ENZYMATIC-ACTIVITIES RESPONSIBLE FOR XYLOGLUCAN DEPOLYMERIZATION IN EXTRACTS OF DEVELOPING TOMATO FRUIT

The capacity of salt-soluble enzymic extracts of young green tomato fr uit to reduce the viscosity and molecular size of tamarind xyloglucan (XG) in solution is stimulated markedly by the addition of XG subunit oligosaccharides. This is not due to end-product activation of an endo -l,4-beta-glucanase or XGase, for the following reasons: non-specific endo-l,4-beta-glucanase (carboxymethylcellulase) activities in develop ing tomato fruit were recovered from buffer and detergent-extracts but were barely detectable in subsequent extracts with 1 M NaCl, where XG -depolymerizing activity predominated. Salt-soluble extracts alone wer e totally unable to degrade high M-r XG from tamarind seed in the abse nce of fragments of low M-r XG or oligosaccharide subunits. The degrad ation catalysed by tomato salt extract plus tamarind XG and XG oligosa ccharides failed to proceed to completion or generate low M-r end prod ucts, as happens in the presence of cellulase. The reactions progresse d with no detectable net increase in total reducing power of the react ion mixtures, but were accompanied by condensation of XG cleavage frag ments with XG [C-14]oligosaccharide, as expected from XG endotransglyc osylase (XET) activity. The activity leading to the depolymerization o f XG, as assayed viscometrically in the presence of XG oligosaccharide , co-chromatographed in various media in the same fractions as XET act ivity, as assayed radiometrically. It is concluded that XET activity i n salt extracts of young green tomato is sufficient by itself to expla in the observed capacity of extracts to depolymerize tamarind XG. In v ivo, 1,4-beta-glucanase or XGase activity would also be required to ge nerate the low M-r XG accepters needed for transglycosylase to catalys e a net depolymerization of XG. (C) 1998 Elsevier Science Ltd. All rig hts reserved.