496
13 The Structure and Metabolism of Carbohydrates
strate is trehalose; the claim that purified trehalase from the brine shrimp Artemia salina also
cleaves cellobiose and lactose should probably be
re-examined. The oligo saccharides and glucosides cleaved by the non-specific a-glucosidases
are not attacked by trehalases. In experiments
with artificial trehalose analogues, the enzymes
of bacteria, insects and mammals all behaved
similarly in that substrates with intact aglucopyranoside residues, such as 6' -deoxy-a,atrehalose or a-galactosyl-a-glucoside, were
cleaved but a,~- or ~,~-trehalose were not.
Despite these results, the mechanism of action of
the insect and mammalian enzymes appear to
differ: the trehalase from the blowfly Sarcophaga
barbata produces equal proportions of a- and ~glucose, whereby the oxygen from the water is
incorporated into ~-glucose; the trehalase from
pig kidney, on the other hand, produces only aglucose. The trehalases of rabbit kidney, Periplaneta and Bombyx and also from the slime mould
Dictyostelium are glycoproteins [202, 259].
Mammalian gut trehalase, like the sucraseisomaltase and the maltase-glucoamylase, is one
of the disaccharidases that is bound to the microvillous layer but has been little investigated. The
enzymes of rat and rabbit gut are dimers with subunits of 66-75 kDa together with a small membrane domain of about 5 kDa [225, 295]. As trehalose is the typical blood sugar of insects, it is
rather amazing that the highest trehalase activities are found in the haemolymph. However, this
situation is explained by the localization of trehalase completely or partially in blood cells. Furthermore, the haemolymph of the cockroach
Periplaneta americana contains a soluble trehalase inhibitor of 86 kDa [107]. In addition to the
easily extractable, soluble trehalase, insect tissues
contain a particulate form. In the flight muscles
of the cockroach Blaberus discoidalis, the moth
Hyalophora cecropia and the locust Locusta migratoria the enzyme is bound to the microsomes,
whereas in the flies Phormia regina, Calliphora
erythrocephala and Sarcophaga barbata it is localized on the inner mitochondrial membrane [22].
The presence of soluble trehalases in flight muscles often results from the presence there of haemolymph; however, in Phormia regina and
Locusta migratoria the soluble enzymes in flight
muscle and haemolymph differ electrophoretically [276]. Trehalose serves as an energy source in
flight muscles, at least for short flight periods; the
regulation of trehalase in the flight muscles is
therefore an interesting, but as yet unsolved,
question [276].
Soluble and particulate trehalases differ not
only in intracellular localization but also in
kinetic properties and organ distribution. In
Bombyx mori, the predominant midgut enzyme is
soluble with a pH optimum of 5.5 and a Km of
1 mmolll, whereas the flight muscle enzyme is
particulate, has a somewhat higher pH optimum
and a lower substrate affinity. It may be assumed
that the two forms have different, tissue-specific
functions; however, both forms are sometimes
found in the same tissue, e.g. in the gut of Bombyx mori and Blaberus, or in the flight muscles of
Apis, Locusta and Periplaneta. In the flight
muscle of Periplaneta americana, the cytosolic
form is an a-glucosidase with a broad specificity,
but the particulate form is highly specific for trehalose [130]. In Bombyx gut, the particulate form
increases at the expense of the soluble form during pupal moulting; intermediate forms which
appear during this period are particle bound but
have the kinetics of the soluble form; the particulate forms can be solubilized with detergents
[259]. Thus, the two forms are apparently products of post-translational modification, and in
Drosophila the soluble and particulate trehalases
are encoded by the same gene. The soluble Bombyx trehalase is a dimer with identical 67-kDa
subunits, whereas the particulate form is monomeric; thus, conversion of the particulate to the
soluble form probably involves dimerization
[259].
Specific fl-glucosidases (ceUobiases) mainly
serve to digest cellulose, and fl-N-acetylglucosaminidases (chitinases) are involved in
chitin metabolism. They will be discussed in relation to the corresponding polysaccharidases.
Multiple lysosomal ~-N-acetylglucosaminidases
have been detected in many mammalian tissues
and also in the carp [274]. The cleavage of cane
sugar (invertase activity) may involve either aglucosidase or ~-fructosidase activity. Genuine flfructofuranosidases, which hydrolyse sucrose,
raffinose and inulin but not maltose, are found in
plants but only rarely in animals, e.g. in the
beetle Dermestes maculatus and the moths Heliothis zea and Erinnyis ella [235].
fl-Galactosidases are involved, together with
other enzymes, in the metabolism of proteoglycans, glycoproteins and glycolipids, and are,
therefore, Ubiquitous. The corresponding bacterial enzymes are among the most investigated of
glycosidases. Animal tissues contain multiple ~galactosidases with acidic pH optima in the lysosomes and cytoplasm. Apart from those of mammalian organs, only the enzymes of Drosophila
13 The Structure and Metabolism of Carbohydrates
strate is trehalose; the claim that purified trehalase from the brine shrimp Artemia salina also
cleaves cellobiose and lactose should probably be
re-examined. The oligo saccharides and glucosides cleaved by the non-specific a-glucosidases
are not attacked by trehalases. In experiments
with artificial trehalose analogues, the enzymes
of bacteria, insects and mammals all behaved
similarly in that substrates with intact aglucopyranoside residues, such as 6' -deoxy-a,atrehalose or a-galactosyl-a-glucoside, were
cleaved but a,~- or ~,~-trehalose were not.
Despite these results, the mechanism of action of
the insect and mammalian enzymes appear to
differ: the trehalase from the blowfly Sarcophaga
barbata produces equal proportions of a- and ~glucose, whereby the oxygen from the water is
incorporated into ~-glucose; the trehalase from
pig kidney, on the other hand, produces only aglucose. The trehalases of rabbit kidney, Periplaneta and Bombyx and also from the slime mould
Dictyostelium are glycoproteins [202, 259].
Mammalian gut trehalase, like the sucraseisomaltase and the maltase-glucoamylase, is one
of the disaccharidases that is bound to the microvillous layer but has been little investigated. The
enzymes of rat and rabbit gut are dimers with subunits of 66-75 kDa together with a small membrane domain of about 5 kDa [225, 295]. As trehalose is the typical blood sugar of insects, it is
rather amazing that the highest trehalase activities are found in the haemolymph. However, this
situation is explained by the localization of trehalase completely or partially in blood cells. Furthermore, the haemolymph of the cockroach
Periplaneta americana contains a soluble trehalase inhibitor of 86 kDa [107]. In addition to the
easily extractable, soluble trehalase, insect tissues
contain a particulate form. In the flight muscles
of the cockroach Blaberus discoidalis, the moth
Hyalophora cecropia and the locust Locusta migratoria the enzyme is bound to the microsomes,
whereas in the flies Phormia regina, Calliphora
erythrocephala and Sarcophaga barbata it is localized on the inner mitochondrial membrane [22].
The presence of soluble trehalases in flight muscles often results from the presence there of haemolymph; however, in Phormia regina and
Locusta migratoria the soluble enzymes in flight
muscle and haemolymph differ electrophoretically [276]. Trehalose serves as an energy source in
flight muscles, at least for short flight periods; the
regulation of trehalase in the flight muscles is
therefore an interesting, but as yet unsolved,
question [276].
Soluble and particulate trehalases differ not
only in intracellular localization but also in
kinetic properties and organ distribution. In
Bombyx mori, the predominant midgut enzyme is
soluble with a pH optimum of 5.5 and a Km of
1 mmolll, whereas the flight muscle enzyme is
particulate, has a somewhat higher pH optimum
and a lower substrate affinity. It may be assumed
that the two forms have different, tissue-specific
functions; however, both forms are sometimes
found in the same tissue, e.g. in the gut of Bombyx mori and Blaberus, or in the flight muscles of
Apis, Locusta and Periplaneta. In the flight
muscle of Periplaneta americana, the cytosolic
form is an a-glucosidase with a broad specificity,
but the particulate form is highly specific for trehalose [130]. In Bombyx gut, the particulate form
increases at the expense of the soluble form during pupal moulting; intermediate forms which
appear during this period are particle bound but
have the kinetics of the soluble form; the particulate forms can be solubilized with detergents
[259]. Thus, the two forms are apparently products of post-translational modification, and in
Drosophila the soluble and particulate trehalases
are encoded by the same gene. The soluble Bombyx trehalase is a dimer with identical 67-kDa
subunits, whereas the particulate form is monomeric; thus, conversion of the particulate to the
soluble form probably involves dimerization
[259].
Specific fl-glucosidases (ceUobiases) mainly
serve to digest cellulose, and fl-N-acetylglucosaminidases (chitinases) are involved in
chitin metabolism. They will be discussed in relation to the corresponding polysaccharidases.
Multiple lysosomal ~-N-acetylglucosaminidases
have been detected in many mammalian tissues
and also in the carp [274]. The cleavage of cane
sugar (invertase activity) may involve either aglucosidase or ~-fructosidase activity. Genuine flfructofuranosidases, which hydrolyse sucrose,
raffinose and inulin but not maltose, are found in
plants but only rarely in animals, e.g. in the
beetle Dermestes maculatus and the moths Heliothis zea and Erinnyis ella [235].
fl-Galactosidases are involved, together with
other enzymes, in the metabolism of proteoglycans, glycoproteins and glycolipids, and are,
therefore, Ubiquitous. The corresponding bacterial enzymes are among the most investigated of
glycosidases. Animal tissues contain multiple ~galactosidases with acidic pH optima in the lysosomes and cytoplasm. Apart from those of mammalian organs, only the enzymes of Drosophila