When mature green tomato fruits are stored at 22°C for 30 days, they ripen normally and soften, but if they are kept at 33°C for 15 days (heat treatment), then stored at 22°C they do not soften. The effect of heat treatment on the development of polygalacturonase (PG, EC 3.2.1.15) activities in tomato fruits during storage therefore was studied. When mature green tomato fruits were stored at 22°C, PG activity, which had not been detectable in the fruits, appeared as the color changed and increased dramatically thereafter. PG activity, however, did not appear during heat treatment. When heat-treated fruits were transferred to 22°C, PG activity appeared after a 6-day lag period and increased during the next 30 days at 22°C to about 15of the value detected in ripe tomato fruits.The PG in ripe tomato fruits was composed of two isoenzymes that had different mol wts. A high molecular form (PG-1, mol wt 100K) appeared during the early stage of ripening and a low molecular form (PG-2, mol wt 44K) a little later. PG-2 increased vigorously during ripening and eventually accounted for most of the enzyme activity in the ripe fruits. Only a single isoenzyme (Y-PG, mol wt 100K), however, was detected in heat-treated tomato fruits stored at 22°C for 30 days. PG-1 and Y-PG gave the same mol wt on Sephacryl S-200 gel nitration, but could be separated by Toyopearl HW-55 F gel filt
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