select few amino acids can also be directly deaminated, e.g., threonine), glu-
tamate formation increases markedly during periods of increased amino acid
catabolism. Ultimately, the majority of this glutamate is oxidatively deami-
nated in the liver, where the released ammonium is incorporated into urea, via
the urea cycle, and subsequently excreted in urine (Figure 39-2).
The carbon skeleton released during amino acid catabolism can undergo
multiple fates, depending on the metabolic situation during which it was
formed, the cell type within which it was generated, and the amino acid from
which it is derived. For example, during periods of excess amino acid inges-
tion, the amino acid-derived carbon skeleton is either used as a metabolic
fuel or converted to glycogen or lipid. In contrast, when rates of amino acid
catabolism are increased as a result of prolonged caloric insufficiency, a large
portion of the carbon skeleton is used by the liver for the synthesis of either
glucose or ketone bodies, depending on the specific amino acid. Indeed,
glucogenic amino acids (those amino acids whose carbon skeleton can gener-
ate net glucose through gluconeogenesis) are essential for maintenance of
blood glucose levels during prolonged caloric insufficiency. Maintenance of
blood glucose is also made possible by increased reliance of skeletal muscle
on amino acids as a fuel source, thereby decreasing glucose utilization.
Certain amino acids are preferentially used in a tissue-specific manner. For
example, skeletal muscle has a relatively high capacity for branched-
chain amino acid (leucine, isoleucine, and valine) utilization. Following
Figure 39-2. General pathway for the degradation of amino acids showing the
relationship between extrahepatic tissues and the liver, which is the site of the
formation of urea.
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