Serine synthesis in rat kidney: studies with perfused kidney and cortical tubules.

Renal serine synthesis was studied in the isolated perfused kidney and in isolated cortical tubules. Serine was produced by the perfused kidney from both glycine and aspartate, indicating flux through at least two separate pathways: serine hydroxymethyltransferase and either the nonphosphorylated or phosphorylated intermediate pathways. The precise nephron site of serine production was determined by measuring serine synthesis from various precursors and the activities of enzymes of both pathways in isolated tubules fractionated on a Percoll gradient into proximal tubule and distal tubule fractions. Both pathways of serine synthesis were located in proximal tubules. Detailed studies of serine synthesis from glycine demonstrated extremely tight coupling between the glycine cleavage enzyme and serine hydroxymethyltransferase, since the rate of 14CO2 production from [2-14C]glycine was less than 5% of that of [1-14C]glycine, whereas the rate of incorporation of 14C into serine from [2-14C]glycine was double that from [1-14C]glycine. These studies demonstrate that the kidney can synthesize serine by two separate pathways, both located in the cells of the proximal tubule.