The supplementation of taurine to all plant–protein, taurine-free basal diets enhances growth and feed efficiency in carnivore fish (e.g., the rainbow trout and the Japanese flounder), but not the common carp, which suggests the suboptimal de novo synthesis of taurine by certain aquatic species ( 6). Pigs, ruminants, and poultry do not need dietary taurine for growth, milk production, or egg production. Human infants, who have relatively low activities of both cysteine dioxygenase and cysteine-sulfinate decarboxylase compared with adults, require the dietary intake of taurine for maintaining normal retinal, cardiac, and skeletal functions. In cats, the conversion of cysteine into taurine is limited due to a low activity of cysteine dioxygenase and of cysteine-sulfinate decarboxylase, which catalyzes the formation of taurine from cysteine-sulfinic acid. In addition to proteinogenic NEAAs, the de novo synthesis of nonproteinogenic AAs should also be considered in nutrition. The rate of Gly synthesis is much lower than the rate of Gly utilization in poultry and young pigs.
TAURINE AMINO ACID ESSENTIAL OR NONESSENTIAL PRO
In contrast to mammals, the synthesis of Pro from Arg in birds and certain fish is limited, and the synthesis of Pro from Glu and Gln is absent in birds and perhaps in most fish.
However, birds and some mammals (e.g., cats and ferrets) cannot synthesize Arg from Glu, Gln, or Pro in the enterocytes of the small intestine, which also may be true in most fish. The de novo synthesis of Arg in animal cells is species specific, with most mammals (e.g., humans, pigs, cattle, sheep, mice, and rats) synthesizing this AA from Glu, Gln, and Pro via the intestinal-renal axis. Rates of NEAA synthesis depend on the availability of EAAs and glucose, as well as species, breed, age, physiologic status, and disease state. Increasing evidence from studies in pigs, poultry, and fish has shown that animals do have dietary requirements of NEAAs to fulfill their genetic potential for maximum growth, reproduction, lactation, and production performance, as well as optimal health and well-being ( 4, 5). The concepts of EAAs and NEAAs have been used for more than a century. In most mammals (e.g., humans, rats, and pigs), the traditionally classified NEAAs are Ala, Arg, Asn, Asp, Cys, Glu, Gln, Gly, Pro, Ser, and Tyr ( 3). In contrast, AAs whose carbon skeletons are synthesized de novo by animal cells were considered to be dispensable in diets and were classified as “nutritionally nonessential” AAs (NEAAs) ( 2). In all animals, the EAAs consist of His, Ile, Leu, Lys, Met, Phe, Thr, Trp, and Val ( 3). The AAs whose carbon skeletons are not synthesized de novo by animal cells were termed “nutritionally essential” AAs (EAAs) in 1912 and must be provided to animals to maintain their growth or nitrogen balance ( 2). l-AAs are much more abundant than d-AAs in nature and are the physiologic isomers in animal and plant proteins. Based on the configuration of glyceraldehyde ( l- or d-isomers as introduced by Emil Fischer in 1908), AAs (except for Gly, taurine, β-alanine, and γ-aminobutyrate, which have no asymmetric carbon) exist as either l- or d-AAs. Amino acids (AAs) 3 are organic compounds that contain amino and acid groups ( 1).
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