Nitrogen balance: The condition wherein the amount of nitrogen ingested
(via protein) is equal to that excreted. Negative nitrogen balance is a
condition wherein more nitrogen is excreted than ingested, usually dur-
ing prolonged periods of calorie restriction. Positive nitrogen balance
results when more nitrogen is ingested than excreted, as in growing
Pyridoxal phosphate: The coenzyme that is required for transaminase
(aminotransferase) reactions, as well as other enzymes. It is the active
form of pyridoxine (vitamin B6).
Proteins are polymers of a-amino acids covalently linked via peptide bonds.
a-Amino acids consist of a central (a) carbon atom to which an amino group, a
carboxylic acid group, a hydrogen atom, and a side-chain group are covalently
linked. Twenty different amino acids are used for protein synthesis, each of
which is encoded by at least one codon (the three nucleotide genetic code)
and differ only in their side-chain group. Amino acids can be divided into two
classes, essential and nonessential amino acids. Essential amino acids cannot
be synthesized by humans (unlike nonessential amino acids) and therefore
must be ingested to meet the requirements of the organism. Certain nonessen-
tial amino acids can become pseudoessential if the starting material from which
they are synthesized becomes limiting (e.g., methionine-derived cysteine).
Amino acids are also used for the synthesis of nonprotein biomolecules (e.g.,
nucleotides, neurotransmitters, and antioxidants), as well as participating in
critical whole body processes, such as interorgan nitrogen transfer and
acid-base balance. Unlike carbohydrate and fatty acids, no storage form of
excess amino acids exists per se. Instead, dietary amino acids in excess to the
body’s synthetic needs are used as an energy source and/or converted to glyco-
gen and lipid. During periods of insufficient nutrient ingestion (e.g., starva-
tion, anorexia nervosa), noncritical skeletal muscle and liver proteins are
preferentially degraded to release utilizable amino acids, to meet both the
biosynthetic and energy needs of the body (Figure 39-1).
The major sites of ingested protein digestion are the stomach and the
small intestine. Gastric, pancreatic, and intestinal peptidases hydrolytically
cleave peptide bonds. The released amino acids, dipeptides, and tripeptides are
transported into small intestinal epithelial cells, wherein dipeptides and tripep-
tides are further degraded to free amino acids. The latter are subsequently
released into the circulation. A healthy, well-fed adult is generally in nitrogen
balance. This means that the amount of nitrogen ingested (as protein) is equal
to that excreted (primarily as urea). When rates of dietary amino acid incor-
poration into new protein exceed rates of amino acid degradation and nitrogen
excretion, the individual is said to be in positive nitrogen balance. Growing
children are normally in positive nitrogen balance. In contrast, an individual is
said to be in negative nitrogen balance when more nitrogen is excreted
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