Mini reviewToxicokinetics and toxicodynamics of ochratoxin A, an update
Introduction
Ochratoxins are a group of secondary metabolites produced by fungi of two genera: Penicillium and Aspergillus. Except ochratoxin α (OTα), the ochratoxins comprise a polyketide-derived dihydroisocoumarin moiety linked via the 7-carboxy group to l-β-phenylalanine by an amide bond. Ochratoxins consist of ochratoxin A (OTA), its methyl ester, its ethyl ester also known as ochratoxin C (OTC), 4-hydroxyochratoxin A (4-OH OTA), ochratoxin B (OTB) and its methyl and ethyl esters and ochratoxin α (OTα), where the phenylalanine moiety is missing (Fig. 1).
OTA is the most toxic member in the group. It was first isolated from Aspergillus ochraceus Wilh. in a laboratory screening for toxigenic fungi [1]. Its configuration was determined using optical rotatory dispersion spectroscopy [2], [3]. The empirical formula is C20H18O6NCl and the molecular weight is 403.82. The IUPAC developed formula of OTA is l-phenylalanine-N-[(5-chloro-3,4-dihydro-8-hydroxy-3-methyl-1-oxo-1H-2-benzopyran-7-yl)carbonyl]-(R)-isocoumarin (Fig. 2).
The chemical abstract specification (CAS) of OTA is 303-47-9. It is a white, crystalline compound, highly soluble in polar organic solvents, slightly soluble in water and soluble in aqueous sodium hydrogen carbonate. The melting points are 90 and 171 °C, when recrystallized from benzene (containing 1 mol benzene/mol) or xylene, respectively [4]. OTA exhibits UV adsorption: [5]. The fluorescence emission maximum is at 467 nm in 96% ethanol and 428 nm in absolute ethanol. The infrared spectrum in chloroform includes peaks at 3380, 1723, 1678 and 1655 cm−1[6]. OTA has weak acidic properties. The pKa values are in the ranges 4.2–4.4 and 7.0–7.3, respectively, for the carboxyl group of the phenylalanine moiety and the phenolic hydroxyl group of the isocoumarin part [7], [8], [9].
OTA production is dependent on different factors such as temperature, water activity (aw) and medium composition, which affect the physiology of fungal producers.
In cool and temperate regions, OTA is mainly produced by Penicillium verrucosum[10], [11], [12], [13] or P. nordicum[12], [13]. P. verrucosum mainly contaminates plants such as cereal crops, whereas P. nordicum has been mainly detected in meat products and cheese [12]. In tropical and semitropical regions, OTA is mainly produced by Aspergillus ochraceus[14], [15], [16]. A. ochraceus is also referred to as A. allutaceus var allutaceus Berkely and Curtis [14]. A. ochraceus have been reported in a large variety of matters like nuts, dried peanuts, beans, spices, green coffee beans and dried fruits, but also in processed meat and smoked and salted fish [16]. Two other species of Aspergillus section Nigri, respectively, A. niger var niger[17], [18] and A. carbonarius[19], [20] have been reported as OTA producers. The OTA contamination of substrata such as cereals, oilseeds and mixed feeds in warm zones is thought to be due to A. niger var niger in addition to A. ochraceus species [21], whereas A. carbonarius seems to be more common on grapes, raisins and coffee [22], [23]. Recently, Samson et al. [24] isolated two new OTA producing Aspergillus species from coffee beans. These species, A. lacticoffeatus and A. sclerotioniger, need further investigations and are provisionally accepted in section Nigri. In addition, another Aspergillus species, A. alliaceus also named Petromyces alliaceus and isolated from onions [25], has been previously reported as OTA producer under laboratory conditions [26]. This species has been suspected to be responsible for the occasional OTA contamination in Californian figs [27], [28] and Argentinean medicinal herbs [29].
The biosynthetic pathway for OTA has not yet been completely established. However, labelling experiments using both 14C- and 13C-labelled precursors showed that the phenylalanine moiety originates from the shikimate pathway and the dihydroisocoumarin moiety from the pentaketide pathway (Fig. 3). The first step in the synthesis of the isocoumarin polyketide consists in the condensation of one acetate unit (acetyl-CoA) to four malonate units. Recent data showed that this step requires the activity of a polyketide synthase [30]. Moreover, the gene encoding polyketide synthase appears to be very different between Penicillium and Aspergillus species [30], [31]. In A. ochraceus, the gene of polyketide synthase is expressed only under OTA permissive conditions and only during the early stages of the mycotoxin synthesis [30]. No such data are presently available on Penicillium. In Penicillium species, Geisen et al. [31] observed that P. nordicum and P. verrucosum use two different polyketide synthases for OTA synthesis. This difference is probably related to the P. verrucosum ability to produce CIT, also a polyketide-based mycotoxin, in addition to OTA. Once formed, the polyketide chain is modified through the formation of a lactone ring (synthesis of mellein) and the addition of a carboxyl group derived from the C1 pool such as S-methylmethionine and sodium formate (synthesis of ochratoxin β) [32]. Subsequently, the chlorine atom is incorporated through the action of chloroperoxidase (synthesis of Ochratoxin α, OTα). Ultimately, ochratoxin A synthetase catalyzes the linking of OTα to phenylalanine (synthesis of OTA) [33], [34].
Section snippets
Toxicokinetics
Both toxicokinetic (the changes of concentrations of a compound in the organism over time) and toxicodynamic (the dynamic interactions of a compound with biological targets and their downstream biological effects) factors determine the toxicity of OTA.
Upon absorption from the gastrointestinal tract, OTA binds to serum proteins. Considerable variations in serum half-lives across species are known to be dependent on the affinity and degree of protein binding. Reabsorption of OTA from the
Toxicodynamics
Several hypotheses on the mechanism of interaction of OTA and its metabolites with endogenous molecules have been put forward to explain its toxicity. They are related to specific interactions, based on highly specific binding onto specific sites of a target molecule and, non-specific interactions, based on the chemical reactivity of OTA and its metabolites and their vicinity to the target molecule.
Conclusions and future perspectives
OTA has been classified as a nephrotoxic, hepatotoxic, immunotoxic and teratogenic compound. As a natural poison contaminating a large variety of plant products and leading to the presence of residues in the products of animal origin, OTA contributes to the contamination of humans. At common concentrations in food, OTA may also be regarded as a modulator of cellular signalling and not as a classical toxin. In addition, OTA is reasonably anticipated to be a possible human carcinogen based on
References (237)
- et al.
The synthesis of ochratoxin A and B metabolites of Aspergillus ochraceus Wilh
Tetrahedron
(1967) Chromatographic methods for the determination of ochratoxin A in animal and human tissues and fluids
J. Chromatogr. A
(1998)- et al.
Molecular characterization of ochratoxin A producing strains of the genum Penicillium
Syst. Appl. Microbiol.
(2002) - et al.
Effect of water activity and temperature on mycelial growth and ochratoxin A production by isolates of Aspergillus ochraceus on irradiated green coffee beans
J. Food Prot.
(2005) - et al.
Influence of water activity and temperature on growth of isolates of Aspergillus section nigri obtained from grapes
Int. J. Food Microbiol.
(2004) - et al.
Occurrence of Aspergillus species in mixed feeds and component raw materials and their ability to produce ochratoxin A
Food Microbiol.
(2004) - et al.
What is the source of ochratoxin A in wine ?
Int. J. Food Microbiol.
(2002) - et al.
Assessment of toxinogenic fungi on Argentinean medicinal herbs
Microbiol. Res.
(2004) - et al.
Development of a real time PCR system for detection of Penicillium nordicum and for monitoring ochratoxin A production in foods by targeting the ochratoxin polyketide synthase gene
Syst. Appl. Microbiol.
(2004) - et al.
Biosynthesis of ochratoxin A
Tetrahedron Lett.
(1971)