Figure 3. A, Pathway of glutamine metabolism in the kidney. 1, Phosphate-dependent glutaminase; 2, glutamate dehydrogenase; 3, reactions of the tricarboxylic acid (TCA) cycle; 4, NADH-malate dehydrogenase; 5, NADP⁺-dependent malic enzyme; 6, pyruvate carboxylase; 7, pyruvate kinase; 8, pathway of gluconeogenesis (cytosol). B, Pathway of glutamine metabolism in the intestine. 1, Phosphate-dependent glutaminase; 2, alanine aminotransferase; 3, reactions of the TCA cycle; 4, NADP⁺-dependent malic enzyme. C, Pathway of glutamine metabolism in the periportal and perivenous cells of the liver. Glutamine nitrogen is utilized for urea synthesis while the carbon skeleton is used for glucose synthesis by periportal cells. Under conditions in which arginine availability is not limiting, glutamine is synthesized in the perivenous cells. 1, Phosphate-dependent glutaminase; 2, glutamate dehydrogenase; 3, enzymes of the gluconeogenesis pathway; 4, carbamoyl-phosphate synthetase; 5, ornithine transcarbamoylase; 6, argininosuccinate synthetase; 7, argininosuccinase; 8, arginase; 9, enzymes of glutamate synthesis; 10, glutamine synthetase.