Material Metabolism of Silkworm

The silkworm absorbs, transforms and utilizes the substances (nutrients) in the environment in the material exchange with the external environment. In the synthesis reaction, it is used as a raw material to enable various tissues and organs in the body to grow and develop: In the decomposition reaction, it is mainly used as an energy substance to release energy through biological oxidation for the needs of life activities; at the same time, waste is generated through the excretion mechanism Excreted from the body and returned to the environment.

The essential amino acids for silkworm growth include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. 10 Kind of, these amino acids cannot be synthesized in the body of silkworms, and must be taken from food. In addition, there are proline and aspartic acid or glutamic acid which can significantly promote the growth of silkworms. Carbohydrates are mainly consumed as energy sources, part of which is used to synthesize storage carbohydrates, such as glycogen. The nutritional value of carbohydrates varies greatly due to different types. Glucose, fructose, sucrose and maltose have high nutritional value, and the utilization rate of pentoses is very low; polysaccharides such as starch and dextrin are due to the amylase activity in the digestive juice The utilization rate has a size, but the silkworm larvae can grow and develop normally as long as there is an efficient carbohydrate in the feed. Lipids are another important component that constitutes the body of silkworms. Among them, only sterols are essential nutrients for silkworms because they cannot be produced in silkworms. For example, cholesterol, β-sitosterol, stigmasterol, etc. are all effective in nutrition. In addition, some vitamins and inorganic salts are indispensable. There are 10 vitamins necessary for silkworms, including biotin, choline, inositol, niacin, pantothenic acid, pyridoxine, riboflavin, thiamine, folic acid and ascorbic acid. The essential inorganic substances are at least potassium, phosphorus, and magnesium. , Calcium, zinc, manganese and iron.

Nibble down the nutrients in the lower mulberry leaves, which basically meets its needs. Mulberry leaf protein (accounting for 5 to 6% of the weight of fresh leaves) is hydrolyzed into peptides or amino acids and then absorbed by proteases in the intestinal juice. Carbohydrates are about 15% of the weight of fresh leaves. Among them, glucose, fructose, sucrose and Both maltose can be directly absorbed by silkworms, but sucrose and maltose must be decomposed into monosaccharides in the midgut tissue by corresponding glycosidase enzymes; the utilization rate of starch and dextrin is very low. The crude fat contained in mulberry leaves is very small, only about 1%. The content of β-sitosterol in sterols is high. The plant sterols ingested by silkworms feed are converted into cholesterol in the body and used.

Protein Metabolism The silkworm is an insect that uses feed protein efficiently. The protein digested and absorbed from mulberry leaves reaches 60 to 70%, and 90% of it remains in the body. This feed protein is decomposed by the hydrolase in the digestive juice to produce peptides or amino acids, and then enters the tube wall cells of the digestive tract. Except for a small part remaining in the tube wall cells to synthesize cell proteins, most of them enter the blood in the form of amino acids. In, and then from the blood into the various tissue cells. The amino acids absorbed by the cells of each tissue synthesize the proteins unique to each tissue in the cells of different tissues. The amino acid composition of silkworm body protein and cocoon silk protein synthesized from mulberry leaf protein conversion is as follows.

Amino acid composition of mulberry leaf protein, silkworm body protein and cocoon silk protein (%)

It can be seen from the above table that the amino acid composition of mulberry leaf protein and silkworm body protein are different, mainly the six kinds of alanine, methionine, glycine, tyrosine, phenylalanine and proline in silkworm body protein. There is more protein than mulberry leaves, especially methionine, which is an essential amino acid. Silkworm body protein is 2.7 times as much as mulberry leaf protein. It must be accumulated from a large amount of food consumed. Other non-essential amino acids can be obtained from mulberry leaves. Produced in silkworms. The cocoon silk protein produced in the silkworm body is different from the silkworm body protein. The percentage of non-essential amino acids such as glycine, alanine, serine and tyrosine is significantly larger. The total of these four amino acids is 79.68%, while the silkworm body protein It only accounts for 43.26% of the total, especially the increase of serine is the largest. It can be seen that the production of cocoon silk protein is not derived from silkworm protein, but directly synthesized in silk gland cells using free amino acids in the blood as materials.

With the participation of transaminase, the alanine and glycine, which are greatly increased in silkworms, can transfer amino groups from another amino acid (glutamic acid or aspartic acid) to pyruvate or glyoxylic acid, respectively. Alanine can be produced under the successive action of glutathione aminotransferase, oxaloacetate decarboxylase and alanine aminotransferase. It can also be produced by ornithine aminotransferase (Xu Tingsen et al., 1980). The silk gland tissue can use ketomalonic acid as the amino receptor to undergo transamination to generate aminomalonic acid, and then, under the catalysis of aminomalonic acid decarboxylase, to generate glycine (Zou Baixiang et al., 1979). Glycine and serine can be converted into each other, glucose can also generate serine through 3-phosphohydroxypyruvate, and serine can generate alanine through pyruvate. Tyrosine can be produced by phenylalanine, and cystine can be produced by the conversion of methionine but not vice versa. Therefore, both phenylalanine and methionine are essential amino acids. When the feed is rich in tyrosine and cystine, the requirement of phenylalanine and methionine can be reduced.

When the content of amino acids in general feed increases within a certain range, the weight of the cocoon layer also increases. However, when the amount of 10 essential amino acids in the total amount of amino acids is large, the weight of the cocoon layer decreases. On the contrary, the non-essential amino acid Tianmen As the content of aspartic acid and glutamic acid increases, the weight of the cocoon layer increases. Therefore, the various amino acids in the feed need to have an appropriate mixing ratio.

The digestive tract digests and absorbs amino acids from food. Except for some of the amino acids that are used for the growth of midgut tissue, most of them are transferred to the blood through the midgut epithelium and transported to various tissues along with the blood circulation to synthesize proteins in each tissue. Excess amino acids, in addition to the conversion to other amino acids as described above, undergo deamination or decarboxylation in midgut epithelial cells, blood or other intermediate metabolic tissues to generate ammonia or amines and other substances. Another example is methionine can be converted to cystathionine, and further generate cystine and its derivative lanthionine. The ability of this kind of transformation is more pronounced in female silkworms than male silkworms, especially in the blood of larvae after 5th instar, the content of lanthionine in the blood increases. This amino acid is related to the formation of egg shells. The tryptophan metabolism of silkworm is quite different from that of mammals and microorganisms. Tryptophan can be oxidized to produce kynurenine, and then anthranilic glycine is produced by anthranilic acid. It can produce 3-hydroxykynurenine, kynurenic acid and kynurenine; from 3-hydroxykynurenine to produce pigments at the end, it can also be metabolized to xanthuric acid or 3-hydroxyaminobenzoic acid in silkworms.

Tyrosine is oxidized by phenolase in silkworms to produce dihydroxyphenylalanine (DOPA). DOPA can be further oxidized and deammonized to produce a quinone substance (such as dihydroxybenzoic acid) that hardens the epidermis. This kind of quinone reacts with epidermal protein to cause so-called quinone tanning to cause hardening. DOPA can be oxidized to produce o-quinone phenylalanine, and finally form 5,6-o-quinone indole, and re-synthesize melanin from o-quinone indole.

The excess amino acids from mulberry leaves, or the amino acids from the decomposition of tissue proteins in the body, are further decomposed in the body to produce ammonia. Ammonia is a toxic substance and must be treated in time. Except for some storage in the form of glutamine and asparagine, it mainly produces uric acid, and some is excreted in urine and spinning in the form of ammonium or urea. Artificial feed breeding reduces urease activity, so that urea accumulates in silkworms, so that urea is also present in the eggs laid.

Uric acid is produced by purine metabolism. Although the uric acid oxidase system in silkworms, such as uricase, allantoinase and allantoidase, exists, the activity of these enzymes is higher than that of the xanthine dehydrogenase system that produces uric acid. Significantly weaker, so the amount of urea produced from uric acid is very small. Since the ornithine cycle metabolism system of the silkworm is incomplete, ornithine cannot be converted to citrulline to produce proline, and the activity of arginase is also very weak. Therefore, the amount of urea as a metabolite is very small, and nitrogen is metabolized. The main product is uric acid.

Carbohydrate metabolism. About 10% of carbohydrates digested and absorbed by silkworm body from mulberry leaves remain in the body in the form of carbohydrates, about 40% is converted into fat, and the remaining 50% is oxidized in life as energy Consumption. According to the determination of related enzymes, it is proved that there are glycolysis system, pentose cycle and tricarboxylic acid cycle in silkworm. In the midgut tissue, due to the strong enzyme activity of the glycolysis system and the strong activity of lactate dehydrogenase, the carbohydrate metabolism in the midgut tissue is dominated by glycolysis, that is, from sugar under hypoxia. The original or glucose is decomposed into lactic acid, and a small amount of lactic acid is formed under aerobic conditions, and pyruvic acid enters the tricarboxylic acid cycle mainly. Glycolysis, pentose cycle and tricarboxylic acid cycle systems have also been found in the fat body. Because the 6-phosphoglucose dehydrogenase and 6-phosphogluconate dehydrogenase have relatively large activities, it can be seen that the pentose cycle is in the fat body. The metabolism of carbohydrates plays an important role. The carbohydrate metabolism of the silkworm body through the pentose cycle accounts for about 35% or more of the total. Another feature of the fat body is the main place for the synthesis and supply of trehalose. It is known that the blood sugar of silkworms is mainly trehalose. Under normal conditions, the blood trehalose content can be kept constant, and the fat body plays an important role in regulating blood sugar concentration. . The trehalose in the 5th instar silkworms accounts for 20 to 35% of the stored carbohydrates, which varies depending on the feed. If glucose is artificially added to the feed, the proportion of trehalose in the stored carbohydrates of the silkworms will be relatively Decrease, which shows that the blood trehalose concentration is not only determined by the amount of glucose supplied in the feed, but also restricted by certain physiological effects. The blood trehalose metabolism and circulation rate that changes due to the glucose content in the feed keeps the blood trehalose content stable. Part of trehalose is used to synthesize amino acids and lipids, and the synthesis amount or synthesis speed of these substances is related to the changes in the trehalose metabolism rate.

The diapause of silkworm egg embryos is also related to carbohydrate metabolism. When silkworm eggs start to enter diapause, the amount of glycogen in the eggs decreases sharply. On the other hand, the amount of sorbitol and glycerol increase correspondingly, and sorbitol accounts for about 2/3 of it. When the diapause is released, the amount of glycogen in the egg decreases sharply. These polyols synthesize glycogen, which can be restored to about 90% of the original level.

Lipid metabolism Lipids include hydrocarbons, long-chain alcohols and fatty acids, fats, oils and waxes, as well as some more complex compounds such as sterols and carotenoids. The lipids in silkworms can be directly derived from food-mulberry leaves, but the lipids (ether extract) in mulberry leaves account for about 3-9% of the dry matter, mainly including chloroplast pigments, phospholipids and sterols, and almost no Fat. The chlorophyll in the chloroplast pigment is subjected to the action of chlorophyllase in the alkaline digestive juice of silkworms to remove the side chain part of phytol and decompose to produce chlorophyllin and pheophorbide, except for the former part, which is neutralized After being absorbed by the intestine, most of the chlorophyll and its derivatives are excreted in the feces, except for the chromophore that becomes the red fluorescent substance in the digestive juice. Phospholipids are mainly lecithin. Lecithin in mulberry leaves accounts for about 16% of the crude fat of mulberry leaves. It is decomposed into glycerol, choline, fatty acids and phosphoric acid in the midgut tissue. Except for a part of these decomposition products, they are transferred to the blood. Part of it is used to synthesize the phospholipids in the midgut tissue and become a component of the protoplast of the midgut cells. The sterol in mulberry leaves is mainly β-sitosterol, in addition to stigmasterol and a small amount of campesterol, these sterols have high nutritional effects on silkworms. The phytosterols in mulberry leaves are absorbed by the digestive tract and converted into cholesterol in the silkworm body to be utilized.

There is almost no fat in mulberry leaves, and the fat content in silkworms, especially in mature silkworms, is very high. For example, the lipid content in fresh tissues of 5th instar larvae is on average more than 3%, mainly distributed in the fat body. Among the lipids of the fat body, triglycerides account for 70-80%, diglycerides and monoesters together account for about 10%, and phospholipids account for about 10%. In addition, it contains about 3% paraffin and sterol esters, as well as trace amounts of free fatty acids. From the fat represented by triglycerides, 10 kinds of caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid and nonanoic acid, tridecanoic acid and pentadecanoic acid can be separated. Fatty acids, of which palmitic acid, stearic acid and unsaturated octadecanoic acid together account for nearly 90%. It can be seen that the fat in the silkworm body is mainly synthesized in the silkworm body except for a part of mulberry leaves. According to the research of artificial feed method, the nutritional value of fatty acids is the highest with linolenic acid and linoleic acid. The nutritional effects of oleic acid, stearic acid and palmitic acid are significantly lower than that of linolenic acid and linoleic acid, so their nutritional value is considered It is related to the degree of unsaturation of fatty acids. Silkworm body can generate acetic acid from the decomposition of sugar, and then synthesize palmitic acid, stearic acid and oleic acid from acetic acid, but cannot synthesize linolenic acid and linoleic acid. The nutritional value of the above fatty acids is obviously consistent with whether the silkworm body can be synthesized. The fatty acid that can be synthesized in the silkworm body also differs according to the amount of fatty acid in the feed. The fatty acid content in the feed is higher, the silkworm body’s ability to synthesize fatty acid will decrease. It can be seen that feed conditions have an effect on the ability of silkworm bodies to synthesize fatty acids.