The "nitrogen or amino acid pool" is a grand
mixture of amino acids available in the cell derived from dietary
sources or the degradation of protein. Since proteins and amino
acids are not stored in the body, there is a constant turnover
of protein. Some protein is constantly being synthesized while
other protein is being degraded. For example, liver and plasma
proteins have a half-life of 180 days or more, while enzymes
and hormones may be recycled in a matter of minutes or hours.
Each day, some of the amino acids are catabolized producing
energy and ammonia. The ammonia is converted to urea and excreted
from the body and represents a drain on the nitrogen pool.
A nitrogen balance is achieved by a healthy person when the
dietary intake is balanced by the excretion of urea wastes. If
nitrogen excretion is greater than the nitrogen content of the
diet, the person is said to be in negative nitrogen balance.
This is usually interpreted as an indication of tissue destruction.
If the nitrogen excretion is less than the content of the diet,
a positive nitrogen balance indicates the formation of protein.
The dynamic balance of the nitrogen pool is summarized in
the graphic on the left.
|Quiz: According to properties studied
earlier, are lipids generally soluble or insoluble in the aqueous
portion of blood? Explain.