Metabolism of the potato saponins α-chaconine and α-solanine by Gibberella pilicaris

Abstract Potato tubers accumulate varying amounts of several saponins preferentially in the peel. These compounds are toxic to living cells containing sterols in their plasma membrane and are therefore thought to be preformed chemical defence compounds. Two strains of the potato pathogen Gibberella pulicaris (Fusarium sambucinum) , R-6380 and R-7843, were analysed for their ability to metabolize the most predominant saponins found in tubers, α-chaconine and α-solanine. The first compound is degraded by both strains via removal of α-1,2- l -rhamnose leading to β 2 -chaconine. This product is converted to the aglycone, solanidine, which is further metabolized to unknown products. The release of α-1,2- l -rhamnose is also the first step in the break down of α-solanine by strain R-6380, followed by the removal of the β-1,3-bound glucose molecule leading to γ-solanine, which is not metabolized any further. Strain R-7843 is not able to metabolize α-solanine. Crude protein extracts of the culture fluid of both strains contained enzymes able to convert α-chaconine to β 2 -chaconine, but with no α-solanine metabolic activity. This result indicates that G. pulicaris excretes enzymes specific for different saponins.