After digestion proteins are hydrolyzed into
amino acids and reach the intracellular amino acid pool, a metabolic
pool limited in size and not expandable (1, 2). From this pool amino
acids can follow 3 major pathways (1):
1. AAs can be used for the synthesis of new
endogenous proteins and other biological substances;
2. AAs can be irremediably oxidized by the body,
yielding urea (+ ammonia) and carbon dioxide (CO2) as terminal end-products
(see process of ureagenesis) and;
3. AAs can be converted into other compounds
(gluconeogenesis).
The free AA pool is maintained within tight
limits (3), even under a variety of conditions the free AA pool are very
similar (4,5). The body tries to maintain body protein stores at constant
levels (6). The concentration of each individual amino acid in the cell is
precisely regulated (4).
The free pool provides individual AAs for
protein synthesis and oxidation, and it is replenished either by protein
breakdown or AAs entering the body from the diet. For example, amino acids
involved in charging of muscle tRNA apparently come from the intracellular pool
(7).
Excluding taurine, the free pool has been
estimated to contain only 100 grams of AAs, and including taurine the free pool
increase up to 130 grams (8), with an additional 5 grams of free AAs
circulating in the bloodstream (8). The free pool is approximately 1% of the
size of the AA stored in tissue.
In an old study, 3g protein/kg only increased
blood concentrations of most AAs by 30% above normal levels, with
concentrations of BCAAs doubling over normal levels (9), which indicates that
the pool is tightly regulated. The concentration of amino acids in the
bloodstream is different from that seen within the muscle (3), changes in
blood AA levels may have no impact on intramuscular AA concentrations.
Protein and amino acids ingested in excess of those needed for
biosynthesis cannot be stored due to the limited size of the
intracellular free amino acid pool, which cannot be much expanded (1). Although
chronic strength exercises can increase the capacity for skeletal muscle
protein storage there is eventually a limit.
When protein intake surpasses the physiological
needs of amino acids, the excess amino acids are disposed of by three
major processes (1):
1. Increased oxidation, with terminal end
products such as CO2 and ammonia
2. Enhanced ureagenesis i. e. synthesis
of urea linked to protein oxidation eliminates the nitrogen radical
3. Gluconeogenesis,
i. e. de novo synthesis of glucose. This is one of the mechanisms developed by the
body to maintain blood glucose within a narrow range, (i. e. glucose
homeostasis). Gluconeogenesis, uses non-glycogenic precursors; in particular
certain specific amino acids (for example, alanine), as well as glycerol
(derived from fat breakdown) and lactate (derived from muscles) to produce
glucose.
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References
1. Yves Schutz. Protein Turnover, Ureagenesis and Gluconeogenesis. Int. J. Vitam. Nutr. Res., 81 (2 – 3), 2011, 101 – 107 101
2. Waterlow, JC. Protein turnover
with special reference to man. Q J Exp Phys (1984) 69: 409-438.
3. Furst, P. Intracellular
muscle free amino acids – their measurement and function. Proc Nutr Soc (1983)
42: 451-462.
4. Scriver, CR et. al. Normal
plasma amino acid value in adults: The influence of some common physiological
variables. Metabolism (1985) 34: 868-873.
5. Waterlow,
JC. Where do we go from here? J Nutr (1994) 124:1524S-1528S
6. Bauman, P.
Q., Stirewalt, W. S., O’Rourke, B. D., Howard, D. & Nair, K. S. (1994)
Precursor pools of protein synthesis: a stable isotope study in swine model.
Am. J. Physiol. 267: E203–E209.
7. Wagenmakers, AJ. Protein
and amino acid metabolism in human muscle. Skeletal Muscle Metabolism in
Exercise and Diabetes. ed. Richter et. al. Plenum Press: New York, 1998.
8. Wahren, J et. al. Effect of
protein ingestion on splanchnic and leg metabolism in normal man and in
patients with diabetes mellitus. J Clin Invest (1976) 57: 990-995.
9. Furst, P. Intracellular
muscle free amino acids – their measurement and function. Proc Nutr Soc (1983)
42: 451-462.