and the hepatopancreas of various molluscs have
been examined in any detail [84]. The Bgalactosidase 1 of Drosophila melanogaster is a
homodimeric glycoprotein of 160 kDa; null mutants are known for the l3-gal locus [84]. The
human and murine lysosomal B-galactosidases
form a high molecular mass complex of
600-700 kDa together with neuraminidase (sialidase) and a "protective protein" [88]. They have
attracted particular attention due to their involvement in glycosphingolipid metabolism and the
demonstration that genetic defects of this enzyme
lead to GM1 gangliosidose, a classical lysosomal
storage disease. The microvillous layer in the
small intestine of the higher mammals contains a
B-galactosidase (lactase) with a pH optimum of 6
and whose natural substrate is lactose [174]. The
microvillous lactase is absent from various kangaroo species of the genus Macropus and from the
monotreme Tachyglossus aculeatus (echidna), in
which lactose plays no important role in the milk.
In contrast to that of the higher mammals, the
lactose of these species is not hydrolysed either
during or before resorption, but is cleaved intracellularly by a lysosomal, acidic B-galactosidase
[58, 254].
a-Mannosidases and a-galactosidases are
widely found in plants and animals and are probably involved mainly in the metabolism of the
organism's own substrates. The a-galactosidase
and a-mannosidase of the hepatopancreas of the
edible mussel Mytilis edulis, however, may also
serve in the digestion of plankton [33, 212]. Similarly, the ubiquitous lysosomal a-L-fucosidases
are required for the metabolism of the organism's
own fucose-containing glycoconjugates [80, 201].
The liver enzymes of various vertebrates are very
similar both immunologically and in their catalytic properties, but show large differences in
their heat stability; the latter differences appear
to have no adaptive significance as, for example,
the activities of the human, rhesus monkey and
armadillo enzymes are completely destroyed after
3 hours at 60°C but those of the zebra and the
fish Racophilus toxotes are reduced by only 14 %.
The activity in human blood, which is high relative to that in various other mammals, is not even
stable at 37°C. Considerable differences in heat
stability are also found between the different afucosidase isozymes of the same organ, e.g. for
the enzymes isolated from the hepatopancreas of
the snail Turbo cornutus [55]. The widely found
enzymes referred to as (l-fucosidases are Bglucosidases with a broader specificity and can
also hydrolyse l3-glucosides and l3-galactosides. It
13.6.2 a-Amylases
497
could be shown with the enzymes from the snail
Achatina balteata and the fly Rhynchosiciara
americana that all three types of substrate bind to
the same active centre [217].
The digestive organs of many marine invertebrates contain (l-glucuronidases, together with
arylsulphatases, which are probably involved in
the digestion of sulphated polysaccharides from
algae. The richest sources of these enzymes are
again found amongst the gastropods [272]. The
very first glycosidase to be crystallized (in 1957)
was the B-glucuronidase of the edible snail. In
mammals, the intracellular localization of the Bglucuronidases is organ specific: in the rat liver,
79 % is found in the lysosomes and 13 % in the
microsomes; in the small intestine 62 % is in the
soluble fraction, 30 % in the lysosomes and only
4 % in the microsomes. The gut enzymes are
responsible for the so-called enterohepatic cycle:
B-glucuronides produced in the liver are hydrolysed in the gut so that the aglycone can be reabsorbed and returned to the liver [63]. Two glucuronidase forms are also found in Drosophila
melanogaster and in the hepatopancreas of the
marine snail Littorina littorea; these, like the
mouse-liver enzymes and other glycosidases, are
glycoproteins [167]. The glycosidic bonds of nucleosides in human tissues and those of other
mammals are cleaved phosphorolytically rather
than hydrolytically. However, almost nothing is
known about nucleoside phosphorylases and
hydrolases (nucleosidases) from a comparative
biochemical standpoint. Phosphorylases specific
for purine and pyrimidine ribonucleosides are
known from the flagellate Trypanosoma cruzi.
However, both here and in other Kinetoplastida
there are also several deoxyribonucleosidases and
ribonucleosidases of different specificity [189].
13.6.2 a-Amylases
The a-amylases are amongst the best-investigated
carbohydrases. They are endoamylases that
cleave a(1-4) glucosidic bonds in the chain and
break down a(1-4), a(1-6)-glucans, such as amylopectin or glycogen, stepwise, at first into
smaller fragments and finally into maltose, isomaltose and glucose. Starch hydrolysis is completed by maltases and isomaltases, e.g. the
maltase-glucoamylase and sucrase-isomaltase in
the mammalian small intestine. The a-amylase
reaction begins with an attack on anyone bond
within the chain; one of the two resulting fragments remains bound to the enzyme and is shor-
been examined in any detail [84]. The Bgalactosidase 1 of Drosophila melanogaster is a
homodimeric glycoprotein of 160 kDa; null mutants are known for the l3-gal locus [84]. The
human and murine lysosomal B-galactosidases
form a high molecular mass complex of
600-700 kDa together with neuraminidase (sialidase) and a "protective protein" [88]. They have
attracted particular attention due to their involvement in glycosphingolipid metabolism and the
demonstration that genetic defects of this enzyme
lead to GM1 gangliosidose, a classical lysosomal
storage disease. The microvillous layer in the
small intestine of the higher mammals contains a
B-galactosidase (lactase) with a pH optimum of 6
and whose natural substrate is lactose [174]. The
microvillous lactase is absent from various kangaroo species of the genus Macropus and from the
monotreme Tachyglossus aculeatus (echidna), in
which lactose plays no important role in the milk.
In contrast to that of the higher mammals, the
lactose of these species is not hydrolysed either
during or before resorption, but is cleaved intracellularly by a lysosomal, acidic B-galactosidase
[58, 254].
a-Mannosidases and a-galactosidases are
widely found in plants and animals and are probably involved mainly in the metabolism of the
organism's own substrates. The a-galactosidase
and a-mannosidase of the hepatopancreas of the
edible mussel Mytilis edulis, however, may also
serve in the digestion of plankton [33, 212]. Similarly, the ubiquitous lysosomal a-L-fucosidases
are required for the metabolism of the organism's
own fucose-containing glycoconjugates [80, 201].
The liver enzymes of various vertebrates are very
similar both immunologically and in their catalytic properties, but show large differences in
their heat stability; the latter differences appear
to have no adaptive significance as, for example,
the activities of the human, rhesus monkey and
armadillo enzymes are completely destroyed after
3 hours at 60°C but those of the zebra and the
fish Racophilus toxotes are reduced by only 14 %.
The activity in human blood, which is high relative to that in various other mammals, is not even
stable at 37°C. Considerable differences in heat
stability are also found between the different afucosidase isozymes of the same organ, e.g. for
the enzymes isolated from the hepatopancreas of
the snail Turbo cornutus [55]. The widely found
enzymes referred to as (l-fucosidases are Bglucosidases with a broader specificity and can
also hydrolyse l3-glucosides and l3-galactosides. It
13.6.2 a-Amylases
497
could be shown with the enzymes from the snail
Achatina balteata and the fly Rhynchosiciara
americana that all three types of substrate bind to
the same active centre [217].
The digestive organs of many marine invertebrates contain (l-glucuronidases, together with
arylsulphatases, which are probably involved in
the digestion of sulphated polysaccharides from
algae. The richest sources of these enzymes are
again found amongst the gastropods [272]. The
very first glycosidase to be crystallized (in 1957)
was the B-glucuronidase of the edible snail. In
mammals, the intracellular localization of the Bglucuronidases is organ specific: in the rat liver,
79 % is found in the lysosomes and 13 % in the
microsomes; in the small intestine 62 % is in the
soluble fraction, 30 % in the lysosomes and only
4 % in the microsomes. The gut enzymes are
responsible for the so-called enterohepatic cycle:
B-glucuronides produced in the liver are hydrolysed in the gut so that the aglycone can be reabsorbed and returned to the liver [63]. Two glucuronidase forms are also found in Drosophila
melanogaster and in the hepatopancreas of the
marine snail Littorina littorea; these, like the
mouse-liver enzymes and other glycosidases, are
glycoproteins [167]. The glycosidic bonds of nucleosides in human tissues and those of other
mammals are cleaved phosphorolytically rather
than hydrolytically. However, almost nothing is
known about nucleoside phosphorylases and
hydrolases (nucleosidases) from a comparative
biochemical standpoint. Phosphorylases specific
for purine and pyrimidine ribonucleosides are
known from the flagellate Trypanosoma cruzi.
However, both here and in other Kinetoplastida
there are also several deoxyribonucleosidases and
ribonucleosidases of different specificity [189].
13.6.2 a-Amylases
The a-amylases are amongst the best-investigated
carbohydrases. They are endoamylases that
cleave a(1-4) glucosidic bonds in the chain and
break down a(1-4), a(1-6)-glucans, such as amylopectin or glycogen, stepwise, at first into
smaller fragments and finally into maltose, isomaltose and glucose. Starch hydrolysis is completed by maltases and isomaltases, e.g. the
maltase-glucoamylase and sucrase-isomaltase in
the mammalian small intestine. The a-amylase
reaction begins with an attack on anyone bond
within the chain; one of the two resulting fragments remains bound to the enzyme and is shor-