Enzymic mechanism of starch synthesis in ripening rice grains VI. Isozymes of starch synthetase

The DEAE-cellulose column chromatography has shown two different forms of starch synthetase, which are referred to as fractions I and II in extracts of rice seeds (non-waxy and waxy varieties) harvested at the milky stage. Similarly treated leaf extracts of rice (non-waxy) and maize (non-waxy) also demonstrate die existence of two major isozyme fractions. In all enzyme preparations tested, ADP-glucose was the sole glucosyl donor and UDP-glucose was totally inactive. Rechromatography, on a DEAE-cellulose column, of two enzyme fractions (I and II) separated from non-waxy rice seed extracts did not alter their elution patterns. Some of their enzymic properties were compared, in particular, their glucosyl-acceptor (primer) specificities. Regardless of potent amylase activities in the two fractions, notable differences were observed in that fraction I utilized the long-chain oligosaccharides [maltododecaose] and various types of high molecular α-glucans more readily than fraction II. However, short-chain oligosaccharides [maltose, maltotriose and maltotetraose] were utilized more readily by fraction II than by fraction I. A possible role for the two starch synthetase isozymes in starch synthesis in plant cells is discussed.