Small Intestine Enterocytes
E i e m
Figure 39-3. Schematic diagram showing the central role of glutamine as a
transporter of amino acid nitrogen to various tissues.
acids. Conditions that influence the tryptophan-to-branched-chain amino acid
ratio in the circulation have been shown to affect serotonin levels in the brain,
because of competition for this transporter. For example, increased skeletal
muscle branched-chain amino acid catabolism during starvation is associated
with an increase in the blood tryptophan-to-branched-chain amino acid ratio,
increased tryptophan uptake in the brain, and increased serotonin synthesis.
The latter then influences wakefulness.
Amino acids also play an important role in cellular signaling. The guanidino
group from arginine is used in the synthesis of nitric oxide, a ubiquitous, highly
reactive signaling molecule that regulates multiple physiologic processes,
including blood pressure, immune response, learning, and metabolism. Much
less understood is the mechanism by which leucine affects cellular signaling.
This amino acid has been termed a “pseudohormone” following the observa-
tions that one or more of its early metabolites (e.g., a-ketoisocaproate) is/are
among the strongest regulators of protein turnover identified to date.