**5. Vital roles of cysteine in broiler poultry nutrition**

Cysteine can be synthesized from methionine and serine by trans-sulfuration [47]. As one of the naturally occurring biogenic amino acids, cysteine plays crucial roles at all the levels of protein structure because it is easily oxidized to cystine, a feature that is very vital for the analysis of the primary structure of proteins; for effects on changes in secondary structure and for stabilization of tertiary and quartenary structure of proteins [69]. It was further noted that it possesses a sulfhydryl group in its side chain, according it a special position that cannot be replaced by any other amino acid. Cysteine, by virtue of its ability to form interand intra-chain disulfide bonds, plays a crucial role in protein structure and in protein-folding pathways. Such bonds, known as disulphide linkages, are common in proteins destined for export or residence on the plasma membrane [70]. He also

**283**

integrity and function.

with fat mass are eloquent testimonies [75].

*Cysteine in Broiler Poultry Nutrition*

reticulum protein).

*DOI: http://dx.doi.org/10.5772/intechopen.97281*

under conditions of impaired L-methionine catabolism.

noted that any mismatched disulfide bonds are rearranged to ensure the correct protein folding under the influence of protein disulfide isomerase (an endoplasmic

Basically, cysteine (and methionine too) is incorporated into structural proteins, and it is also required for normal growth. The two are major protein constituents of feathers and hair, with methionine occurring in greater percentage in muscle while cysteine is higher in feather keratin [61]. Cysteine has been reported to be thirteen (13) times higher in broiler feathers than methionine [71, 72]. This indicates their importance in growth and feather development of broiler poultry. Reduced feed intake and weight gain have been associated with L-cysteine supplementation in young animals. Its anoretic effects have been reported to manifest as reduced final body weight, body weight gain, feed intake, and feed efficiency in rats [73]. This was attributed to the bitter taste imparted by L-cysteine. However [74], reported that reduced (Cys) and oxidized (cystine) forms of cysteine support animal growth equally when provided in a cyst(e)ine deficient and methionine adequate diet. Since L-cysteine is a spare amino acid for Methionine, as the adverse effects of L-methionine deficiency can be ameliorated by L-cysteine supplementation in the diet of animals [75]. In the opinion of [54], whole body protein synthesis and physiological Homeostasis can be maintained by dietary supplementation of L-cysteine

Cysteine is involved in the biosynthesis of methionine by accelerating the pathway leading to the formation of pheomelanin, thereby blocking the formation of eumelanin that produces dark colors [76]. Cysteine itself is a powerful antioxidant and has the potential to trap reactive oxygen species (ROS) [5]. It plays a central role in the antioxidant protection system of the body such as glutathione (GSH), by functioning as a precursor of some constituents [77, 78]. GSH is a potent antioxidant which protects the body against toxic effects of elevated levels of endogenous and exogenous electrophiles [79]. Taurine, another SAA, and Hydrogen sulphide are also produced from dietary L-cysteine and they play vital roles in the reduction of oxidative stress and protection against several environmental toxins [80].

It has the capacity to improve intestinal histomorphometric indices of broiler chickens with a consequent increase in absorption of nutrients [81]. High L-cysteine concentration has been observed in proteins and mucins that contribute to the maintenance of gut integrity and plays key roles in intestinal structure and function [82, 83]. The indications are, therefore, that L-cysteine deficiency causes certain degrees of intestinal distortions and is essential in the maintenance of intestinal

Lipid metabolism is also mediated by L-cysteine and its derivatives such as S-methyl L-cysteine with hypoglyemic and antihyperlipidemic characteristics, through reduction in fasting blood sugar and total triglycerides [84], and N-acetlycysteine which improves lipid metabolism by affecting serum cholesterol, triglycerides, Very High Density Lipoproteins (VHDL), and High Density Lipoproteins (HDL) levels [85]. The mode of action of L-cysteine underlying these effects are not clear, but it is believed to be partially accounted for by its target on gene expression of certain biochemical substances such as element-binding protein and fatty acid synthase [85]. Its roles in lipid metabolism and positive correlation

Cysteine is, in fact, a rather potent reducing agent in addition to its capacity of being capable of either chelating or complexing trace elements [6]. Its reducing agent bio-activity when supplemented in diets at 0.38%, is capable of converting pentavalent to trivalent organic arsenic, which is up to 100 times more toxic and of great significance in poultry and animal nutrition [86]. The authors also opined that this has great implication in the use of certain poultry drugs e.g. coccidiostats

#### *Cysteine in Broiler Poultry Nutrition DOI: http://dx.doi.org/10.5772/intechopen.97281*

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

**4. Cysteine digestibility and bioavailability**

acid for endogenous protein synthesis [53].

**5. Vital roles of cysteine in broiler poultry nutrition**

converted into Cys [61].

Although there is adequate physiological concentrations of cysteine, many cells still rely on the trans-sulfuration pathway for a minimum of 47% of their cysteine requirements [60]. Cysteine requirement is therefore subsumed in the total SAAs requirement as captured by [35, 55] for different age ranges in poultry. Commercial diets are traditionally formulated to meet broiler requirements for methionine + cystine (Met + Cys), based on the assumption that amounts of dietary Met are

Following a combination of heat treatment and alkaline food processing some alterations occur in the chemical nature of cysteine leading to some effects in its digestibility and subsequent absorption. These two processes are vital in ensuring the assimilation of amino acids by broilers. Heat processing causes the oxidation of a significant portion of protein-bound cysteine to cystine, which has lower digestibility [62]. This may probably be due to the formation of disulphide bridges during the transformation process. Dietary cystine is also converted to lanthionine under the influence of heat and alkali treatment [63]. The reduced SAA activity of lanthionine results in reduced availability of protein-bound cysteine [64]. Since protein metabolism continues even when no protein is being consumed, some of the amino acids released are oxidized and are not available for re-synthesis of new proteins. Feeding a protein-free diet to broilers, therefore, elicit a cysteine response (reduction in body weight loss and improves nitrogen balance) [65], indicating that it could be substantially depleted in the body pool making it the first limiting amino

All the nutrients ingested by an animal via its diet cannot be utilized by the animal because some are undigested. Furthermore, some are either absorbed in forms that cannot be utilized for physiological and metabolic functions in the body or are not absorbed at all. The available nutrients refer to the portion of the nutrients that are digested, absorbed and metabolized [20]. The same is true of amino acids, which are bioavailable if they occur in forms that can be utilized by the cells for maintenance or production. Digestibility of amino acids, therefore, is the digestion of the amino acids consumed in the diets and their subsequent absorption from the lumen of the small intestine into the bloodstream [66]. The portion of the absorbed amino acids present in chemical forms amenable to protein synthesis indicates their bioavailability [17]. The concept of digestible amino acids is critical to establish ideal protein ratios [67], and broiler diets are now formulated based on digestible proteins and amino acids [68].

Cysteine can be synthesized from methionine and serine by trans-sulfuration [47]. As one of the naturally occurring biogenic amino acids, cysteine plays crucial roles at all the levels of protein structure because it is easily oxidized to cystine, a feature that is very vital for the analysis of the primary structure of proteins; for effects on changes in secondary structure and for stabilization of tertiary and quartenary structure of proteins [69]. It was further noted that it possesses a sulfhydryl group in its side chain, according it a special position that cannot be replaced by any other amino acid. Cysteine, by virtue of its ability to form interand intra-chain disulfide bonds, plays a crucial role in protein structure and in protein-folding pathways. Such bonds, known as disulphide linkages, are common in proteins destined for export or residence on the plasma membrane [70]. He also

**282**

noted that any mismatched disulfide bonds are rearranged to ensure the correct protein folding under the influence of protein disulfide isomerase (an endoplasmic reticulum protein).

Basically, cysteine (and methionine too) is incorporated into structural proteins, and it is also required for normal growth. The two are major protein constituents of feathers and hair, with methionine occurring in greater percentage in muscle while cysteine is higher in feather keratin [61]. Cysteine has been reported to be thirteen (13) times higher in broiler feathers than methionine [71, 72]. This indicates their importance in growth and feather development of broiler poultry. Reduced feed intake and weight gain have been associated with L-cysteine supplementation in young animals. Its anoretic effects have been reported to manifest as reduced final body weight, body weight gain, feed intake, and feed efficiency in rats [73]. This was attributed to the bitter taste imparted by L-cysteine. However [74], reported that reduced (Cys) and oxidized (cystine) forms of cysteine support animal growth equally when provided in a cyst(e)ine deficient and methionine adequate diet. Since L-cysteine is a spare amino acid for Methionine, as the adverse effects of L-methionine deficiency can be ameliorated by L-cysteine supplementation in the diet of animals [75]. In the opinion of [54], whole body protein synthesis and physiological Homeostasis can be maintained by dietary supplementation of L-cysteine under conditions of impaired L-methionine catabolism.

Cysteine is involved in the biosynthesis of methionine by accelerating the pathway leading to the formation of pheomelanin, thereby blocking the formation of eumelanin that produces dark colors [76]. Cysteine itself is a powerful antioxidant and has the potential to trap reactive oxygen species (ROS) [5]. It plays a central role in the antioxidant protection system of the body such as glutathione (GSH), by functioning as a precursor of some constituents [77, 78]. GSH is a potent antioxidant which protects the body against toxic effects of elevated levels of endogenous and exogenous electrophiles [79]. Taurine, another SAA, and Hydrogen sulphide are also produced from dietary L-cysteine and they play vital roles in the reduction of oxidative stress and protection against several environmental toxins [80].

It has the capacity to improve intestinal histomorphometric indices of broiler chickens with a consequent increase in absorption of nutrients [81]. High L-cysteine concentration has been observed in proteins and mucins that contribute to the maintenance of gut integrity and plays key roles in intestinal structure and function [82, 83]. The indications are, therefore, that L-cysteine deficiency causes certain degrees of intestinal distortions and is essential in the maintenance of intestinal integrity and function.

Lipid metabolism is also mediated by L-cysteine and its derivatives such as S-methyl L-cysteine with hypoglyemic and antihyperlipidemic characteristics, through reduction in fasting blood sugar and total triglycerides [84], and N-acetlycysteine which improves lipid metabolism by affecting serum cholesterol, triglycerides, Very High Density Lipoproteins (VHDL), and High Density Lipoproteins (HDL) levels [85]. The mode of action of L-cysteine underlying these effects are not clear, but it is believed to be partially accounted for by its target on gene expression of certain biochemical substances such as element-binding protein and fatty acid synthase [85]. Its roles in lipid metabolism and positive correlation with fat mass are eloquent testimonies [75].

Cysteine is, in fact, a rather potent reducing agent in addition to its capacity of being capable of either chelating or complexing trace elements [6]. Its reducing agent bio-activity when supplemented in diets at 0.38%, is capable of converting pentavalent to trivalent organic arsenic, which is up to 100 times more toxic and of great significance in poultry and animal nutrition [86]. The authors also opined that this has great implication in the use of certain poultry drugs e.g. coccidiostats

containing pentavalent organic arsenic, whose toxicity is accentuated by pharmacologic cysteine. It is well established that modest excesses of SAAs, particularly cysteine, can have marked pharmacologic effects on trace-mineral utilization, but far less is known about effects of excess cysteine ingestion [87].

#### **6. Methionine-cysteine balance**

Methionine and cysteine are closely related in that the latter is endogenously synthesized from the former via the trans-sulfuration pathway by L-methionine degradation [43]. In this pathway, methionine is converted to homocysteine, which in turn donates a sulfur group to serine (a non-essential amino acid) to ultimately form cysteine. The production of cysteine accounts for 47% of methionine dietary requirement [48]. L-cysteine can furnish up to 47% and 77% of the requirements for SAAs in young and older animals, respectively [88]. Nevertheless, the practice of formulating commercial diets to contain adequate methionine+cysteine, with the assumption that dietary methionine is converted to cysteine, is common. This may lead to reduced efficiency of amino acid utilization, since methionine will be supplied in excess. This can be addressed by adequate knowledge of methionine:cysteine ratio in relation to total sulfur amino acids (TSAAs), and the quantity of methionine converted to cysteine [61]. Another condition of imbalance is created when excess cysteine is provided in methionine deficient diets, with growth depressing effects in chicks [14]. Such imbalances need to be addressed to ensure efficient utilization of the SAAs by broilers.

As broilers age or increase in weight, maintenance needs for amino acids, including methionine and cysteine, and ideal amino acid ratios will alter. However, not much is known about the methionine:cysteine ratio, although a ratio of 52:43 [89] and a minimum of 49:45 [61] has been recommended for poultry and growing broilers, respectively. Also, little is known about the effects of excess cyst(e)ine on chicks, but among the EAAs, excess methionine is known to have the most adverse effects on growth [71, 90]. Variations in the ratio of these amino acids affect growth responses in broilers, and the utilization and efficacy of hydroxyl analogues of methionine or its precursors [6]. Therefore, determination of the optimum methionine:cysteine ratio in relation to TSAAs is necessary to foster proper growth and development of broilers.
