498
13 The Structure and Metabolism of Carbohydrates
tened at the end by one to three glucose residues
at a time during several cleavage reactions. The
chain is then released and a new reaction initiated. a-Amylase also catalyses the transfer of
maltose and glucose residues to appropriate
receptors. Exoamylases, such as the ~-amylases
of higher plants and the lysosomal exo-a(1-4)glucosidases, attack the chain at the non-reducing
end and each time remove one molecule of ~maltose or glucose. ~-Amylases have been
detected in many bacteria, fungi and higher
plants. In view of their localization in lysosomes
and their acid pH optima, the starch-hydrolysing
enzymes involved in intracellular digestion in the
Protozoa, Porifera and Enidaria are probably
exo-a(I-4)-glucosidases; in contrast, all animals
with extracellular digestion processes probably
possess genuine a-amylases. These enzymes are
usually secreted into the gut lumen by associated
glands. A remarkable specialization is the gelatinous crystalline style of the mussels and some gastropods; amongst other digestive enzymes, this
contains mainly a-amylase and is gradually solubilized while it is introduced into the stomach.
The induction of a-amylase by a starch-rich diet
has been demonstrated in the mammals and also
in Drosophila [110].
In several vertebrates, a-amylases with rather
different properties are produced in the pancreas
and in the salivary glands. The pancreas type is
probably present in all vertebrates, whereas the
salivary gland amylase is found only in the anurans, the monotreme Tachyglossus aculeatus,
rodents and primates. a-Amylase is found in the
blood and urine of many mammals; just how the
enzyme comes to be present in these fluids and
what function, if any, it has remains a mystery. The
production of a-amylases in other tissues, particularly in mammalian liver, was for a long time controversial. In the meantime, a-amylase mRNA
was identified with certainty in mouse liver [240].
a-Amylases have been isolated and characterized not only from various mammals but also from
a series of invertebrates, especially insects, spiders, crabs and molluscs [11, 269]. The molecular
masses of the animal a-amylases are all about
52 kDa but may be somewhat higher as the result
of glycosylation. The a-amylases are metalloenzymes which are stabilized and activated by calcium
ions. Many are glycoproteins; the carbohydrate
content of the human salivary gland enzyme is
higher than that of the pancreatic enzymes of
humans and other mammals; the chicken pancreatic enzyme is carbohydrate free. The aamylases of mammals and molluscs have neutral
pH optima, whereas the optima in insects are
more acidic (pH 5-6) and in the spiders are more
alkaline (pH 7.2-7.8). Systematic differences in
pH optima are found in the crustaceans: pH 6.3
and lower in the Isopoda, Amphipoda and Astacura, and pH 6.8 or higher in the Natantia and
Brachyura. All a-amylases are activated by Clions; the effectiveness of the halogen ions decreases in the order Cl- > Be > 1- > p--. The optimal
Ct concentration varies greatly with the species
and there is often no clear relationship with
physiological ion concentrations: the optimal
NaCI concentration is about 0.1 mmolll for the
marine crustaceans Homarus americanus and Carcinus maenas and for the crayfish Orconectes virilis; 0.5 mmolll for the marine mussel Mya arenaria; and 1.0 mmolll for the freshwater isopod AselIus aquaticus. The beetle Callosobruchus sinensis
is the only known exception where chloride is inhibitory [11, 269]. The catalytic properties of the
amylases of various Drosophila species vary
widely with the habitat [188]. The a-amylase of the
porcine pancreas is a polypeptide of 496 amino
acids and has five internal disulphide bridges. The
sequence shows differences of 14 % to the rat and
mouse enzymes and 17 % to the human enzyme.
The difference between sequences in the rat and
mouse is 6 % and between those in the rodents and
humans is 17.3 %. The complete sequence is also
known for the 475 amino acids of Drosophila
melanogaster and for 228 amino acids of the Cterminus of the enzyme from the beetle Tribolium
castaneum. The difference between these two
insect species is about 40 %, which is only slightly
less than the 46 % difference between the insects
and humans [111, 196, 206].
Isoenzymes of the a-amylases have been found
in almost all investigated vertebrates and invertebrates. This is partly the result of posttranslational modifications (glycosylation, deamidation) and partly determined by postsecretory changes (deglycosylation, deamidation). Genetic analysis has already shown that
many animals possess several a-amylase loci. In
the mammals, there are separate loci for the
pancreas and salivary gland enzymes, and Drosophila and Musca also have at least two loci. A
more detailed picture of the number of a-amylase
genes has been obtained by mRNA and DNA
analysis. In the mouse, the a-amylase genes lie
within a region of only 78 kb on chromosome 3;
only one gene is present for the salivary gland
enzyme, which is also expressed in the liver, but
at least four genes exist for the pancreas amylase,
although only two of these are apparently tran-
13 The Structure and Metabolism of Carbohydrates
tened at the end by one to three glucose residues
at a time during several cleavage reactions. The
chain is then released and a new reaction initiated. a-Amylase also catalyses the transfer of
maltose and glucose residues to appropriate
receptors. Exoamylases, such as the ~-amylases
of higher plants and the lysosomal exo-a(1-4)glucosidases, attack the chain at the non-reducing
end and each time remove one molecule of ~maltose or glucose. ~-Amylases have been
detected in many bacteria, fungi and higher
plants. In view of their localization in lysosomes
and their acid pH optima, the starch-hydrolysing
enzymes involved in intracellular digestion in the
Protozoa, Porifera and Enidaria are probably
exo-a(I-4)-glucosidases; in contrast, all animals
with extracellular digestion processes probably
possess genuine a-amylases. These enzymes are
usually secreted into the gut lumen by associated
glands. A remarkable specialization is the gelatinous crystalline style of the mussels and some gastropods; amongst other digestive enzymes, this
contains mainly a-amylase and is gradually solubilized while it is introduced into the stomach.
The induction of a-amylase by a starch-rich diet
has been demonstrated in the mammals and also
in Drosophila [110].
In several vertebrates, a-amylases with rather
different properties are produced in the pancreas
and in the salivary glands. The pancreas type is
probably present in all vertebrates, whereas the
salivary gland amylase is found only in the anurans, the monotreme Tachyglossus aculeatus,
rodents and primates. a-Amylase is found in the
blood and urine of many mammals; just how the
enzyme comes to be present in these fluids and
what function, if any, it has remains a mystery. The
production of a-amylases in other tissues, particularly in mammalian liver, was for a long time controversial. In the meantime, a-amylase mRNA
was identified with certainty in mouse liver [240].
a-Amylases have been isolated and characterized not only from various mammals but also from
a series of invertebrates, especially insects, spiders, crabs and molluscs [11, 269]. The molecular
masses of the animal a-amylases are all about
52 kDa but may be somewhat higher as the result
of glycosylation. The a-amylases are metalloenzymes which are stabilized and activated by calcium
ions. Many are glycoproteins; the carbohydrate
content of the human salivary gland enzyme is
higher than that of the pancreatic enzymes of
humans and other mammals; the chicken pancreatic enzyme is carbohydrate free. The aamylases of mammals and molluscs have neutral
pH optima, whereas the optima in insects are
more acidic (pH 5-6) and in the spiders are more
alkaline (pH 7.2-7.8). Systematic differences in
pH optima are found in the crustaceans: pH 6.3
and lower in the Isopoda, Amphipoda and Astacura, and pH 6.8 or higher in the Natantia and
Brachyura. All a-amylases are activated by Clions; the effectiveness of the halogen ions decreases in the order Cl- > Be > 1- > p--. The optimal
Ct concentration varies greatly with the species
and there is often no clear relationship with
physiological ion concentrations: the optimal
NaCI concentration is about 0.1 mmolll for the
marine crustaceans Homarus americanus and Carcinus maenas and for the crayfish Orconectes virilis; 0.5 mmolll for the marine mussel Mya arenaria; and 1.0 mmolll for the freshwater isopod AselIus aquaticus. The beetle Callosobruchus sinensis
is the only known exception where chloride is inhibitory [11, 269]. The catalytic properties of the
amylases of various Drosophila species vary
widely with the habitat [188]. The a-amylase of the
porcine pancreas is a polypeptide of 496 amino
acids and has five internal disulphide bridges. The
sequence shows differences of 14 % to the rat and
mouse enzymes and 17 % to the human enzyme.
The difference between sequences in the rat and
mouse is 6 % and between those in the rodents and
humans is 17.3 %. The complete sequence is also
known for the 475 amino acids of Drosophila
melanogaster and for 228 amino acids of the Cterminus of the enzyme from the beetle Tribolium
castaneum. The difference between these two
insect species is about 40 %, which is only slightly
less than the 46 % difference between the insects
and humans [111, 196, 206].
Isoenzymes of the a-amylases have been found
in almost all investigated vertebrates and invertebrates. This is partly the result of posttranslational modifications (glycosylation, deamidation) and partly determined by postsecretory changes (deglycosylation, deamidation). Genetic analysis has already shown that
many animals possess several a-amylase loci. In
the mammals, there are separate loci for the
pancreas and salivary gland enzymes, and Drosophila and Musca also have at least two loci. A
more detailed picture of the number of a-amylase
genes has been obtained by mRNA and DNA
analysis. In the mouse, the a-amylase genes lie
within a region of only 78 kb on chromosome 3;
only one gene is present for the salivary gland
enzyme, which is also expressed in the liver, but
at least four genes exist for the pancreas amylase,
although only two of these are apparently tran-