Ruminants, cellulose digestion

Cellulase is an enzyme found primarily in fungi, lower protozoans, and some symbiotic bacteria in termites and ruminants that digests the / l->4 bonds between adjacent glucose units in cellulose. 

While government-supported research now emphasizes the production of liquid fuels from biomass, commercialization might be reached sooner by cooperation with those interested in cellulose digestion by ruminants. Experience in the practical upgrading of coarse fodder would be directly applicable to the hydrolysis of biomass by cellulases. 

Animal feedstuffs (Table 2) are a natural by-product of vegetable oil manufacture because this latter results in a variety of residues such as palm fruit pulp fibre and Brazil nut or other kernel oil-cake. Most cellulosic fibre wastes can be converted by steam explosion (sudden release of steam pressure to separate lignin from cellulose) to a cellulose digestible by ruminants. The oil-cakes often have high protein and carbohydrate contents and can be used as a major component of nonruminant feeds. 

Ruminants can digest cellulose in crop residues because the microorganisms in their rumen... 

Animals lack the enzymes necessary to catalyze the hydrolysis of cellulose and so can t digest it. Cattle and other ruminants use cellulose as a food source indirectly. 

The nature of the diet sets the basic pattern of metabohsm. There is a need to process the products of digestion of dietary carbohydrate, lipid, and protein. These are mainly glucose, fatty acids and glycerol, and amino acids, respectively. In ruminants (and to a lesser extent in other herbivores), dietary cellulose is fermented by symbiotic microorganisms to short-chain fatty acids (acetic, propionic, butyric), and metabohsm in these animals is adapted to use these fatty acids as major substrates. All the products of digestion are metabohzed to a common product, acetyl-CoA, which is then oxidized by the citric acid cycle (Figure 15-1). 

Cellulose differs from amylose principally in the stereochemistry of the acetal linkages, which are a in amylose but P in cellulose. a-Amylase is specific for al 4 bonds and is not able to hydrolyse pi 4 bonds. An alternative enzyme, termed cellulase, is required. Animals do not possess cellulase enzymes, and thus cannot digest wood and vegetable fibres that are predominantly composed of cellulose. Ruminants, such as cattle, are equipped to carry out cellulose hydrolysis, though this is dependent upon cellulase-producing bacteria in their digestive tracts. 

In the higher animals, including humans, cellulose is indigestible, but important as roughage (see p.273). Many herbivores (e.g., the ruminants) have symbiotic unicellular organisms in their digestive tracts that break down cellulose and make it digestible by the host. 

Animal Nutrition. Sulfur in the diets of ruminant animals is beneficial to the animals growth (see Feeds and feed additives). Sulfur increases feed intake, cellulose and dry matter digestion, and the synthesis of microbial protein. This results in increased meat, milk, and wool production (43). The special uses for sulfur in agriculture demonstrate a significant and continuing need for increased use of sulfur (44). 

Propionic (propanoic) acid-producing bacteria are numerous in the digestive tract of ruminants. Within the rumen some bacteria digest cellulose to form glucose, which is then converted to lactate and other products. The propionic acid bacteria can convert either glucose or lactate into propionic and acetic acids which are absorbed into the bloodstream of the host. Usually some succinic acid is also formed. 

Humans and most other animals cannot digest cellulose. Ruminant animals (cattle, sheep, goats, moose) have bacteria in their stomachs that break down cellulose into products that can be used by the animal. Chemical processes are available to convert cellulose to simple sugars by the reaction... 

Digestion can be a limiting factor in the ability of organisms to utilize saccharides. Many adults lack the lactase enzyme required to hydrolyze lactose. When these individuals consume milk products, the lactose remains undigested in the intestine, where it is acted upon by bacteria. These bacteria produce gas and intestinal pain, and diarrhea may result. The lack of a digestive enzyme for cellulose in humans and virtually all other animals means that these animals cannot metabolize cellulose. The cellulosic plant material eaten by ruminant animals such as cattle is actually digested by the action of enzymes produced by specialized rumen bacteria in the stomachs of such animals. 

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