8. POLYSACCHARIDASES
397
paper, mercerized cotton, regenerated cellulose trinitrate in which the
nitrate groups had been removed by treatment with alcoholic ammonium sulfide, cellulose that had been reprecipitated from a xanthate
solution (viscose) or from a solution in cuprammonium hydroxide, and
alkali-soluble celluloses obtained from the triacetate or after treatment
with hydrochloric acid. In addition, hydrocellulose and swollen cellulose fibers, obtained by the action of phosphoric acid (254), have been
used as substrates. Recently, substituted cellulose derivatives, such as
carboxymethylcellulose
(CMC),
carboxymethylhydroxyethylcellulose
(CMHEC), hydroxyethylcellulose (HEC), sulfoethylcellulose, and
cellulose sulfate (255), have been introduced for the assay of cellulose
activity (256, 257), which thus can be measured by the decrease in
viscosity of the substrate solution rather than by determining the increase in reducing power, as was necessary with the other substrates.
B. MECHANISM OF ACTION
At least three different groups of enzymes are known to be involved
in the biological breakdown of cellulose. The first group of cellulases,
called C x enzyme by Reese and Levinson (258) is believed to initiate
the attack of cellulose in fibers, resulting in a rapid loss of their tensile
strength. Enzymes of this kind have been reported to occur in certain
fungi (Chaetomium globosum), in which they are apparently not accompanied by other cellulases (259). The substrate, thus modified, or
partially soluble cellulose, hydrocellulose, or carboxymethylcellulose, is
then hydrolyzed by the C^ enzyme into water-soluble fragments of low
molecular weight. The latter enzyme, being the cellulase proper, is believed to split the chains at random, and the mechanism of its action
resembles closely that of «-amylase. Finally, ß-glucosidases such as
cellobiase continue the breakdown to glucose, which has been found to
be the sole reducing product of exhaustive biological cellulose degradation (260), and it has been reported by Karrer (253) that the conversion of cellulose to glucose may exceed 93%.
Although the ß-glucosidase usually occurs in association with the
cellulases (C x ), these two enzymes have been separated from one another by the adsorption of the /?-glucosidase on aluminum metahydroxide at pH 3.5 and subsequent elution in slightly alkaline medium
(261). The same authors have shown that the cellulase fraction is distinctly specific for high molecular substrates only, whereas the ßglucosidase attacks mainly the low molecular weight cellodextrins, as
seen from the data in Table XVIII. It is also evident that the ß-glucosidase fraction is not exclusively a cellobiase, i.e., an enzyme specific
for the disaccharide only, but that it is capable of hydrolyzing also oligo-
397
paper, mercerized cotton, regenerated cellulose trinitrate in which the
nitrate groups had been removed by treatment with alcoholic ammonium sulfide, cellulose that had been reprecipitated from a xanthate
solution (viscose) or from a solution in cuprammonium hydroxide, and
alkali-soluble celluloses obtained from the triacetate or after treatment
with hydrochloric acid. In addition, hydrocellulose and swollen cellulose fibers, obtained by the action of phosphoric acid (254), have been
used as substrates. Recently, substituted cellulose derivatives, such as
carboxymethylcellulose
(CMC),
carboxymethylhydroxyethylcellulose
(CMHEC), hydroxyethylcellulose (HEC), sulfoethylcellulose, and
cellulose sulfate (255), have been introduced for the assay of cellulose
activity (256, 257), which thus can be measured by the decrease in
viscosity of the substrate solution rather than by determining the increase in reducing power, as was necessary with the other substrates.
B. MECHANISM OF ACTION
At least three different groups of enzymes are known to be involved
in the biological breakdown of cellulose. The first group of cellulases,
called C x enzyme by Reese and Levinson (258) is believed to initiate
the attack of cellulose in fibers, resulting in a rapid loss of their tensile
strength. Enzymes of this kind have been reported to occur in certain
fungi (Chaetomium globosum), in which they are apparently not accompanied by other cellulases (259). The substrate, thus modified, or
partially soluble cellulose, hydrocellulose, or carboxymethylcellulose, is
then hydrolyzed by the C^ enzyme into water-soluble fragments of low
molecular weight. The latter enzyme, being the cellulase proper, is believed to split the chains at random, and the mechanism of its action
resembles closely that of «-amylase. Finally, ß-glucosidases such as
cellobiase continue the breakdown to glucose, which has been found to
be the sole reducing product of exhaustive biological cellulose degradation (260), and it has been reported by Karrer (253) that the conversion of cellulose to glucose may exceed 93%.
Although the ß-glucosidase usually occurs in association with the
cellulases (C x ), these two enzymes have been separated from one another by the adsorption of the /?-glucosidase on aluminum metahydroxide at pH 3.5 and subsequent elution in slightly alkaline medium
(261). The same authors have shown that the cellulase fraction is distinctly specific for high molecular substrates only, whereas the ßglucosidase attacks mainly the low molecular weight cellodextrins, as
seen from the data in Table XVIII. It is also evident that the ß-glucosidase fraction is not exclusively a cellobiase, i.e., an enzyme specific
for the disaccharide only, but that it is capable of hydrolyzing also oligo-