Introduction to Human Nutrition

54 Introduction to Human Nutrition

For example, glycine is an important anti-infl amma-
tory, immunomodulatory, and cytoprotective agent
through the glycine receptor on the cell surface. The
role of cysteine in regulating glutathione synthesis
and its role in protection against oxidative damage

has been well established. The physiology of the argi-

nine–nitric oxide pathway has also been an active area

of investigation. In general, these nonprotein func-

tions of amino acids serve important functions in the

maintenance of (1) immune and other protective

functions; (2) digestive function; and (3) cognitive

and neuromuscular function. It is also worth noting

that these functions are primarily exerted by nutri-

tionally dispensable amino acids. Hence, the de novo

synthesis pathways and/or the amount of exogenous

supply of these amino acids or their precursors are

important in modulating the physiological and patho-

physiological conditions.

4.4 Classifi cation of amino acids

“Indispensability” as a basis of

classifi cation

For most of the past 65 years amino acids have been

divided into two general, nutritional categories: indis-

pensable (essential) and dispensable (nonessential).

This categorization provided a convenient and gener-

ally useful way of viewing amino acid nutrition at the

time. The original defi nition of an indispensable

amino acid was:

One which cannot be synthesized by the animal

organism out of materials ordinarily available

to the cells at a speed commensurate with the

demands for normal growth.

There are three important phrases in this defi nition:

ordinarily available, at a speed and for normal

growth.

The phrase “ordinarily available” is an important

qualifi er within this defi nition because a number

of nutritionally essential amino acids, for example

the branched-chain amino acids, phenylalanine and

methionine, can be synthesized by transamination of

their analogous α-keto acids. However, these keto

acids are not normally part of the diet and so are not

“ordinarily available to the cells.” They may be used

in special situations such as in nitrogen-accumulating

diseases, including renal failure, where they may assist

in maintaining a better status of body nitrogen

metabolism.

The phrase “at a speed” is equally important

because there are circumstances in which the rate of

synthesis of an amino acid may be constrained, such

CH 2

CH 2

CH 2

A

CH 3

CH 3

HN

NH

N

C

NG,NG-dimethyl-L-arginine
(ADMA)

NG,NG-dimethyl-L-arginine

(SDMA)

CH 2

CH 2

CH 2

A

CH 3

HN

NH

N

C

H 3 C

Ornithine

Figure 4.3 Some amino acids that arise via a post-translational modi-
fi cation of a polypeptide-bound amino acid. These amino acids are not
coded by DNA but are important determinants of the structural and
functional characteristics of proteins. Shown are (1) the formation of
hydroxyproline, from proline, involved in the maturation of the differ-
ent types of collagens in cells; (2) the methylation of a specifi c histidine
in the muscle protein actin (it could be that this modifi cation gives this
protein its ability to function effectively in the contractile activities of
the skeletal muscles that help us to move about); and (3) the methyla-
tion of arginine to form asymmetric and symmetric dimethylarginine,
which serve as an endogenous nitric oxide synthase inhibitor and play
important roles in modulating nitric oxide production and organ blood
fl ow in health and diseased conditions.