This article explains the conference given by Ing. Alfredo R. Palomino of the Universidad Científica del Sur in Lima, Peru, at the XXXI Biovet International Symposium in Tarragona, during the fourth session dedicated to enzymes and mycotoxin binders, which took place on May 29th, 2018.

With the title, “Digestive enzymes in fishes” the lecture explored the different types of digestions presented by fishes (depending on their species) and the participation of enzymes in digestion. The veterinarian of the Biovet S.A. team, Antón Tallero, has made the adaptation of the paper.

Introduction

The ability of an organism to digest food particles depends on the presence and the appropriate number of digestive enzymes (Smith, 1980).

The digestion of food in the digestive tract of teleost fish is carried out by enzymes like other farm animals, although these will vary depending on the physiological, anatomical and behavioral differences of these aquatic animals.

Gastric digestion

Digestion begins in the stomach, since teleost fish do not have salivary glands like mammals or birds. Therefore, the mouth of the fish has a mere mechanical function and of apprehension in the digestion of feed.

Within the fish, we distinguish those who perform a gastric and intestinal digestion, and those who do not have a proper stomach and are able to carry out a first digestion in the esophagus. An example of this physiological phenomenon takes place on the species Neogobius gymnotrachelus, which is classified as an agastric fish.

The lack of stomach is not uncommon among teleost fishes, but our understanding of this reductive specialization is still scarce. The absence of a stomach does not restrict the trophic preference, which results in fish with a very similar anatomy capable of digesting different diets.

Thus, within the stomach proteolytic and non-proteolytic digestion are distinguished:

Proteolytic digestion

It occurs at a very acidic pH thanks to the concentration of hydrochloric acid and its optimal value for protein digestion varies in different species. The low pH level activates the pepsinogen which is the precursor of pepsin, an enzyme responsible for hydrolyzing proteins in the stomach. The amount of pepsin secreted in the gut is regulated by the concentration of protein in the diet.

Pepsin breaks down proteins and releases protein fragments and some free amino acids that will pass to the intestine, where they will then be absorbed.

Non-proteolytic digestion

In certain fish, another type of proteolytic digestion may occur, such as the digestion of chitin. Chitin is a structural carbohydrate that gives shape to the exoskeleton of crustaceans, among other marine species.

The chitinase is, therefore, an enzyme that can digest the main component of the exoskeleton of crustaceans, to enable a subsequent intestinal digestion and thus avoid possible obstructions in the gastrointestinal tract.

Another type of enzyme that digests structural carbohydrates is cellulase, present in the stomachs of fish with herbivore type of feeding.

Intestinal digestion

The digestion of carbohydrates, lipids and proteins continues in the small bowel. Most digestive enzymes are excreted by pancreas, passing from this organ to the digestive tract by the pancreatic duct.

Pancreatic digestion

It takes place at neutral or basic pH, thanks to the secretion of bicarbonate (bile juice that comes from the gallbladder). The main enzymes that come into play are:

• Endopeptidases: Trypsin and chymotrypsin. They hydrolyze proteins and polypeptides.
• Endopeptidase: elastase I and I I. They digest elastin and other structural proteins.
• Exopeptidase: carboxypeptidase A and B: cut terminal junctions of proteins and polypeptides.

In the agastric fishes, the hydrolysis of proteins begins with pancreatic trypsin.

As for the digestion of fats, the pancreatic enzymes that act are:

Pancreatic lipase

Active and unique form. It performs the hydrolysis or breakage of triglycerides ingested in the diet into monoglycerides and free fatty acids. To make this possible, bile salts act by emulsifying fats and creating smaller drops that promotes the action of lipase.

Non-specific lipase

They digest neutral lipids, cholesterol esters and waxes.

Finally, the enzymes responsible for hydrolyzing complex carbohydrates and excreted by the pancreas are the glucosidases:

Pancreatic amylase

Responsible for hydrolyzing glycogen and starch.

Chitinase

Chitin (arthropod exoskeleton).

Digestion in pyloric caeca

It is observed in all species and the physiological role of these diverticula is still controversial. Within them, trypsin-like activity has been found in tuna species, Sebastis maurins, chinook salmon and rainbow trout. Polypeptidase activity as well It has been observed.

There is a complementary action between trypsin and polypeptidase of the pyloric caeca, allowing to reach the free amino acids for absorption in the intestinal wall.

Proteolytic intestinal digestion

There are other proteolytic enzymes in the intestine that take part in digestion. These are found in the cell membranes of the cells of the intestinal wall. The characteristics of these membrane peptidases are:

• They are trigger molecules of pancreatic coenzymes.
• They are amino, di and tripeptidases in the cytoplasm of enterocytes.
• They enable the absorption of free amino acids or small peptides (2 to 6 amino acids )

In agastric fishes this is where protein hydrolysis (carboxypolipeptidase and leucine aminopeptidase) is completed.

Absorption and transportation of nutrients

The absorption of amino acids and short chain peptides is carried out by the enterocytes in two different ways: Simple and facilitated diffusion; and active transport.

For the absorption of fats to occur, mixed micelles must be formed – a product of lipid digestion – which are insoluble in water and can be brought into contact with the microvilli of the small intestine and absorbed into the cell membrane by diffusion.

Once inside the enterocytes, the lipid digestion products bind to a protein, which leads to the smooth endoplasmic reticulum. In this one, the   resynthesis   of triglycerides, lecithins and esterified cholesterol.

The different lipids are subsequently grouped and surrounded by a cover of lipoproteins, giving rise to the appearance of the chylomicrons. Its composition would be: 87% triglycerides, 9% phospholipids and free cholesterol, 3% esterified cholesterol and 1% fat-soluble vitamins and proteins.

The chylomicrons subsequently pass into the lymphatic system.

And, finally, regarding the sugars: fructose, galactose and glucose, resulting from the digestion of complex carbohydrates in the intestinal lumen, cross the enterocyte by facilitated diffusion or cotransport to reach the bloodstream.

Conclusions

The concentration of all these enzymes varies depending on the feeding habits of the fish.

Specializing fish or s in a specific diet exhibit significant differences in type, concentration and enzyme activity may be determined not enzymes or have a very reduced activity

Specialized fish, which are fed with different types of diets, can find a greater diversity of enzymes.

It is important to know the digestive habits and characteristics of the digestive tract, since this allows us to formulate adequate diets for an optimal development of the fish and at the same time it will help us to choose functional ingredients that could be incorporated in the diet.

Pictures by Ing. Alfredo R. Palomino.