PHOSPHOLIPASE-D DURING TOMATO FRUIT RIPENING

Phospholipase D (PLD; EC 3.1.4.4) was found in tomato fruit pericarp. Some biochemical properties of PLD extract were investigated so as to establish the best conditions for tomato PLD activity determination. T omato PLD activity depends strictly on the presence of Ca2+ ions. The effects of sodium dodecyl sulphate (SDS) and Triton X-100 on the enhan cement of tomato PLD activity is evident in a limited range of concent rations. The stimulating effect of SDS is much more evident at higher concentrations of Ca2+:5-fold increase in activity in the presence of 120 mM Ca2+ and 0.35% of SDS in the incubation mixture. The pH optimum of tomato PLD was between 5.5 and 6.0 and the optimal temperature was 40 degrees C, although PLD was more stable at 30 degrees C. No polymo rphism of PLD was detected by isoelectric focusing or exclusion chroma tography of the enzyme prepared by ammonium sulphate precipitation of the extract. The pI was 4.7. The molecular mass of tomato PLD was prov isionally estimated as 190 kDa. PLD was mainly soluble; little activit y was located in the microsomal fraction. Changes in PLD activity in t omato fruits during on-plant and post-harvest ripening could be correl ated with the total phospholipid decline previously reported to occur during the ripening process. Comparison of PLD activity changes during ripening in normal tomato fruits and in mutant fruits (unable to ripe n) did not reveal any direct relationship between the regulation of ri pening and PLD activity.