FIGURE 1.

Pathway of glutamine utilization for the synthesis of citrulline. The infusion of 2-[¹⁵N]-glutamine has the potential to label the amido (5-) group of glutamine and the amino (2-), δ (5-) and ureido nitrogen of citrulline. The contribution of glutamine to citrulline synthesis was determined as the conversion of glutamine M+1 to citrulline M+1 (dashed boxes), the conversion of 2-[¹⁵N]-glutamine to 2-[¹⁵N]-citrulline (solid rectangle), and the conversion of [²H₅]-glutamine to [²H₅] citrulline (not shown for clarity). The reactions shown in the gray-shaded center box take place in multiple organs and cell types. However, the synthesis of CP and citrulline occurs in enterocytes, the site of production of most of the circulating citrulline. CP, carbamoyl phosphate; GSA, glutamate semialdehyde; α-KG, α-ketoglutarate.