Figure 3.

Enzymes that catabolize or use essential amino acids. Each of the amino acids that are considered essential for mammalian cells, because they are unable to synthesize them, are either catabolized (red arrows) or used to synthesize various products (blue arrows) by specific enzymes. Many of these enzymes are up-regulated in dendritic cells in response to inflammation and cytokines (both pro- and anti-inflammatory) or by the action of regulatory T (Treg) cells (highlighted in bold face). For example, arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS/NOS2) can both consume arginine, the availability of which is sensed through the RAG/mammalian target of rapamycin (mTOR) pathway, leading to the inhibition of T-cell proliferation and the promotion of forkhead box P3 (FOXP3) expression (Fig. 1).