1. Introduction
As a high-quality protein source for fish feed, fish meal is popular for its rich and balanced nutrients, easy digestibility and palatability [
1]. With the rapid development of global fisheries, aquaculture has seen an increasing demand for fish meal, which is mainly composed of dried fish carcasses [
1,
2]. Dependence on fish meal exerts enormous pressure on limited marine resources and finding alternative resources of fish meal remains urgent [
2]. The alternative resources of fish meal can be obtained not only from animals but also from plants. However, compared with fish meal, plant protein sources have some defects, such as anti-nutritional factors enrichment and amino acid imbalance [
3,
4]. Animal protein sources are the ideal component in animal feed formulation, and they are superior to plant protein sources [
1]. Some animal protein sources, such as meat and bone meal (MBM), hydrolyzed animal protein and blood meal (BM), are commonly used. It was reported that no significant difference was discovered in the specific growth rate (SGR) of large yellow croaker (
Pseudosciaena crocea) fed with MBM replacing 45% fish meal [
5]. In juvenile Nile tilapia (
Oreochromis niloticus), up to 50% of fish meal could be replaced with hemoglobin powder, which did not affect SGR [
6]. Dietary 12.5% hydrolyzed feather meal (76% fish meal replacement) was possible in European seabass (
Dicentrarchus labrax) without growth decline [
7]. Notably, there were many studies on fish meal replaced with various animal protein sources in aquatic feed, while few studies were carried out on hydrolyzed collagen.
At present, three main sources are used to extract hydrolyzed collagen: terrestrial animal by-products, leather waste and fish by-products [
1,
8,
9]. However, infectious diseases existing in terrestrial animal by-products (such as foot-and-mouth disease) and chromium in leather waste limit the extraction of hydrolyzed collagen from the above two raw materials [
1,
8], thus fish by-products are gradually welcomed by the public. Collagen is abundant in fish by-products (such as fish skin and bone) [
10]. In deep-sea redfish (
Sebastes mentella), the collagen content in skin, scale and bone were 47.5%, 6.8% and 10.3%, respectively [
9]. In carp (
Cyprinus carpio), the yields of acid-soluble collagens in skin, scale and bone were 41.3%, 1.35% and 1.06%, respectively [
11]. Fish by-products account for about 25% of the global harvest, of which 13% is used for fish meal production, and the rest is discarded [
12]. The discarded fish by-products not only pollute the environment but also waste resources. The key to extracting hydrolyzed collagen from fish by-products is to break the peptide bonds of the collagen, which can be achieved using heat, acids, bases, enzymes or a combination of these physical, chemical and biological methods [
13]. Hydrolyzed collagen obtained from fish by-products consists of low molecular weight peptides such as Pro-Hyp [
14]. The Pro-Hyp is proven to improve the dysfunction of the skin barrier and promote the growth and differentiation of skin fibroblasts [
15]. In addition, hydrolyzed collagen helps to lower blood pressure, improve immunity and promote calcium absorption [
10]. However, the investigation of the effect of hydrolyzed collagen on fish muscle quality and glycolipid metabolism remains insufficient.
Triploid crucian carps (3n = 150) were obtained from crossing the male allotetraploid (4n = 200) (intercrossing between
Carassius auratus red var. (♀) and
Cyprinus carpio L. (♂)) with the female Japanese crucian carp (
Carassius auratus cuvieri) (2n = 100) [
16]. The triploid crucian carp is sterile, and its sterility makes it possible for reproductive energy to be transferred to growth performance [
17]. Therefore, triploid crucian carps are expected to show the advantages of faster growth and longer life span [
17]. The sterility of triploid crucian carp also hinders them from mating with other fish in nature, which is of great significance for safeguarding fish genetic resources [
16]. In addition, the advantages of strong disease resistance and good quality flesh of triploid crucian carp make it readily accepted by customers and farmers [
16]. As far as we know, few studies have been carried out on the feeding and nutrition of triploid crucian carp. In the present study, we aimed to explore the effects of fish meal replaced with hydrolyzed collagen on the growth performance, muscle quality and glycolipid metabolism of triploid crucian carp. This will contribute to the application of fish by-products and the development of an environmentally friendly economy.
4. Discussion
Fish by-products, containing some bioactive peptides (such as Hyp-Pro), are one of the main sources of collagen extraction [
10,
11]. In the present study, the crude protein content of hydrolyzed collagen extracted from by-products of tilapia was as high as 88.51%, while that of fish meal was 55.25%. The content of glycine, alanine, arginine and proline in hydrolyzed collagen was higher than those in fish meal. Glycine and alanine have the effect of stimulating fish feeding [
24]. Arginine, as a precursor, is involved in the synthesis of urea, polyamines, agmatine, proline and glutamate [
25]. Proline is proven to be crucial in collagen synthesis and cell differentiation [
26]. In the present research, the WGR and SGR of fish decreased significantly when the proportion of fish meal replaced with hydrolyzed collagen reached 4%. There might be two reasons for the decline of SGR and WGR. One was the lower content of essential amino acids of hydrolyzed collagen. In the present study, the EAAs content in fish meal and hydrolyzed collagen were 26.21% and 23.18%, respectively. With the increase in the proportion of hydrolyzed collagen replacing fish meal, the content of essential amino acids in 4% and 6% HC groups decreased. The content of methionine, valine, lysine, isoleucine, phenylalanine, leucine, threonine and histidine in the 4% and 6% HC groups were lower than those in the control group and 2% HC group, which may cause the nutritional requirements of triploid crucian carp in 4% and 6% HC groups to not be satisfied. Ai et al. [
5] found that the decline of the growth of large yellow croaker was related to the imbalance of essential amino acids in MBM. The other was the decreased digestion ability of fish to feed. The activities of intestinal amylase, trypsin and lipase decreased significantly when the proportion of fish meal replaced with hydrolyzed collagen reached 4%. Starch, protein and fat in food are hydrolyzed under the action of intestinal amylase, trypsin and lipase, respectively [
27], which help nutrients in food to be absorbed by the body. In the present experiment, the SGR of triploid crucian carp decreased significantly when the proportion of fish meal replaced with hydrolyzed collagen reached 4%, and there were no significant differences in morphometric indexes and nutrition components of muscle among all the treatments. The above results were in conformity with some former research. Zhao et al. [
28] found that when 4% hydrolyzed collagen was added to the feed, it significantly decreased the SGR of grass carp (
Ctenopharyngodon idellus), while affecting muscle nutrition components in a not significant way. There was no significant difference in liver weight between mice fed with a normal diet and oral administration of fish collagen hydrolysates for 14 days, respectively [
8].
Umami, sweet, and bitter tastes are closely related to consumers’ acceptance or rejection of food [
29]. Glutamates releases a unique taste, which is the original definition of umami taste [
30]. The α-glutamyl dipeptides and tripeptides, especially peptides containing aspartic acid, threonine and serine, present a taste of umami, and the umami molecules endow salty taste and increase the intensities of other tastes [
29]. The taste of sweetness is largely related to the content of glycine and alanine [
30]. The name “glycine” comes from the Greek word “glykys”, which means sweet, and its sweetness is similar to glucose [
26]. Bitter food is usually rejected by consumers, however, limited bitterness in food may be desirable [
29]. The increased glutamic acid content in 4% HC and 6% HC groups and serine content in all HC groups might be related to the fact that hydrolyzed collagen was rich in proline and glycine, which could be converted into glutamic acid and serine, respectively [
26,
31]. The free proline content of muscle in all HC groups decreased significantly, which might be related to the conversion of proline into glutamic acid and other amino acids [
31]. The arginine content in hydrolyzed collagen was higher than that in fish meal, while the free arginine content of muscle in all HC groups decreased significantly with hydrolyzed collagen replacement, which needed to be further studied. The results that the replacement of fish meal with hydrolyzed collagen significantly increased the umami amino acids content and significantly decreased the bitter amino acids content of muscle showed that hydrolyzed collagen was helpful to improve the taste of the triploid fish.
Fatty acids are hydrocarbon chains with 2–36 carbon atoms, one end of which is methyl and the other end is carboxyl [
32]. As the substantial components of lipids, fatty acids are important energy substrates, meanwhile, they are the components of phospholipids involved in the formation of cell membranes [
33]. Fatty acids can be divided into two categories: saturated fatty acids (SFAs) without double bonds and unsaturated fatty acids with double bonds. Unsaturated fatty acids families include n-3, n-6 and n-9 families, and the position of the first double bond is different among these three fatty acids [
32]. Human can synthesize most kinds of fatty acids, except for α-linolenic acid (ALA, C18:3 n-3) and linoleic acid (LA, C18:2 n-6), because human lacks the desaturase enzymes that catalyze the formation of the double bond at the n-3 or n-6 position of the hydrocarbon chain (calculating from the methyl carbon), respectively [
34]. The ALA and LA can only be obtained from food, therefore, they are termed essential fatty acids [
35]. When the uptake of SFAs increases, the LDL-C content in the body increases, which makes the human body more prone to coronary heart disease [
36]. The n-3 PUFAs are vital in decreasing lipogenesis, and the n-6 PUFAs have certain benefits in decreasing total cholesterol, LDL-C and HDL-C content [
33]. In the present research, the ALA and LA content increased significantly and the SFAs content decreased significantly with hydrolyzed collagen replacement, indicating that hydrolyzed collagen was beneficial in improving fatty acids composition of fish muscle.
Texture profile analysis (TPA) is used to obtain a series of parameters, which include hardness, adhesiveness and so on, to evaluate the texture properties of muscle by simulating human oral movements [
37]. Texture characteristics are closely related to the acceptability of consumers [
38]. Crispy grass carp (
Ctenopharyngodon idellus C. ET V), belonging to freshwater fish, is popular with consumers because of its high crispness, hardness and chewiness of muscle [
38]. In the present research, compared with the control group, the 4% HC and 6% HC groups have significantly higher muscle chewiness, and the 6% HC group has significantly higher muscle hardness. The increased hardness in 6% HC group might be linked to the increased myofiber density. The higher the myofiber density, the harder the muscle [
22]. In the present experiment, the myofiber density in 6% HC group was highest and significantly higher than that in other HC groups. In addition to this reason, the increase in muscle hardness and chewiness might be related to the increase in muscle collagen cross-linking. The content of hydroxylysine in hydrolyzed collagen (1.38%) was much higher than that in fish meal (0.39%). Hydroxylysine has the function of forming collagen cross-linking, which improves the mechanical, thermic stableness and tractile intensity of collagen fiber [
39]. The hardness of Atlantic salmon fillets was positively correlated with cross-linking concentration [
40]. Compared with the control group, all HC groups have significantly lower muscle adhesiveness. Adhesiveness is inversely proportional to cell binding force [
41]. The decrease in muscle adhesiveness might be related to the increase in intercellular binding force by hydrolyzed collagen. Muscle pH is related to glycolysis, fatty acid composition and some biological reactions, and the decrease in pH will lead to a soft texture [
21]. In the present research, no significant difference was detected in muscle pH, and the muscle chewiness and myofiber density increased significantly when the proportion of fish meal replaced with hydrolyzed collagen reached 4%, suggesting that hydrolyzed collagen had the function of improving muscle texture of the triploid fish, which will make it more acceptable to customers.
Superoxide dismutase (SOD), a kind of metalloenzyme, is capable of catalyzing superoxide radicals (O
2•−) into hydrogen peroxide (H
2O
2) and oxygen (O
2), which effectively protects the body from the damage of reactive oxygen species (ROS) [
42]. Glutathione peroxidase (GSH-Px) can eliminate organic hydroperoxides [
43]. SOD and GSH-Px are crucial in protecting the body from antioxidant damage. Malondialdehyde (MDA) is produced by chemical reactions and enzymatic catalysis of PUFAs, and it is a biomarker of oxidative stress [
44]. Compared with the control group, the activities of serum GSH-Px in all HC groups and the serum SOD in the 6% HC group increased significantly, and the content of serum MDA in all HC groups decreased significantly. The increased serum GSH-Px and SOD activities, and the decreased MDA content might be attributed to the antioxidant function of hydrolyzed collagen [
10]. Peptides isolated from gelatin hydrolysate of tilapia skin have been proven to be capable of reducing the level of intracellular ROS and increasing the expression of antioxidant factors [
45]. In the present study, serum GSH-Px activity increased significantly and serum MDA content decreased significantly with hydrolyzed collagen replacement, which coincided with the former research that the GSH-Px and SOD activities in the skin of mice were significantly increased after oral administration of the diet containing collagen hydrolysate [
46]. Excessive ROS would cause the oxidation of subcellular membrane and structural proteins, and ultimately affect muscle texture, water-holding capacity and other quality characteristics, which was harmful to muscle quality [
47]. When common carp (
Cyprinus carpio) was exposed to oxidative stress, the muscle physicochemical properties decreased significantly [
48]. The relationship between oxidative stress and muscle quality indicated that hydrolyzed collagen improved muscle quality probably by improving the antioxidant capacity of triploid crucian carp.
Glucose is an important source of energy for physiological activities [
49]. Glucose in the blood is absorbed by cells in the liver and muscle under the action of insulin, and stored in the form of glycogen [
50]. Hepatic glycogen is crucial for maintaining normal glucose homeostasis in the body, and it is decomposed to keep the blood glucose level in starvation in a normal range, and the content of hepatic glycogen is related to metabolic pathways such as gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis [
51]. The serum glucose content decreased significantly when the proportion of fish meal replaced with hydrolyzed collagen reached 4%. The decreased serum glucose content might be attributed to the increased expression level of
IGF-1 in the liver. In the present experiment, the expression level of
IGF-1 in the liver increased significantly when the proportion of hydrolyzed collagen replacing fish meal reached 2%. Insulin-like growth factor-1, a peptide hormone, has similar amino acid sequences and functions to insulin, and it can promote glucose uptake in peripheral tissues [
52]. In addition, the decreased serum glucose content in 4% HC and 6% HC groups might also be related to the enhancement of antioxidant capacity. In the present research, the serum GSH-Px activity in all HC groups increased significantly, and the serum MDA content decreased significantly compared with the control group. There is a tight relationship between oxidative stress and insulin resistance, a situation in which cells failed to utilize insulin normally, and increased GSH-Px activity helps the body to resist oxidative stress [
50,
53]. Glycogen phosphorylase (
GPase) is vital in the process of glycogen decomposition, which releases glucose-1-phosphate (G-1-P) by cleaving α-1,4 glycosidic bridges, and G-1-P participates in the glycolytic pathway by conversion to glucose-6-phosphate (G-6-P) [
54]. Glucokinase (
GK) is the first enzyme in liver glycolysis, and it is also the speed-limiting enzyme, which catalyzes the conversion of glucose into G-6-P [
55]. The decreased expression levels of
GPase and
GK in the liver might be attributed to the decreased hepatic glycogen content, resulting in the decreased activities of glycogen decomposition and glycolysis-related enzymes, but the mechanism of hepatic glycogen decrease needs to be further studied. High glucose level may stimulate vascular cells to produce ROS through the activation of NAD(P)H oxidase, which depends on protein kinase C (PKC) pathway, and ultimately affect the quality of fish [
47,
56]. In the present experiment, the decrease in serum glucose content may be beneficial in improving the muscle quality of fish to some extent.
Lipids are a class of organic molecules, including triglycerides, sterols, and so on, and they are crucial in energy storage and cell signal transmission [
49]. When the lipid level in the body is too high, there may be various diseases, such as hypertriglyceridemia [
57]. Lipids cannot flow freely in the blood because of their insolubility in water, and lipoproteins (such as HDL-C, LDL-C, very-low-density lipoproteins (VLDLs), and chylomicrons) are the main carriers of lipids transported in the blood [
49]. Cholesterol, a kind of sterol, is mainly synthesized in the liver and is the precursor of steroid hormones, bile acids, etc. [
58]. Low-density lipoprotein (LDL) is the main carrier of cholesterol transportation from the liver to body tissues, and a high level of LDL in serum will greatly increase the risk of arterial disease [
58]. High-density lipoprotein (HDL) is the main carrier of cholesterol transportation from organism tissues to the liver, and HDL is decomposed in the liver or excreted as waste, which is essential to prevent the body from suffering from arterial diseases [
58,
59]. In the present experiment, the content of serum triglyceride and total cholesterol decreased significantly with hydrolyzed collagen replacement. Serum LDL-C content and liver TG content decreased significantly when the proportion of hydrolyzed collagen replacing fish meal reached 4% and 6%, respectively. The decrease in lipid content might be attributed to the increased expression levels of genes related to lipolysis and the decreased expression levels of genes related to lipogenesis in the liver. The
HADH,
LPL and
CPT 1 genes are related to lipolysis. Lipoprotein lipase is in the position to hydrolyze triglycerides in chylomicrons and VLDL [
60]. Both Hydroxyacyl-coenzyme A dehydrogenase and carnitine O-palmitoyltransferase 1 catalyze the β-oxidation of fatty acids [
61,
62]. The
FAS and
SREBP 1 genes are related to fat synthesis. Long-chain fatty acids, the principal component of triglyceride, are synthesized under the catalysis of fatty acid synthase [
60]. The synthesis of cholesterol and fatty acids is controlled by the transcription factor of SREBP [
63], of which SREBP-1 promotes the expression of adipogenic genes [
60]. In the present research, the expression levels of
HADH,
LPL and
CPT 1 in the liver increased significantly, and the expression levels of
FAS and
SREBP 1 decreased significantly with hydrolyzed collagen replacement, indicating that hydrolyzed collagen decreased the body lipid content probably by promoting fat decomposition and inhibiting fat synthesis. In addition, the decrease in serum lipid content might be related to the decrease in SFAs content and the increase in PUFAs content in muscle. Previous studies have proved that the higher the SFAs content, the higher the LDL-C content in the body [
36], and the n-3 PUFAs and n-6 PUFAs are effective in reducing LDL-C and T-CHO content [
33]. In the present experiment, the content of serum TG, T-CHO and LDL-C decreased significantly with fish meal replaced with hydrolyzed collagen, and the results coincided with the former research [
8], which indicated that the levels of plasma total lipids and triglycerides decreased significantly in mice treated with oral collagen hydrolysates.