PECTINESTERASES FROM MATURE, UNRIPE PEACH FRUIT BIND STRONGLY TO PECTIC POLYSACCHARIDES

Contrary to previous findings, the level of the pectin de-esterifying enzyme, pectinesterase (PE; EC 3.1.1.11), is shown to be much higher i n mature, green peach fruit than in ripe fruit. Aqueous buffers readil y extracted three pectinesterase isoforms from ripe fruit but only a p ortion of the activity from mature, green fruit. In mature, green frui t extracts the enzyme precipitated when the ionic strength was lowered ; consequently isoforms could not be recovered by ion exchange chromat ography. In extraction residues from mature, green fruit, residual PE could be measured as active enzyme and, when denatured, could be detec ted by immunological techniques. Extraction of the enzyme was enhanced after digestion of the tissue with pectin lyase. The extracted enzyme fractionated as a large molecular weight complex rich in uronic acid, rhamnose, galactose and arabinose. After further digestion with endo- beta-1,4-galactanase, the enzyme was in two fractions of smaller size but with residual carbohydrate. When mature, green and ripe fruit tiss ue were co-extracted, the recovered activity was as predicted from ind ependently extracted tissues demonstrating that enzyme activity was no t influenced by inhibitors contributed by either tissue type. Isoforms known to be present in the ripe fruit were recovered from extracts of the mixed tissues. It is concluded that PE in extracts of mature, gre en fruit has a strong association with pectic polymers and this has le ad to its underestimation in previous studies. It is suggested that su ch an association with pectin polymers in vivo may regulate enzyme act ivity and enzyme turnover.