Conversion of the polygalacturonase isoenzymes from ripening tomato fruits.

From pericarp tissue of ripening tomato (Lycopersicon esculentum Mill. cv. Sonato), two isoenzymes of polygalacturonase, PG1 and PG2, can be extracted. With water hardly any polygalacturonase activity is extracted; with 0.5 M NaCl predominantly PG2 is found and subsequent extraction with 1.25 M NaCl delivers mainly PG1. A partly purified PG1 solution gradually decomposes into PG2. Conversion of PG2 into an isoenzyme that resembles PG1, but differs from it, can be brought about by a factor (convertor) that occurs at low levels in free form in unripe and early-ripe fruits as well as in leaves. Convertor (CV) can be set free from PG1 by a short treatment at 100°C, at which temperature the convertor activity itself also decreases. The in vitro activities and several characteristics of the isoenzymes and CV as found by us differ from that found by others, probably because of carefully optimized methods. It is suggested that the CV anchors PG2 onto the cell wall, forming PG1. Thus PG1 would constitute the form that depolymerizes the pectins in the middle lamellae. [ABSTRACT FROM AUTHOR]