Flux of the l-Serine Metabolism in Rat Liver

Abstract

L-Serine metabolism in rat liver was investigated, focusing on the relative contributions of the three pathways, one initiated by L-serine dehydratase (SDH), another by serine: pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT), and the other involving serine hydroxymethyltransferase and the mitochondrial glycine cleavage enzyme system (GCS). Because serine hydroxymethyltransferase is responsible for the interconversion between serine and glycine, SDH, SPT/AGT, and GCS were considered to be the metabolic exits of the serine-glycine pool. In vitro, flux through SDH was predominant in both 24-h starved and glucagontreated rats. Flux through SPT/AGT was enhanced by glucagon administration, but even after the induction, its contribution under quasi-physiological conditions (1 mM L-serine and 0.25 mM pyruvate) was about 1 ⁄10 of that through SDH. Flux through GCS accounted for only several percent of the amount of L-serine metabolized. Relative contributions of SDH and SPT/AGT to gluconeogenesis from L-serine were evaluated in vivo based on the principle that 3 H at the 3 position of L-serine is mostly removed in the SDH pathway, whereas it is largely retained in the SPT/AGT pathway. The results showed that SPT/AGT contributed only 10-20% even after the enhancement of its activity by glucagon. These results suggested that SDH is the major metabolic exit of L-serine in rat liver.

Flux of the l-Serine Metabolism in Rabbit, Human, and Dog Livers

“…The flux through SPT/AGT was enhanced by glucagon administration, but even after the induction, its contribution was about 1 ⁄10 of that through SDH both in vitro and in vivo. The flux through GCS was comparable with that through SPT/AGT in glucagon-treated rats (7). However, this pattern of L-serine metabolism in rat liver may not be extrapolated to other animals, because the SDH activity is known to decrease drastically as the body size of animals increases (8).…”

“…The reactions for the L-serine metabolism and the decarboxylation from [1-14 C]glycine in vitro were carried out under the same conditions as those described in the preceding paper (7 .95 Ci/ml; 14 C, 1.14 Ci/ml) was infused into the portal veins of separate rabbits continuously for 15 min at a rate of ϳ4 ml/15 min, and after termination of the infusion, the rabbits were allowed to metabolize the infused substrates for another 5 min as described for the infusion into rats. Then the livers were isolated and immersed in liquid nitrogen immediately.…”

“…Then the livers were isolated and immersed in liquid nitrogen immediately. Radioactive glucose was isolated from the frozen livers, and its 3 H and 14 C radioactivities were differentially counted as described (7). Table I, the activities of SPT/ AGT, SDH, and SHMT in rabbit, dog, and human livers are compared with those in 24-h starved and glucagon-treated rat livers.…”

“…Materials-Sources of all reagents and male Wistar rats are described in the preceding paper (7). Japanese white male rabbits weighing ϳ2 kg were obtained from a local dealer.…”

“…Procedures for in Vitro Experiments-Cytoplasmic extracts, Mit-Ps suspensions, soluble fractions, and reconstituted cytoplasmic extracts were prepared from rabbit, human, dog, and rat livers as described (7). To obtain a mitochondrial suspension, a portion of the cytoplasmic extract (650 ϫ g supernatant) was centrifuged at 8200 ϫ g for 10 min, and the pellet was washed once and suspended in the homogenizing sucrose solution (0.25 M sucrose, 3 mM imidazole and HCl, and 0.1 mM EDTA, pH 7.4) to give a 2.5 ml suspension/g of original liver.…”