Degradation of native starch granules by barley alpha-glucosidases

The initial hydrolysis of native (unboiled) starch granules in germinating cereal kernels is considered to be due to alpha-amylases. We report that barley (Hordeum vulgare L.) seed alpha-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 alpha-amylase isozymes. Two alpha-glucosidase charge isoforms were used individually and in combination with purified barley alpha-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 alpha-glucosidase was combined with either high or low pl alpha-amylase. Synergism was also found when low pl alpha-glucosidase was combined with alpha-amylases. Scanning electron micrographs revealed that starch granule degradation by alpha-amylases alone occurred specifically at the equatorial grooves of lenticular granules. Granules hydrolyzed by combinations of alpha-glucosidases and alpha-amylases exhibited larger and more numerous holes on granule surfaces than did those granules attacked by alpha-amylase alone. As the presence of alpha-glucosidases resulted in more areas being susceptible to hydrolysis, we propose that this synergism is due, in part, to the ability of the alpha-glucosidases to hydrolyze glucosidic bonds other than alpha-1,4- and alpha-1,6- that are present at the granule surface, thereby eliminating bonds which were barriers to hydrolysis by alpha-amylases. Since both alpha-glucosidase and alpha-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.