[Ochratoxins: Molecular strategies for developing an antidote]

Ochratoxin A (OcA) is a prominent member of a group of mycotoxins which display nephrotoxic, genotoxic, teratogenic, carcinogenic and immunosuppressive effects and which have also been linked to Balkan Endemic Nephropathy. The toxicity of OcA is thought to be primarily due to its inhibition of phenylalanine-t-RNA synthetase, a phenylalanine-metabolizing enzyme. Based on the three-dimensional structure of phenylalanine-t-RNA synthetase, we have analyzed its interactions with OcA by means of molecular-dynamical simulations and identified three quite different binding modes, all of which suggest an affinity only in the millimolar range. This would seem to be in conflict with toxicological findings frequently cited in textbooks but is in agreement with recent in vitro studies on purified phenylalanine-t-RNA synthetase, which also exclude this enzyme as the main target for OcA action. In vivo, OcA binds preferentially to serum albumin, a plasma protein, with a corresponding effect on its toxicokinetics (retention). Antagonizing this effect would lead to an enhanced elimination rate, thereby reducing all adverse effects of OcA, as has been demonstrated using albumin-deficient mice. Based on the three-dimensional structure of serum albumin, we have simulated its interaction with OcA. The long-term goal is the animal-free identification of a synthetic antagonist with an affinity between that of the endogenous ligands (e.g. billirubin) and OcA. Such a substance could - by reducing the retention time of the toxin in the body - potentially eliminate all toxic effects of OcA.