390
PETER BERNFELD
branching and contains shorter branches with uninterrupted sequences
of a-l,4-glucosidic bonds than amylopectin, occurs almost exclusively in
animals; i.e., glycogen is always synthesized in the absence of ß-amylase.
The less-branched amylopectin, however, containing longer a-l,4-glucosidic chains than glycogen, and the unbranched amylose with very
long «-1,4-glucosidic chains are exclusively synthesized in plants. It
appears logical to assume, therefore, that /?-amylase occurring only in
plants, like starch, may in some way be connected with the less frequent occurrence in starch than in glycogen of the a-l,6-glucosidic
branching points. It is realized, however, that other mechanisms for
the regulation of the degree of branching in the starch polysaccharides
must exist.
Inasmuch as a comparison between the two pure /?-amylases is permitted on the basis of the few data available, it appears that their action on the substrate is identical, as seen from the close agreement between their ratios of saccharifying to dextrinizing activity (see Table
XVI). The chemical composition of the cereal enzyme differs, however, from that of the tuber enzyme; the nitrogen contents, the isoelectric points, and the values for specific activity per protein differ markedly for the two enzymes.
C. «-GLUCOSIDASES
In contrast to the amylases, which produce appreciable amounts of
glucose only during the final stages of starch and glycogen breakdown,
the enzymes that liberate glucose as the initial and sole product of
hydrolytic action on these polysaccharides are generally called «-glucosidases. Because of their common specificity for the starch polysaccharides, this class of enzymes is also frequently referred to as amyloglucosidases. «-Glucosidases that split low-molecular oligosaccharides,
e.g., the maltose-hydrolyzing enzyme maltase and others, are rather
similar to the amyloglucosidases; they frequently occur together with
them, and some of them are able to attack polysaccharides as well as
oligosaccharides, so that a clear distinction between amyloglucosidases
and «-oligoglucosidases is not always possible.
«-Glucosidases may be divided into three classes, i.e., those that
break only «-1,4-glucosidic bonds, «-glucosidases specific for «-1,6-glucosidic linkages, and enzymes that may hydrolyze either type of bond.
In many instances the specificity of the enzyme for one or the other of
these two bonds has not been exactly established; other «-glucosidases
are actually able to hydrolyze both 1,4- and 1,6-glucosidic linkages,
and some «-glucosidases, though probably specific for only one type of
PETER BERNFELD
branching and contains shorter branches with uninterrupted sequences
of a-l,4-glucosidic bonds than amylopectin, occurs almost exclusively in
animals; i.e., glycogen is always synthesized in the absence of ß-amylase.
The less-branched amylopectin, however, containing longer a-l,4-glucosidic chains than glycogen, and the unbranched amylose with very
long «-1,4-glucosidic chains are exclusively synthesized in plants. It
appears logical to assume, therefore, that /?-amylase occurring only in
plants, like starch, may in some way be connected with the less frequent occurrence in starch than in glycogen of the a-l,6-glucosidic
branching points. It is realized, however, that other mechanisms for
the regulation of the degree of branching in the starch polysaccharides
must exist.
Inasmuch as a comparison between the two pure /?-amylases is permitted on the basis of the few data available, it appears that their action on the substrate is identical, as seen from the close agreement between their ratios of saccharifying to dextrinizing activity (see Table
XVI). The chemical composition of the cereal enzyme differs, however, from that of the tuber enzyme; the nitrogen contents, the isoelectric points, and the values for specific activity per protein differ markedly for the two enzymes.
C. «-GLUCOSIDASES
In contrast to the amylases, which produce appreciable amounts of
glucose only during the final stages of starch and glycogen breakdown,
the enzymes that liberate glucose as the initial and sole product of
hydrolytic action on these polysaccharides are generally called «-glucosidases. Because of their common specificity for the starch polysaccharides, this class of enzymes is also frequently referred to as amyloglucosidases. «-Glucosidases that split low-molecular oligosaccharides,
e.g., the maltose-hydrolyzing enzyme maltase and others, are rather
similar to the amyloglucosidases; they frequently occur together with
them, and some of them are able to attack polysaccharides as well as
oligosaccharides, so that a clear distinction between amyloglucosidases
and «-oligoglucosidases is not always possible.
«-Glucosidases may be divided into three classes, i.e., those that
break only «-1,4-glucosidic bonds, «-glucosidases specific for «-1,6-glucosidic linkages, and enzymes that may hydrolyze either type of bond.
In many instances the specificity of the enzyme for one or the other of
these two bonds has not been exactly established; other «-glucosidases
are actually able to hydrolyze both 1,4- and 1,6-glucosidic linkages,
and some «-glucosidases, though probably specific for only one type of