Biosynthesis of ochratoxin A

Ochratoxin A Producing Fungi, Biosynthetic Pathway and Regulatory Mechanisms

Ochratoxin A (OTA), mainly produced by Aspergillus and Penicillum species, is one of the most important mycotoxin contaminants in agricultural products. It is detrimental to human health because of its nephrotoxicity, hepatotoxicity, carcinogenicity, teratogenicity, and immunosuppression. OTA structurally consists of adihydrocoumarin moiety linked with l-phenylalanine via an amide bond. OTA biosynthesis has been putatively hypothesized, although several contradictions exist on some processes of the biosynthetic pathway. We discuss recent information on molecular studies of OTA biosynthesis despite insufficient genetic background in detail. Accordingly, genetic regulation has also been explored with regard to the interaction between the regulators and the environmental factors. In this review, we focus on three aspects of OTA: OTA-producing strains, OTA biosynthetic pathway and the regulation mechanisms of OTA production. This can pave the way to assist in protecting food and feed from OTA contamination by understanding OTA biosynthetic pathway and regulatory mechanisms.

Production of(14)C-ochratoxin A byPenicillium verrucosum sp. 1761 in liquid culture

The synthesis of(14)C-labelled ochratoxin A was carried out by biosynthesis withPénicillium verrucosum sp. 1761 in liquid culture using 2-(14)C-Na-acetate and 2-(14)C-malonic acid as precursor. The largest amount of ochratoxin A produced byPénicillium verrucosum was detected in a minimal medium after an incubation period of 28 days. By addition of citric acid and 2,4-dichlorophenoxy acetic acid larger ochratoxin A contents could be measured. For transformation studies of ochratoxin A, a molecule which is exclusively labelled in the isocoumarin moiety is best suitable. This labelling could be obtained by using labelled malonic acid as precursor, whereas with 2-(14)C-Na-acetate as precursor the whole molecule was labelled. Incorporation rates of radioactivity into the molecule were from 0.04% to 0.11%, specific radioactivities from 0.02 μCi/mg to 0.07 μCi/mg.

Biodegradation of ochratoxin a for food and feed decontamination

Ochratoxin A (OTA) is one of the most important mycotoxins that is found in food and feed products. It has proven toxic properties, being primarily known for its nephrotoxicity and carcinogenicity to certain animal species. OTA is produced by several species of Aspergillus and Penicillium that can be found in a wide variety of agricultural products, which makes the presence of OTA in these products common. Many countries have statutory limits for OTA, and concentrations need to be reduced to as low as technologically possible in food and feed. The most important measures to be taken to control OTA are preventive in order to avoid fungal growth and OTA production. However, these measures are difficult to implement in all cases with the consequence of OTA remaining in agricultural commodities. Remediation processes are often used to eliminate, reduce or avoid the toxic effects of OTA. Biological methods have been considered increasingly as an alternative to physical and chemical treatments. However, examples of practical applications are infrequent. This review will focus on the (i) known microorganisms and enzymes that are able to biodegrade OTA; (ii) mode of action of biodegradation and (iii) current applications. A critical discussion about the technical applicability of these strategies is presented.

Metabolism of glutamic acid in Aspergillus ochraceus during the biosynthesis of ochratoxin A

The uptake and utilization of glutamic acid in the biosynthesis of ochratoxin A by Aspergillus ochraceus were studied. Uniformly labeled L[14C]glutamic acid was incorporated into both the phenylalanine and isocoumarin moieties of ochratoxin A. Penicillic acid was also labeled. During the early stages of development, the amino acid was used mainly for the synthesis of ribonucleic acid and protein. A portion of glutamic acid was oxidized and was recovered as metabolic 14CO-2. The initial uptake velocity of glutamic acid decreased with age and was pH and temperature dependent. No relationship was found between the initial uptake velocities and ochratoxin A biosynthesis.

Incorporation of chlorine-36 into ochratoxin A

Incubation of Aspergillus ochraceus NRRL 3174 in a medium containing Na(36)Cl incorporated (36)Cl into ochratoxin A. The highest incorporation (0.75 and 0.70%, respectively) was obtained when the radioactive chloride was added to the medium by the 2nd or 3rd day of incubation.

Acute toxicity of ochratoxins A and B in chicks

Ochratoxins A and B were given to 1-day-old Babcock B-300 cockerels to evaluate acute toxic effects. Two trials with ochratoxin A gave 7-day oral median lethal dose estimates of 116 mug (3.3 mg/kg) and 135 mug (3.9 mg/kg) per chick. Chicks given daily oral doses of 100 mug of ochratoxin A died on the second day. Single subcutaneous doses of 400 mug of ochratoxin A were also lethal. The 7-day oral median lethal dose of B was estimated at 1,890 mug (54 mg/kg) per chick. Chicks given oral doses of 100 mug of ochratoxin B daily for 10 days survived. Sublethal doses of both ochratoxins A and B resulted in growth suppression which was proportional to the amount of ochratoxin given. Visceral gout was the principal gross finding. Microscopic examinations revealed acute nephrosis, hepatic degeneration or focal necrosis, and enteritis. Suppression of hematopoiesis in the bone marrow and depletion of lymphoid elements from the spleen and bursa of Fabricius were frequently seen. Both ochratoxins appeared to have similar pathological effects. This is the first report on the toxicity of ochratoxin B.

Production of ochratoxin A by Aspergillus ochraceus isolated in Japan from moldy rice

A simple thin-layer chromatography-fluorodensitometric method for quantitative analysis of ochratoxin A was developed. This method proved to be of use in investigating the production of the toxin and the nutritional factors affecting the toxin production by two strains of Aspergillus ochraceus isolated from moldy rice in Japan. These fungi produced large amounts of ochratoxin A in a nutrient solution containing 1% l-phenylalanine and 2% yeast extract.