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chapter 17
Protein and Amino Acid Metabolism
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FIGURE 17-1
Overall metabolism of proteins and amino acids. Body protein is
maintained by the balance between the rates of protein synthesis and
breakdown. These processes are influenced by hormones and energy
supply.
some nonessential amino acids may become essential. For
example, when liver function is compromised by disease
or premature birth, cysteine and tyrosine become essential
because they cannot be formed from their usual precursors
(methionine and phenylalanine).
Glutamine, a nitrogen donor in the synthesis of purines
and pyrimidines required for nucleic acid synthesis (Chap-
ter 27), aids in growth, repair of tissues, and promotion
of immune function. Enrichment of glutamine in enteral
and parenteral nutrition augments recovery of seriously
ill patients. Arginine may be considered as a semiessen-
tial amino acid. It participates in a number of metabolic
pathways, namely, formation of urea and ornithine, crea-
tine and creatinine, spermine, agmatine and citrulline, and
nitric oxide (NO). The endothelial cells lining the blood
vessels produce NO from arginine, which has a major role
in vasodilator function (discussed later). Dietary arginine
supplementation improves coronary blood flow, reduces
episodes of angina, and helps in patients with walking
pain due to claudication.
Nitrogen Balance
For protein synthesis to occur, all 20 amino acids must
be present in sufficient quantities. Absence of any one
essential amino acid leads to cessation of protein synthe-
sis, catabolism of unused amino acids, increased loss of
nitrogen in urine, and reduced growth. Negative nitrogen
balance exists when the amount of nitrogen lost from the
body (as nitrogen metabolites excreted in urine and feces)
exceeds that taken in. This state continues until the essen-
tial amino acid deficiency is corrected. Negative nitrogen
balance also occurs in malabsorption syndromes, fever,
trauma, cancer, and excessive production of catabolic hor-
mones (e.g., hypercortisolism; see Chapter 32). When the
dietary nitrogen intake equals nitrogen losses, the body
is in nitrogen balance. In normal adults, anabolism equals
catabolism. When nitrogen intake exceeds nitrogen losses,
there is a positive nitrogen balance, with anabolism ex-
ceeding catabolism. The body retains nitrogen as tissue
protein, which is a characteristic of active growth and tis-
sue repair (e.g., growth in children, pregnancy, recovery
from an emaciating illness).
Quality and Quantity of Dietary
Protein Requirement
Dietary protein provides organic nitrogen and the essen-
tial amino acids. The quantitative estimation of protein
requirement must take into account the quality of protein,
as determined by its essential amino acid composition.
Dietary protein should provide all of the essential amino
acids in the appropriate amounts. If the concentration of
one amino acid is significantly greater or less than that of
the others (in a protein or amino acid mixture), utilization
of the others may be depressed and will be reflected in
growth failure.
A procedure for assessment of protein quality consists
of feeding growing rats various levels of the test protein
and assessing the slope of regression lines relating growth
rate and protein intake. Wheat protein is deficient in ly-
sine when compared with lactalbumin, which contains all
of the essential amino acids in desirable concentrations.
Wheat protein is therefore assessed to be 20% as effec-
tive as an equivalent amount of lactalbumin. Similarly,
proteins from corn, which are also deficient in lysine, do
not support optimal growth. However, genetic selection
and breeding programs have yielded strains of com with
higher lysine content. Proteins of animal origin—namely,
meats, eggs, milk, cheese, poultry, and fish—are of good
quality since they provide all of the essential amino acids.
Gelatin,
the protein derived from collagen, lacks trypto-
phan and is of poor quality. In general, plant proteins are
of poor quality because they lack one or more essential
amino acids. The best quality plant proteins are found in
legumes and nuts. Therefore, the diet of a pure vegetarian
requires careful planning to achieve a combination of pro-
teins that provide necessary amounts of all essential amino
acids. Combinations of complementary vegetable proteins
include rice and black-eyed peas; whole wheat or parched
crushed wheat (bulgur) with soybeans and sesame seeds;
