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

The C-14-labelled glucan synthesised by soluble starch synthase (ADPgl ucose: 1,4-alpha-D-glucan-4-glucosyl transferase, EC 2.4.1.21) is part ially degraded to glucose and maltose in assays of crude extracts of s ome plant organs. As these degradation products are not precipitated b y treatment with aqueous methanol-KCl (75% v/v: 1% w/v), the conventio nal means by which the product of the starch synthase reaction is sepa rated from the substrate, ADP[U-C-14]glucose, the activity of the enzy me can be seriously underestimated. An alternative procedure involving the absorption of unreacted ADP[U-C-14]glucose with anion exchange re sin provides a more accurate estimate of the activity.