CAUTION ON THE USE OF THE GENERALLY ACCEPTED METHANOL PRECIPITATION TECHNIQUE FOR THE ASSAY OF SOLUBLE STARCH SYNTHASE IN CRUDE EXTRACTS OF PLANT-TISSUES

The C-14-labelled glucan synthesised by soluble starch synthase (ADPglucose: 1,4-alpha-D-glucan-4-glucosyl transferase, EC 2.4.1.21) is partially degraded to glucose and maltose in assays of crude extracts of some plant organs. As these degradation products are not precipitated by treatment with aqueous methanol-KCl (75% v/v: 1% w/v), the conventional means by which the product of the starch synthase reaction is separated from the substrate, ADP[U-C-14]glucose, the activity of the enzyme can be seriously underestimated. An alternative procedure involving the absorption of unreacted ADP[U-C-14]glucose with anion exchange resin provides a more accurate estimate of the activity.