Intracellular glycosidases are found in all tissues. Their biological role is probably related to
the conversion and degradation of bound carbohydrates and glycoconjugates. In the case of lysosome enzymes, the pH optima are characteristically 3-5; however, enzymes with such pH optima
are also found extracellularly, e.g. in mammalian
blood plasma. There have been few comparative
investigations of glycosidases with functions other
than digestion, but such enzymes are known in all
animals down to the Protozoa. For example, at
least nine different glycosidase activities have
been detected in the blood forms of Trypanosoma
brucei and these are probably involved in the
turnover of the variable surface glycoproteins
[44].
The literature is full of information about the
hydrolytic effects of digestive secretions, other
body fluids, and tissue extracts of many different
vertebrates and invertebrates on a wide range of
glycosides, oligosaccharides and polysaccharides.
In many cases, not only have the activities been
demonstrated but functional properties of the
enzymes, such as substrate affinity and specificity,
cofactor dependence, pH optimum and temperature dependence, have been investigated. However, there is as yet little information about the
molecular structures of the carbohydrases and little can be said, therefore, about their evolution.
13.6.1 Glycosidases
The human and mammalian guts contain several
a-glucosidases which are referred to as maltase,
isomaltase and sucrase, according to their substrate specificities; there is also a trehalase (Table 13.4). The enzyme activities sucrase-isomaltase
and maltase-glucoamylase are involved in the last
steps of starch digestion, during which they cleave
glucose residues from dextrins, maltose, isomaltose and linear glucans. Both disaccharidases,
together with various peptidases, are bound to
the microvillous membrane of intestinal cells.
Their substrates are cleaved by "membranecontact digestion" and the products are transferred into the cells by an Na + -dependent, hydrolase-linked transport system. All enzymes bound to
the microvillous membrane are anchored via a
non-polar, N-terminal domain and form homodimeric complexes. The maltase-glucoamylase is
synthesized as a single polypeptide of about
240 kDa but is cleaved at one position by a pancreatic protease after insertion into the membrane
[199]. Maltase-glucoamylase and trehalase have
13.6.1 Glycosidases
495
also been isolated from the brush border of
kidney tubules of various mammals, and sucraseisomaltase has been isolated from the kidney of
the goose Anser anser; apparently, the kidney trehalase is anchored to the membrane via phosphatidylinositol [87]. a-Glucosidases are also found
in the liver and other organs, where they occur as
either acidic lysosomal or neutral cytosolic enzymes. Human lysosomal a-glucosidase agrees in
36-41 % of its 951 amino acids with the isomaltases and sucrases [114].
In invertebrates, the digestive glycosidases
may be freely solubilized in the gut or more-orless firmly bound to the gut cells. In caterpillars
of the moth Erinnyis ello, these enzymes are
localized in the glycocalyx of the gut cells; in larvae of the fly Rhynchosciara americana they are,
like the aminopeptidases, integral membrane
proteins. The soluble enzymes are usually monomers and the bound forms dimers or oligomers
[267]. The a-glucosidases present in the midgut
and haemolymph of the cockroach Periplaneta
americana differ considerably in pH optimum,
affinity for the p-nitrophenyl substrate, and substrate specificity; in contrast to the haemolymph
enzyme, that of the gut also cleaves trehalose.
The a-glucosidase in the midgut of Erinnyis
ello caterpillars (Lepidoptera: Sphingidae) is
found as a tetramer of 151 kDa and as monomers,
whereas the enzyme from the imago has a mass of
86 kDa and is immunologically unrelated to the
larval enzyme; thus E. ello possesses at least two
a-glucosidase genes [235]. In contrast to most
other a-glucosidases, the haemolymph enzyme of
the honey bee shows the typical kinetics of an
allosteric enzyme with cooperativity. Interestingly, the Hill constant increases from 0.65 to
1.21 during pupa development. Cooperativity has
also been demonstrated for other insect aglucosidases.
'ftehalases have been found in many organisms
from the bacteria, yeasts and slime moulds to the
mammals. Of the animal trehalases, those from
mammalian kidney and gut [225, 295], from the
crustaceans Artemia and Macrobrachium, and
from various insects [22, 202, 259] have been isolated and characterized. The trehalases from different organisms differ in many properties: the
pH optima vary between 3.5 and 7.5, and the Km
values vary between 0.4 and 20 mmolll; Mg2+
inhibits the yeast trehalase but not that of the
beetle Harmonia axyridis; and the mammalian
enzyme is inhibited by phloridzin but the cockchafer enzyme is not. However, all these enzymes
show agreement in that their only natural sub-
the conversion and degradation of bound carbohydrates and glycoconjugates. In the case of lysosome enzymes, the pH optima are characteristically 3-5; however, enzymes with such pH optima
are also found extracellularly, e.g. in mammalian
blood plasma. There have been few comparative
investigations of glycosidases with functions other
than digestion, but such enzymes are known in all
animals down to the Protozoa. For example, at
least nine different glycosidase activities have
been detected in the blood forms of Trypanosoma
brucei and these are probably involved in the
turnover of the variable surface glycoproteins
[44].
The literature is full of information about the
hydrolytic effects of digestive secretions, other
body fluids, and tissue extracts of many different
vertebrates and invertebrates on a wide range of
glycosides, oligosaccharides and polysaccharides.
In many cases, not only have the activities been
demonstrated but functional properties of the
enzymes, such as substrate affinity and specificity,
cofactor dependence, pH optimum and temperature dependence, have been investigated. However, there is as yet little information about the
molecular structures of the carbohydrases and little can be said, therefore, about their evolution.
13.6.1 Glycosidases
The human and mammalian guts contain several
a-glucosidases which are referred to as maltase,
isomaltase and sucrase, according to their substrate specificities; there is also a trehalase (Table 13.4). The enzyme activities sucrase-isomaltase
and maltase-glucoamylase are involved in the last
steps of starch digestion, during which they cleave
glucose residues from dextrins, maltose, isomaltose and linear glucans. Both disaccharidases,
together with various peptidases, are bound to
the microvillous membrane of intestinal cells.
Their substrates are cleaved by "membranecontact digestion" and the products are transferred into the cells by an Na + -dependent, hydrolase-linked transport system. All enzymes bound to
the microvillous membrane are anchored via a
non-polar, N-terminal domain and form homodimeric complexes. The maltase-glucoamylase is
synthesized as a single polypeptide of about
240 kDa but is cleaved at one position by a pancreatic protease after insertion into the membrane
[199]. Maltase-glucoamylase and trehalase have
13.6.1 Glycosidases
495
also been isolated from the brush border of
kidney tubules of various mammals, and sucraseisomaltase has been isolated from the kidney of
the goose Anser anser; apparently, the kidney trehalase is anchored to the membrane via phosphatidylinositol [87]. a-Glucosidases are also found
in the liver and other organs, where they occur as
either acidic lysosomal or neutral cytosolic enzymes. Human lysosomal a-glucosidase agrees in
36-41 % of its 951 amino acids with the isomaltases and sucrases [114].
In invertebrates, the digestive glycosidases
may be freely solubilized in the gut or more-orless firmly bound to the gut cells. In caterpillars
of the moth Erinnyis ello, these enzymes are
localized in the glycocalyx of the gut cells; in larvae of the fly Rhynchosciara americana they are,
like the aminopeptidases, integral membrane
proteins. The soluble enzymes are usually monomers and the bound forms dimers or oligomers
[267]. The a-glucosidases present in the midgut
and haemolymph of the cockroach Periplaneta
americana differ considerably in pH optimum,
affinity for the p-nitrophenyl substrate, and substrate specificity; in contrast to the haemolymph
enzyme, that of the gut also cleaves trehalose.
The a-glucosidase in the midgut of Erinnyis
ello caterpillars (Lepidoptera: Sphingidae) is
found as a tetramer of 151 kDa and as monomers,
whereas the enzyme from the imago has a mass of
86 kDa and is immunologically unrelated to the
larval enzyme; thus E. ello possesses at least two
a-glucosidase genes [235]. In contrast to most
other a-glucosidases, the haemolymph enzyme of
the honey bee shows the typical kinetics of an
allosteric enzyme with cooperativity. Interestingly, the Hill constant increases from 0.65 to
1.21 during pupa development. Cooperativity has
also been demonstrated for other insect aglucosidases.
'ftehalases have been found in many organisms
from the bacteria, yeasts and slime moulds to the
mammals. Of the animal trehalases, those from
mammalian kidney and gut [225, 295], from the
crustaceans Artemia and Macrobrachium, and
from various insects [22, 202, 259] have been isolated and characterized. The trehalases from different organisms differ in many properties: the
pH optima vary between 3.5 and 7.5, and the Km
values vary between 0.4 and 20 mmolll; Mg2+
inhibits the yeast trehalase but not that of the
beetle Harmonia axyridis; and the mammalian
enzyme is inhibited by phloridzin but the cockchafer enzyme is not. However, all these enzymes
show agreement in that their only natural sub-