**Acknowledgements**

*Bioactive Compounds - Biosynthesis, Characterization and Applications*

far less is known about effects of excess cysteine ingestion [87].

**6. Methionine-cysteine balance**

ensure efficient utilization of the SAAs by broilers.

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

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

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.

The primary ingredients used in broiler poultry nutrition are limiting in SAAs in particular. Methionine and cysteine as the major SAAs, are α-amino acids which are important in animal nutrition. The recent increase in growth potentials of modern commercial broilers has been attributed to genetic improvement resulting in increased appetite and early market weight. They are more responsive to proteins (amino acids) than to energy concentration due to reduced maintenance need. Amino acids play vital nutritional and physiological roles in all livestock and must be supplied in appropriate ratios to foster protein accretion, the major objective in broiler poultry production. Nutritionally, amino acids are equivalent to proteins,

Dietary amino acids must be in the ideal ratios for efficient use of proteins in diet formulation, maximum utilization and minimum excretion of nitrogen. Among the SAAs, methionine and cysteine are gluconeogenic and need to be balanced to circumvent the lethality associated with excess cysteine, an apparently irreplaceable amino acid. However, biosynthesis of cysteine occurs in animals and plants via the

warranting a shift in focus from proteins to individual amino acids.

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**7. Conclusions**

We are deeply indebted to the omniscience God, the Fountain of all wisdom, who we worship and adore, for endowing and equipping us with the necessary wisdom and skills to write this chapter. We also thank our colleagues friends and wellwishers for their encouragement. Special appreciation to Joan Business Services, Azikiwe Road, Port Harcourt, Nigeria, for skillfully typesetting the manuscript under short notice.

Finally, we wish to thank our wives and children, very specially, for their encouragement, and for having to bear with our apparent momentary unavailability during the preparation of the manuscript: For Nte - Ruth (wife) and Awaji-inyiemi (son); and for Gunn - Regina (wife), Ewomazino (daughter), Oghenemaro (daughter), and Okgariorieze (son). We love you all unconditionally.
