Agriculture Reference
In-Depth Information
AmIno AcIds
Protein is required in the diet to supply the indispensable amino acids that cannot
be synthesized in the body and to supply nitrogen for synthesis of the dispensable
amino acids. Dietary protein quality is largely dependent on how closely the quan-
tity and proportions of biologically available indispensable amino acids match the
needs of the consumer. Whole egg and milk proteins are particularly high in quality,
whereas grain proteins tend to be low in lysine, with corn also low in tryptophan.
By combining two or more protein sources that have complementary amino acid
concentrations, a high-quality food may be created from items that are individually
poor. Such a combination can result by using appropriate proportions of grain and
legume proteins.
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Those (indispensable) amino acids that cannot be synthesized by normal human tis-
sues at a rate sufficient to meet needs are histidine, isoleucine, leucine, lysine, methio-
nine, phenylalanine, threonine, tryptophan, and valine. Thus, required amounts must
be present in the diet. Preformed cystine (or cysteine) can meet some of the need for
methionine, and preformed tyrosine can meet some of the need for phenylalanine.
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Premature infants or humans with disease-associated organ malfunction may have
dietary requirements for amino acids that would otherwise be dispensable. Their
rates of synthesis appear to be inadequate under certain physiological or pathological
circumstances to meet cellular needs. As a consequence, cysteine, tyrosine, proline,
arginine, or glutamine may be conditionally indispensable. For example, metabolic
functions of the organs of premature infants may not be fully developed, and the
ability to synthesize dispensable amino acids, such as cysteine and proline, is often
limited. There is evidence that activity of the liver enzyme cystathianase, required
for conversion of methionine to cysteine, does not reach adult levels even in full-
term infants until they are at least 4 months old. Cirrhosis of the liver in adults, from
excessive long-term alcohol consumption, may impair the synthesis of tyrosine from
phenylalanine and of cysteine from methionine and, as a consequence, impair syn-
thesis of metabolically important proteins. The genetic disorder phenylketonuria is
associated with limited phenylalanine hydroxylase activity required for conversion
of phenylalanine to tyrosine.
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Amino acids that can be synthesized from tissue or dietary precursors at a rate suf-
ficient to meet needs (if protein-nitrogen supplies are adequate) are considered dis-
pensable. These normally include alanine, arginine, asparagine, cysteine, cystine,
glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
