POLYGALACTURONASE ISOENZYMES OF FUNGI INVOLVED IN THE BREAKDOWN OF SULFITED STRAWBERRIES

The polygalacturonases of four fungi, Rhizopus stolonifer, Rhizopus sexualis, Mucor piriformis and Aureobasidium pullulans, suspected of involvement in the breakdown of sulphited strawberries, have been resolved into a number of distinct enzyme forms. These isoenzymes differ from each other with respect to isoelectric points, molecular weights, resistance to extremes of pH and temperature, and susceptibility to inhibitors. The isoenzymes produced by any one organism in culture were identical to those produced in inoculated strawberries, although the relative proportions of the different forms was altered. Ten separate samples of microbially contaminated fruit from commercial sources, all of which subsequently suffered disintegration during storage in sulphite liquor, were analysed for polygalacturonase activity. The main isoenzyme present in each batch had an isoelectric point close to 7.1. Comparisons between the properties of polygalacturonases from field sources and those of known origin, revealed that Rhizopus sexualis was the most likely source organism for nine of the ten enzymes, Rhizopus stolonifer probably being responsible for the tenth. The polygalacturonase of isoelectric point near 7.1 was much more effective at macerating sulphited strawberries than the other isoenzymes tested. It also retained activity considerably longer than the others when added to sulphite liquor. It is concluded that a single polygalacturonase isoenzyme secreted by Rhizopus sexualis was responsible for disintegration of all but one of the strawberry samples examined. Copyright © 1979 John Wiley & Sons, Ltd