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Metabolism and Enzymology

Degradation of Native Starch Granules by Barley -Glucosidases ¹

Department of Agronomy, University of Wisconsin, Madison, Wisconsin 53706, U.S. Department of Agriculture, Agricultural Research Service, Cereal Crops Research Unit, Madison, Wisconsin 53706

The initial hydrolysis of native (unboiled) starch granules in germinating cereal kernels is considered to be due to -amylases. We report that barley (Hordeum vulgare L.) seed -glucosidases (EC 3.2.1.20) can hydrolyze native starch granules isolated from barley kernels and can do so at rates comparable to those of the predominant -amylase isozymes. Two -glucosidase charge isoforms were used individually and in combination with purified barley -amylases to study in vitro starch digestion. Dramatic synergism, as much as 10.7-fold, of native starch granule hydrolysis, as determined by reducing sugar production, occurred when high pl -glucosidase was combined with either high or low pl -amylase. Synergism was also found when low pl -glucosidase was combined with -amylases. Scanning electron micrographs revealed that starch granule degradation by -amylases alone occurred specifically at the equatorial grooves of lenticular granules. Granules hydrolyzed by combinations of -glucosidases and -amylases exhibited larger and more numerous holes on granule surfaces than did those granules attacked by -amylase alone. As the presence of -glucosidases resulted in more areas being susceptible to hydrolysis, we propose that this synergism is due, in part, to the ability of the -glucosidases to hydrolyze glucosidic bonds other than -1,4- and -1,6- that are present at the granule surface, thereby eliminating bonds which were barriers to hydrolysis by -amylases. Since both -glucosidase and -amylase are synthesized in aleurone cells during germination and secreted to the endosperm, the synergism documented here may function in vivo as well as in vitro.

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