13.6.3 Digestion of Cellulose, Lichenin and Laminarin
499
scribed. The gene for the salivary glandlliver
enzyme contains two tissue-specific leader sequences that are separated from the coding
sequence by an intron of almost 10 kb; the genes
for the pancreas enzyme lack this long intron. In
contrast, the other introns of the transcribed genes
are all found at homologous positions. The rat possesses nine a-amylase genes, each with at least
seven introns [289]. Drosophila melanogaster and
other species of the melanogaster subgroup have
two amylase genes. As the result of frequent gene
conversion, the coding sequences for the two
genes are identical, but the flanking DNA sequences vary markedly [112]. Considerable polymorphism of a-amylase has been observed in almost all
species examined, e.g. in Drosophila, Bombyx,
and the salivary gland and pancreatic enzymes of
man and mammals. In the domestic pig there are
one to three isoenzymes of the pancreatic amylase
(PPA) with identical catalytic properties but differing frequencies. In one population of 50 animals,
10 had only PPA-2, and 39 had PPA-1 and PPA-2;
only 1 animal had all three isoenzymes [258].
13.6.3 Digestion of Cellulose, Lichenin
and Laminarin
Cellulose is the most abundant organic substance
in the biosphere (even more abundant than chitin)
and represents an enormous food reservoir. Many
animals can exploit cellulose directly in metabolism, although this occurs mostly with the cooperation of protozoans, bacteria or fungi. The vertebrates have no cellulolytic enzyme systems, and cellulose can only be used indirectly for nutrition;
this occurs via anaerobic degradation by symbiontic bacteria in particular regions of the alimentary tract. The products of bacterial metabolism,
e.g. volatile fatty acids, as well as vitamins and
essential amino acids are then taken up by the animal. Cellulose fermentation and metabolism of
the volatile fatty acids formed is widely found in
the mammals as well as, for example, in the tortoise Chelonia mydas and some herbivorous birds.
In most mammals, the cellulolytic bacteria inhabit
the large intestine and the appendix. However,
the bacterial products are exploited much more
efficiently where the cellulose fermentation
occurs in a spacious rumen, as is found in the
ruminants, hippopotamuses, rock hyraxes, sloths,
certain ape species and kangaroos [16, 26]. The
ciliates present in the stomach of the ruminants
are not important for cellulose digestion. The genera Isotricha and Dasytricha from the Holotricha
have no capacity whatsoever for cellulose hydrolysis; several genera of the Entodiniomorpha, e.g.
Eudiplodinium and Eremoplastron, can hydrolyse
cellulose but others, e.g. Entodinium, Metadinium
and Ophryoscolex, cannot [56]. Bacterial cellulose
fermentation and utilization of the resulting
volatile fatty acids has also been demonstrated in
beetles of the family Scarabaeidae [19].
There are several different indications for cellulose utilization in animals: comparison of the
cellulose content of the diet and the faeces; the
incorporation of carbon from 14C-Iabelled cellulose into carbon dioxide and tissue components;
the capacity to survive on an exclusively cellulose
diet; and the in vitro demonstration of the hydrolysis of native (microcrystalline) cellulose [176].
Based upon these parameters, cellulose utilization has been shown in earthworms, pulmonate
snails, insects and mammals. Amongst the
insects, cellulose digestion is found especially in
the wood-eating species, whereas it is often
absent from the leaf-eaters [266]. It has been
shown in thysanurans (Thermobia, Ctenolepisma) , cockroaches, termites, beetles (Anobiidae, Cerambycidae, Coccinellidae, Scarabeidae),
hymenopterans (Siricidae), and the floor mites of
the family Oribatidae. The efficiency of cellulose
utilization is especially high in the thysanurans
and the termites [176, 299]. The cellulose fibres of
wood are embedded in a matrix of the phenolic
polymer lignin, which is covalently bound to
hemicellulose. Unlike the fungi, animals have no
enzymes for the hydrolysis of lignin and can
remove it only by mechanical means. However,
not all wood-eating insects make use of the cellulose; for example, the scoletids and buprestids
use the low molecular weight sugars, starch, pectins and xylans of wood [47].
The complete digestion of native cellulose
requires at least three types of enzyme (the cellulase complex):
1. Endo-B(1-4)-glucanase or Cx enzyme (EC
3.2.1.4) hydrolyses internal B(1-4)-glucosidic
bonds in carboxymethy1cellulose or other
soluble celluloses. The activity against corresponding oligosaccharides (cellodextrins) declines with decreasing chain length. It is inactive
against crystalline cellulose.
2. Cellobiohydrolase or C1 enzyme (EC 3.2.1.91)
cleaves cellobiose from the non-reducing ends
of linear ~(1-4)-glucan chains.
3. Cellobiase (EC 3.2.1.21) cleaves cellobiose and
cellodextrins. The activity declines markedly with
an increase in the number of sugar residues.
499
scribed. The gene for the salivary glandlliver
enzyme contains two tissue-specific leader sequences that are separated from the coding
sequence by an intron of almost 10 kb; the genes
for the pancreas enzyme lack this long intron. In
contrast, the other introns of the transcribed genes
are all found at homologous positions. The rat possesses nine a-amylase genes, each with at least
seven introns [289]. Drosophila melanogaster and
other species of the melanogaster subgroup have
two amylase genes. As the result of frequent gene
conversion, the coding sequences for the two
genes are identical, but the flanking DNA sequences vary markedly [112]. Considerable polymorphism of a-amylase has been observed in almost all
species examined, e.g. in Drosophila, Bombyx,
and the salivary gland and pancreatic enzymes of
man and mammals. In the domestic pig there are
one to three isoenzymes of the pancreatic amylase
(PPA) with identical catalytic properties but differing frequencies. In one population of 50 animals,
10 had only PPA-2, and 39 had PPA-1 and PPA-2;
only 1 animal had all three isoenzymes [258].
13.6.3 Digestion of Cellulose, Lichenin
and Laminarin
Cellulose is the most abundant organic substance
in the biosphere (even more abundant than chitin)
and represents an enormous food reservoir. Many
animals can exploit cellulose directly in metabolism, although this occurs mostly with the cooperation of protozoans, bacteria or fungi. The vertebrates have no cellulolytic enzyme systems, and cellulose can only be used indirectly for nutrition;
this occurs via anaerobic degradation by symbiontic bacteria in particular regions of the alimentary tract. The products of bacterial metabolism,
e.g. volatile fatty acids, as well as vitamins and
essential amino acids are then taken up by the animal. Cellulose fermentation and metabolism of
the volatile fatty acids formed is widely found in
the mammals as well as, for example, in the tortoise Chelonia mydas and some herbivorous birds.
In most mammals, the cellulolytic bacteria inhabit
the large intestine and the appendix. However,
the bacterial products are exploited much more
efficiently where the cellulose fermentation
occurs in a spacious rumen, as is found in the
ruminants, hippopotamuses, rock hyraxes, sloths,
certain ape species and kangaroos [16, 26]. The
ciliates present in the stomach of the ruminants
are not important for cellulose digestion. The genera Isotricha and Dasytricha from the Holotricha
have no capacity whatsoever for cellulose hydrolysis; several genera of the Entodiniomorpha, e.g.
Eudiplodinium and Eremoplastron, can hydrolyse
cellulose but others, e.g. Entodinium, Metadinium
and Ophryoscolex, cannot [56]. Bacterial cellulose
fermentation and utilization of the resulting
volatile fatty acids has also been demonstrated in
beetles of the family Scarabaeidae [19].
There are several different indications for cellulose utilization in animals: comparison of the
cellulose content of the diet and the faeces; the
incorporation of carbon from 14C-Iabelled cellulose into carbon dioxide and tissue components;
the capacity to survive on an exclusively cellulose
diet; and the in vitro demonstration of the hydrolysis of native (microcrystalline) cellulose [176].
Based upon these parameters, cellulose utilization has been shown in earthworms, pulmonate
snails, insects and mammals. Amongst the
insects, cellulose digestion is found especially in
the wood-eating species, whereas it is often
absent from the leaf-eaters [266]. It has been
shown in thysanurans (Thermobia, Ctenolepisma) , cockroaches, termites, beetles (Anobiidae, Cerambycidae, Coccinellidae, Scarabeidae),
hymenopterans (Siricidae), and the floor mites of
the family Oribatidae. The efficiency of cellulose
utilization is especially high in the thysanurans
and the termites [176, 299]. The cellulose fibres of
wood are embedded in a matrix of the phenolic
polymer lignin, which is covalently bound to
hemicellulose. Unlike the fungi, animals have no
enzymes for the hydrolysis of lignin and can
remove it only by mechanical means. However,
not all wood-eating insects make use of the cellulose; for example, the scoletids and buprestids
use the low molecular weight sugars, starch, pectins and xylans of wood [47].
The complete digestion of native cellulose
requires at least three types of enzyme (the cellulase complex):
1. Endo-B(1-4)-glucanase or Cx enzyme (EC
3.2.1.4) hydrolyses internal B(1-4)-glucosidic
bonds in carboxymethy1cellulose or other
soluble celluloses. The activity against corresponding oligosaccharides (cellodextrins) declines with decreasing chain length. It is inactive
against crystalline cellulose.
2. Cellobiohydrolase or C1 enzyme (EC 3.2.1.91)
cleaves cellobiose from the non-reducing ends
of linear ~(1-4)-glucan chains.
3. Cellobiase (EC 3.2.1.21) cleaves cellobiose and
cellodextrins. The activity declines markedly with
an increase in the number of sugar residues.