Pectic polysaccharides are attacked by hydroxyl radicals in ripening fruit: evidence from a fluorescent fingerprinting method

Background and aims Many fruits soften during ripening, which is important commercially and in rendering the fruit attractive to seed-dispersing animals. Cell-wall polysaccharide hydrolases may contribute to softening, but sometimes appear to be absent. An alternative hypothesis is that hydroxyl radicals ((OH)-O-center dot) non-enzymically cleave wall polysaccharides. We evaluated this hypothesis by using a new fluorescent labelling procedure to ` fingerprint' (OH)-O-center dot-attacked polysaccharides. Methods We tagged fruit polysaccharides with 2-(isopropylamino)-acridone (pAMAC) groups to detect (a) any mid-chain glycosulose residues formed in vivo during (OH)-O-aEuro cent action and (b) the conventional reducing termini. The pAMAC-labelled pectins were digested with Driselase, and the products resolved by high-voltage electrophoresis and high-pressure liquid chromatography. Key Results Strawberry, pear, mango, banana, apple, avocado, Arbutus unedo, plum and nectarine pectins all yielded several pAMAC-labelled products. GalA-pAMAC (monomeric galacturonate, labelled with pAMAC at carbon-1) was produced in all species, usually increasing during fruit softening. The six true fruits also gave pAMAC center dot UA-GalA disaccharides (where pAMAC center dot UA is an unspecified uronate, labelled at a position other than carbon-1), with yields increasing during softening. Among false fruits, apple and strawberry gave little pAMAC center dot UA-GalA; pear produced it transiently. Conclusions GalA-pAMAC arises from pectic reducing termini, formed by any of three proposed chain-cleaving agents ((OH)-O-aEuro cent, endopolygalacturonase and pectate lyase), any of which could cause its ripening-related increase. In contrast, pAMAC center dot UA-GalA conjugates are diagnostic of mid-chain oxidation of pectins by (OH)-O-aEuro cent. The evidence shows that (OH)-O-aEuro cent radicals do indeed attack fruit cell wall polysaccharides non-enzymically during softening in vivo. This applies much more prominently to drupes and berries (true fruits) than to false fruits (swollen receptacles). (OH)-O-aEuro cent radical attack on polysaccharides is thus predominantly a feature of ovary-wall tissue.