The Effect of Carbohydrates on the Growth of Silkworms

The carbohydrates absorbed by the silkworm are mainly consumed in the body as energy, and the surplus can be used to synthesize storage carbohydrates (such as glycogen), lipids and chitin, etc. Carbohydrates also play an important role in aspects of body function and cell construction.

The nutritional value of carbohydrates

According to the nutrition test of more than 20 kinds of carbohydrates and related substances on the silkworm, it is found that a variety of carbohydrates have nutritional effects and can replace each other, and there is no “essential carbohydrates”. The nutritional value of various carbohydrates to silkworms is shown in the table below. It can be seen from the table that the nutritional value of carbohydrates to silkworms varies greatly depending on the species. The nutritional value of pentoses is very low, and the nutritional value of hexoses and their oligosaccharides are high. For silkworms, the most nutritious sugars are sucrose and glucose, followed by fructose. Except for sorbitol, which has higher nutritional value, other sugar alcohols have very low nutritional value or are ineffective. Due to the different activity of amylase in the digestive juice of different silkworm species, the nutritional value of polysaccharides is also different.

The carbohydrate requirement is related to the protein or amino acid content. Sufficient carbohydrates in the feed can improve the utilization efficiency of protein and amino acids by silkworms and promote fatty acid synthesis. Protein can be fully used to build silkworm body and synthesize silk protein, so that silkworm grows well and the quality of silkworm cocoons is improved.

The content of sugar in silkworms is affected by feed conditions. If glucose and sucrose are added to the mulberry leaf breeding silkworm, the glycogen, body fluid reducing sugar and fat in the silkworm will increase. If the amount of glucose added in the synthetic feed is increased, the amount of glycogen in the fat body of the artificial feed and trehalose in the body fluid will also increase, but when the amount of starch is increased, the amount of sugar in the body will increase very little.

The weak (or lacking) and strong amylase activity of silkworm digestive juice is a trait controlled by a pair of alleles, which is recessively inherited. For silkworms lacking amylase activity, starch and dextrin have no nutritional value. The activity of amylase in the digestive juice of current silkworm varieties is generally very low, so the utilization rate of starch and dextrin in mulberry leaves is very low, but there are monosaccharides and oligosaccharides that can be directly utilized by silkworms in mulberry leaves. According to the actual comparison results of mulberry leaf breeding, silkworm’s starch utilization ability is not related to growth and development and cocoon silk quality.

Carbohydrates have a feeding promotion effect on silkworms, but the promotion effect varies greatly due to their different types. The most effective is sucrose, followed by fructose and raffinose, and the third is inositol, galactose, and rhamnose. Glucose is less effective in promoting food intake. Electrophysiological studies have shown that the two nodular protrusions on the tumor-like body of the larvae have the function of a chemical sensor. Among them, the nodular protrusion I is 20 times more sensitive to sucrose response than glucose.

Carbohydrate metabolism

The carbohydrates in the body fluid of the silkworm are mainly trehalose, which accounts for more than 90% of the blood sugar. Others such as glucose, fructose, and sucrose are only present in a small amount. When the silkworm hunger strikes, the fat body glycogen decreases rapidly, but the trehalose content of body fluid is almost unchanged. Only when the fat body glycogen drops to the lowest level, the body fluid trehalose content slowly decreases. If the mulberry is given again after the hunger strike, the first is the increase of trehalose, and the second is the increase of glycogen. The content of trehalose in body fluids is relatively stable, which indicates that the blood sugar level of silkworm bodies has an auto-regulation phenomenon like that of mammals.

The biosynthetic pathway of trehalose in the body fluid of the silkworm is the same as that of other insects, which is converted from glucose. Fat body is the main place to synthesize trehalose, but its synthesis ability varies with different feed conditions. The tracer test with ¹⁴ C-glucose showed that during the normal eating period, ¹⁴ C of the radioactive glucose fed can be bound to fat body glycogen and body fluid trehalose, and the metabolic turnover rate of body fluid trehalose is high; if hunger strikes , ¹⁴ C only binds to body fluid trehalose, and the sugar’s metabolic turnover rate decreases. When the added amount of glucose in the artificial feed, ¹⁴ CO ₂  exhaled volume from ¹⁴ C-L-glucose increases, and ¹⁴ CO ₂ exhaled amount from ¹⁴ C-L-trehalose is almost constant, trehalose biosynthetic capability drops, the conversion of trehalose to fat is promoted. Trehalose accounts for about 20%-35% of carbohydrates stored in the 5th instar silkworm bred by artificial feed. Although the blood sugar level of insects remains constant, large changes also occur during the growth metamorphosis period, and this is no exception for silkworms. The decrease in blood sugar during the metamorphosis phase of the silkworm is due to the change in the activity of trehalose synthase.

Trehalase, which decomposes trehalose, is present in various tissues and organs of larvae, especially in the midgut tissues. This enzyme has a high activity. This enzyme is mainly membrane-bound and confined to the plasma membrane on the side of the bottom membrane. This implies that this enzyme is actively involved in the use of trehalose in body fluids. The 5th instar larvae fasted for more than 3 days, their activity decreased, and trehalose injection could restore their vitality; if the swallowing ganglia were removed, the enzyme activity increased by 1.2-1.3 times. This indicates that midgut trehalose is involved in maintaining the function of midgut cells, and its activity is regulated by the endocrine system.

In the body fluid of silkworm, in addition to free trehalose, there is also more sorbitol-6-phosphate. At the end of the fifth instar, the content of the latter is about twice that of the former, but overall, the two The content of the pigtails increases with the elapse of the 5th age, and decreases to about half of the original at the end of spinning.

Glycolysis and the pentose cycle

There are glycolysis, pentose cycle and tricarboxylic acid cycle in silkworm. All the enzymes involved in glycolysis in the midgut tissue of the silkworm have been discovered, and the enzyme activity is high, and lactate dehydrogenase and α-glycerol phosphate dehydrogenase also exist. Therefore, the carbohydrate metabolism of the midgut tissue is dominated by glycolysis, that is, glycogen or glucose is broken down into lactic acid under anaerobic conditions, and a small amount of lactic acid is formed under aerobic conditions, and pyruvate enters the tricarboxylic acid cycle. The activity of lactate dehydrogenase in midgut tissue is stronger than that of α-glycerol phosphate dehydrogenase, while the activity of lactate dehydrogenase in larval fat body and muscle and adult breast muscle is lower than that of α-glycerol phosphate dehydrogenase. In the midgut tissue, it has been confirmed that glucose 6-phosphate, an intermediate product of glycolysis, can also be metabolized via the pentose cycle. The 4th instar silkworm was fed with glucose marked at a specific location, and compared with the exhaled ¹⁴ CO ₂ , it was found that about 35% of the glucose was metabolized through the pentose cycle. The activity of 6-phosphate glucose dehydrogenase and 6-phosphate gluconate dehydrogenase in the fat body is particularly high, and both activities generate reduced coenzyme II. In the presence of acetyl C_oA and malonyl C_oA, a large amount of soluble components of fat body redox coenzyme II, and its redox is conjugated with the reduction phase of coenzyme II in the presence of glucose 6-phosphate. It shows that fat body carbohydrate metabolism is closely related to fatty acid biosynthesis.