT A enters the proximal tubule cells by the common organic anion transport system.

Distribution of 14C-ochratoxin A in the mouse monitored by whole-body autoradiography

The tissue distribution of 14C-labeled ochratoxin A was studied in mouse using whole-body autoradiography. The distribution was followed for 18 days after one single intravenous injection of 5 microCi/animal, corresponding to 160-230 ng toxin/g body weight. Very long persistence of 14C-ochratoxin A in the circulation was noticed and the toxin was detected in the blood even after 18 days when the experiment was finished. The radioactivity in the kidney was unequally distributed with a slightly higher concentration in the inner cortical and medullary parts. This was seen from 24 hrs and on after injection. Very high concentrations of radioactivity were found in the bile of treated animals. The radioactivity extracted from several sections was chemically characterized with thin-layer chromatography and was found to represent 14C-ochratoxin A.

Effects of plasma ochratoxin A and luminal pH on the jejunal absorption of ochratoxin A in rats

The effects of iv injection of ochratoxin A (OA) on its absorption from the jejunum in rats were studied in vivo to demonstrate the jejunal absorption of OA in relation to the OA level in blood plasma. In addition the effects of the pH of the medium on OA uptake by the everted jejunum of the rat were studied in vitro to assess the contribution made by transfer of the non-ionized form of OA to the jejunal absorption of the toxin. The in vivo study showed that OA was absorbed from the jejunum even when its level was higher in the plasma than in the jejunal lumen. Jejunal uptake of OA in vitro was increased with a decrease in medium pH, the increase in uptake coinciding with an increase in the proportion of OA present in the non-ionized form. These results suggest that OA absorption is attributable mainly to transfer of the non-ionized form, and that a concentration gradient of the non-ionized form from the jejunal lumen to the blood and lymph may be achieved even when the total OA level is higher in the blood plasma than in the lumen.

Evidence for an enterohepatic circulation of ochratoxin A in mice

The distribution and elimination of [3H]ochratoxin A (OTA) from stomach content and tissue, intestine content and tissue, liver, bile, serum and urine of Swiss male mice which had received a single low dose of OTA by intubation was followed as a function of time. The profiles of radioactivity do not show a smooth decline after the absorption period, but an oscillating pattern with rapid declines followed by increases which favour the assumption of an enterohepatic circulation. Between 28% and 68% of conjugated OTA together with OTA cleavage products were found in bile giving evidence for biliary excretion of OTA and its metabolites in mice. When given i.m. to mice [3H]OTA is already found after 30 min in bile and intestine contents and its elimination patterns show several peaks confirming the biliary excretion and the enterohepatic circulation. Cholestyramine, which is known to prevent the enterohepatic circulation of drugs and toxins, changes the profile of elimination of OTA which no longer presents the cyclic pattern. This result is also in favour of an enterohepatic circulation of OTA. When phenylalanine is given together with OTA by oral gavage the toxicokinetics of the mycotoxin change completely in the different body fluids, in stomach and intestine content and tissues. Phenylalanine seems to facilitate the gastric absorption of OTA and the gastro-intestinal transit. It increases also its early excretion into urine and bile. However, its elimination pattern no longer shows the oscillating pattern. Thus phenylalanine seems to inhibit the intestinal reabsorption of OTA conjugates.

Placental transfer of ochratoxin A and its cytotoxic effect on the mouse embryonic brain

Pregnant ICR mice were given a single ip injection of 5 mg ochratoxin A/kg on day 11 or 13 of pregnancy. Concentrations of ochratoxin A in the maternal serum and tissues reached maximum levels within 2 hr of the injection and then decreased rapidly. The half-life of the toxin in serum was 28.7 hr on day 11 and 23.6 hr on day 13. On the other hand, the concentrations of ochratoxin A in the embryos were very low 2 hr after injection and then gradually increased up to 48 and 30 hr after injection on day 11 and 13, respectively. Pharmacokinetically, the embryo was found to be a 'deep' compartment. In mice treated with ochratoxin A on day 10 of pregnancy, the incidence of pyknotic cells in the telencephalon of the embryos began to increase 12 hr after injection to a peak between 36 and 48 hr, coinciding with the time of peak concentration of the toxin in the embryo.

Distribution of the [3H]-label from low doses of radioactive ochratoxin A ingested by rats, and evidence for DNA single-strand breaks caused in liver and kidneys

The distribution of a single low dose of [3H]-ochratoxin A (OTA) in different tissues of male Wistar rats, after administration by intubation, was investigated after 5 h, 24 h and 48 h. This dose corresponds to concentrations encountered in naturally contaminated feed (4 ppm). The distribution of [3H]-label varied with the time elapsed after administration; at 5 h the highest specific label was found in the stomach contents and in decreasing order in: intestinal contents, lung, liver, kidney, heart, fat, intestine, testes, and the lowest in muscles, spleen and brain. With exception of brain, fat, stomach and lung, all tissues showed maximum levels at 24 h, after which time the label decreased steadily, whereas in fat it increased. After a 12-week feeding experiment, with doses of 288.8 micrograms/kg corresponding to an intake of 4 ppm in feed each 48 h, the DNA in liver and kidneys was investigated for damage. By the alkaline elution method combined with micro-spectrofluorimetric determinations of DNA, evidence for DNA single-strand breaks was obtained. These findings support reports on the carcinogenic action of OTA.

Changes of renal mRNA species abundance by ochratoxin A

Ochratoxin A is a nephrotoxin produced by certain species of Aspergillus and Penicillium. We have found previously that renal but not hepatic P-enolpyruvate carboxykinase, and the mRNA for this enzyme, are rapidly decreased in rats and swine fed 0.1 to 1 mg/kg body weight for a few days. In the present study, we isolated kidney mRNA from rats fed ochratoxin A for 2-5 days. By screening a rat kidney cDNA library with [32P]RNA, we have identified several renal mRNAs whose concentration is changed within 2 days by the toxin. The transcription rate of each mRNA was measured in nuclei isolated from kidneys of rats fed ochratoxin A. The incorporation of [32P]UMP into P-enolpyruvate carboxykinase mRNA and the synthesis of other RNAs were not affected. Therefore, the toxin changes mRNA abundance at the post-transcriptional level.

Ochratoxin A: plasma concentration and excretion into bile and urine in albumin-deficient rats

The fate of ochratoxin A (OA) injected iv was studied in both albumin-deficient and normal rats. The OA concentration in plasma decreased to a level below 0.5 micrograms/ml within 10 min of the injection in albumin-deficient rats, but remained above 50 micrograms/ml for 90 min in normal rats. The OA concentrations in bile and urine, and the rate of OA excretion in these fluids were 20-70-fold higher in albumin-deficient than in normal rats. The results demonstrate that a primary effect of albumin binding on OA is to retard its elimination by restricting the entry of OA into the hepatic and renal cells.

Distribution of 14C-labelled ochratoxin A in pregnant mice

Autoradiography was used to study the distribution of 14C-labelled ochratoxin A for up to 4 hr after its iv administration to mice at various stages of pregnancy. The highest 14C concentration was consistently found in the bile throughout the experimental period. The concentration of radioactivity in the tissues was found, in decreasing order, in the liver, kidney, blood, salivary glands, large vessels, brown fat, myocardium, uterus and lymphatic tissues. The toxin was shown to cross the placental barrier on day 9 of pregnancy, at which time it is most effective in producing foetal malformations.

Decrease of renal phosphoenolpyruvate carboxykinase RNA and poly(A)+ RNA level by ochratoxin A

Ochratoxin A, a nephrotoxin produced by Aspergillus ochraceus, decreases the activity of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK) in the cytosol of rat kidneys, as well as inhibits renal gluconeogenesis (Meisner, H., and Meisner, P. (1981) Arch. Biochem. Biophys. 208, 146-153). Ochratoxin A greatly reduces the level of translatable mRNA for PEPCK in kidneys of rats fed the toxin for 2 days, while the efficiency of translation of poly(A)+ RNA is not affected. A species of poly(A)+ RNA coding for a 72,000 Mr protein is increased in relative amount. Although similar in molecular weight to PEPCK, this protein is not precipitated by an antibody to PEPCK, and has a different peptide map. The sequence abundance of PEPCK mRNA, as measured by either Northern blotting or the dot blot technique, is reduced, while the hepatic level of PEPCK mRNA is not changed. The total poly(A)+ RNA level is reduced 50% in kidneys, but not livers, of rats fed a standard dose of ochratoxin A for 3-5 days. In nuclei isolated from toxin-fed rats, the rate of transcription of total RNA or PEPCK mRNA, as measured by incorporation of [32P]UTP, is not reduced by the toxin. Ochratoxin A therefore lowers total renal mRNA concentration, and certain species, notably PEPCK, are reduced to a greater extent than the bulk of the RNA pool.

Metabolism of ochratoxin A by rats

Albino rats were given ochratoxin A (6.6 mg/kg body weight) intraperitoneally or per os. Independent of route administration, 6% of a given dose was excreted as the toxin, 1 to 1.5% as (4R)-4-hydroxyochratoxin A, and 25 to 27% as ochratoxin alpha in the urine. The metabolite (4S)-4-hydroxyochratoxin A, which is formed by rat liver microsomes in the presence of NADPH, was not detected. Only traces of ochratoxins A and alpha were found in feces. Identical experiments were carried out with brown rats, since the Km value for the formation of the 4S epimer was considerably lower when brown rat microsomes were used. About the same ratios of metabolites and metabolite recoveries as those found for albino rats were found for brown rats. Brown rats were also given the two hydroxylated metabolites and ochratoxin alpha (0.66 mg/kg body weight) intraperitoneally. The three compounds were excreted in the urine; within 48 h, 90% recovery of ochratoxin alpha and 54 and 35%, respectively, of the 4R and 4S isomers were observed.

Formation of 4-hydroxyochratoxin A from ochratoxin A by rat liver microsomes

Hydroxyochratoxin A was isolated and identified from the urine of rats after injection with ochratoxin A. By incubating ochratoxin A with rat liver microsomes and reduced nicotinamide adenine dinucleotide phosphate, one major (90%) and two minor metabolites, more polar than ochratoxin A, were formed. Thin-layer chromatography revealed that the major metabolite had Rf values identical to those of hydroxyochratoxin A in six different solvent systems. Formation of the metabolites in vitro was inhibited by carbon monoxide and by metyrapone, and the rate of formation increased after pretreatment of the rats with phenobarbital. A type I spectrum appeared upon binding of ochratoxin A to microsomes with a spectral dissociation constant (Ks) of 37.6 microM. These findings strongly suggest the involvement of a cytochrome P-450 in the hydroxylation of ochratoxin A by rat liver microsomes. Apparent Km and Vmax values for the formation of hydroxyochratoxin A were determined to 50 microM and 5.5 nmol/mg of protein per h, respectively.

Tissue distribution of radioactivity from ochratoxin A-14C in rats

Examination of the distribution of radioactivity in rat tissues during the first 24 hr after administration of ochratoxin A-14C demonstrated maximum accumulation in stomach and kidney. The highest counts were observed in stomach, lung, kidney, thymus, spleen and heart during the first 6 hr after treatment, whereas the brain, liver muscle, duodenum, jejunum, ileum, and cecum exhibited the greatest counts at 18 hr after toxin exposure.