Metabolism of the potato saponins alpha-chaconine and alpha-solanine by Gibberella pulicaris

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, alpha-chaconine and alpha-solanine. The first compound is degraded by both strains via removal of alpha-1,2-L-rhamnose leading to beta(2)-chaconine. This product is converted to the aglycone, solanidine, which is further metabolized to unknown products. The release of alpha-1,2-L-rhamnose is also the first step in the break down of a-solanine by strain R-6380, followed by the removal of the beta-1,3-bound glucose molecule leading to gamma-solanine, which is not metabolized any further. Strain R-7843 is not able to metabolize alpha-solanine. Crude protein extracts of the culture quid of both strains contained enzymes able to convert alpha-chaconine to beta(2)-chaconine, but with no alpha-solanine metabolic activity. This result indicates that G. pulicaris excretes enzymes specific for different saponins. (C) 1997 Elsevier Science Ltd.