8. POLYSACCHARIDASES
393
of corn amylopectin by ß-amylase. The molecular weight of the limit
dextrin was not significantly reduced by the glucosidase action. After
treatment of limit dextrin with yeast glucosidase and subsequent heat
inactivation of the latter enzyme, the polysaccharide had become again
susceptible to β-amylase action. Thus, a second limit dextrin was obtained which still stained with iodine. In this manner, alternate degradation of limit dextrin by ß-amylase and by yeast glucosidase led
to a progressive, step-by-step degradation of the substrate, which maintained its iodine staining capacity and its macromolecular properties
during several steps. This stepwise degradation of amylopectin by the
two enzymes was the basis that led Meyer and Bernfeld to advance
the bushlike structure of amylopectin (233).
The yeast «-glucosidase has not been sufficiently purified to claim
its specificity for 1,6-bonds alone. Evidence to prove or to disprove the
ability of this enzyme to split a-l,4-glucosidic bonds is not available.
The existence of a-l,6-glucosidases in higher plants (barley malts)
has been demonstrated by Kneen (127, 234) and Schwimmer (235),
while Kerr and co-workers (236, 237) have shown a similar enzyme to
occur in molds. The enzyme was called "limit dextrinase" because of its
ability to make limit dextrins available to saccharification by /?-amylase,
though the mechanism of their action is not completely established. Although it is not known whether barley limit dextrinase action is confined to the 1,6-glucosidic bonds, a mold amyloglucosidase from Aspergillus niger has been shown to break down amylose and amylopectin,
as well as limit dextrins (238) and, thus, to hydrolyze both types of
bonds. The action of a limit dextrinase from A. niger on dextran was
found to be extremely slow, though isomaltase was hydrolyzed rapidly
(239). A crystalline preparation of limit dextrinase from A. oryzae has
been obtained by Underkofler and Roy (166).
Another enzyme that appears to be capable of hydrolyzing both
types of bonds is the maltase of Clostridium acetobutylicum, first described by Hockenhull and Herbert (154). The action of this enzyme
was found by French and Knapp (153) to consist in the hydrolytic removal of individual glucose units from the nonreducing ends of amylopectin, amylose, or oligosaccharide. It is capable of hydrolyzing at
least the majority of the branch linkages, resulting in extensive hydrolysis of amylopectin, amylose, or dextrins to glucose.
Black-fco/ϊ molds (Aspergillus usanii) have been found to contain a
"debranching" enzyme (240), which has not been further defined.
4. Conclusions on a-Glucosidases
The physiological significance of α-glucosidases lies obviously in the
tendency of the organism to break down starch or glycogen to the level
393
of corn amylopectin by ß-amylase. The molecular weight of the limit
dextrin was not significantly reduced by the glucosidase action. After
treatment of limit dextrin with yeast glucosidase and subsequent heat
inactivation of the latter enzyme, the polysaccharide had become again
susceptible to β-amylase action. Thus, a second limit dextrin was obtained which still stained with iodine. In this manner, alternate degradation of limit dextrin by ß-amylase and by yeast glucosidase led
to a progressive, step-by-step degradation of the substrate, which maintained its iodine staining capacity and its macromolecular properties
during several steps. This stepwise degradation of amylopectin by the
two enzymes was the basis that led Meyer and Bernfeld to advance
the bushlike structure of amylopectin (233).
The yeast «-glucosidase has not been sufficiently purified to claim
its specificity for 1,6-bonds alone. Evidence to prove or to disprove the
ability of this enzyme to split a-l,4-glucosidic bonds is not available.
The existence of a-l,6-glucosidases in higher plants (barley malts)
has been demonstrated by Kneen (127, 234) and Schwimmer (235),
while Kerr and co-workers (236, 237) have shown a similar enzyme to
occur in molds. The enzyme was called "limit dextrinase" because of its
ability to make limit dextrins available to saccharification by /?-amylase,
though the mechanism of their action is not completely established. Although it is not known whether barley limit dextrinase action is confined to the 1,6-glucosidic bonds, a mold amyloglucosidase from Aspergillus niger has been shown to break down amylose and amylopectin,
as well as limit dextrins (238) and, thus, to hydrolyze both types of
bonds. The action of a limit dextrinase from A. niger on dextran was
found to be extremely slow, though isomaltase was hydrolyzed rapidly
(239). A crystalline preparation of limit dextrinase from A. oryzae has
been obtained by Underkofler and Roy (166).
Another enzyme that appears to be capable of hydrolyzing both
types of bonds is the maltase of Clostridium acetobutylicum, first described by Hockenhull and Herbert (154). The action of this enzyme
was found by French and Knapp (153) to consist in the hydrolytic removal of individual glucose units from the nonreducing ends of amylopectin, amylose, or oligosaccharide. It is capable of hydrolyzing at
least the majority of the branch linkages, resulting in extensive hydrolysis of amylopectin, amylose, or dextrins to glucose.
Black-fco/ϊ molds (Aspergillus usanii) have been found to contain a
"debranching" enzyme (240), which has not been further defined.
4. Conclusions on a-Glucosidases
The physiological significance of α-glucosidases lies obviously in the
tendency of the organism to break down starch or glycogen to the level