Chemical and biochemical reactions of solanapyrone A, a toxin from the chickpea pathogen, Ascochyta rabiei (Pass.) Labr.

Abstract

The absence of solanapyrone A in chickpea plants infected by Ascochyta rabiei or fed the toxin was investigated. Toxin disappeared when incubated with protein extracts of chickpea, either in the presence or absence of NADPH or even if the extracts were boiled. When solanapyrone A was incubated with proteins precipitated by ammonium sulphate (30–70% saturation fraction) a new compound was detected on reversed phase HPLC with a longer retention time than solanapyrone A. This was identified as demethylated solanapyrone A and was subsequently shown to be formed as a result of the reaction of the toxin with residual ammonium sulphate. When ammonium sulphate was rigorously removed from the protein fraction, solanapyrone A still disappeared but the demethylated compound was not found. Incubation with other proteins also caused the disappearance of the toxin as did incubation with NaCl and Tris–HCl. In the latter instance, when the reaction mixture was separated by reversed phase chromatography another compound appeared on the HPLC trace with a λ max of 321 which eluted later than solanapyrone A. These data show that solanapyrone A is a labile compound, reacting with proteins, possibly by binding to them, and with low molecular weight compounds. It is not surprising therefore that it is neither recovered from infected plants nor from plants fed the toxin, where its reactivity is likely to be the reason for its toxicity.