Flux of the L-serine metabolism in rat liver. The predominant contribution of serine dehydratase.

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 glucagon-treated 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 3H 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.

Inhibition by 2,5-anhydromannitol of glycolysis in isolated rat hepatocytes and in Ehrlich ascites cells.

2,5-Anhydromannitol decreases lactate formation and 3H2O formation from [5-3H]glucose in isolated rat hepatocytes metabolizing high concentrations of glucose. The inhibition of glycolysis is accompanied by a slight decrease in the cellular content of fructose-6-P and a more substantial decrease in the cellular content of fructose-1,6-P2, with no change in the content of glucose-6-P. The 3H2O release data and changes in hexosephosphate distribution indicate possible inhibitions at phosphofructokinase-1 and phosphoglucose isomerase. 2,5-Anhydromannitol also inhibits glycolysis in Ehrlich ascites cells, but the tumor cells, unlike hepatocytes, must be treated with 2,5-anhydromannitol prior to exposure to glucose to obtain the inhibition. The decrease in 3H2O formation from [5-3H]glucose and the metabolite pattern that results from the addition of low concentrations (less than or equal to 0.25 mM) of 2,5-anhydromannitol indicate an inhibition at phosphofructokinase-1 that cannot be attributed to a decrease in the cellular content of fructose-2,6-P2. Higher concentrations (greater than or equal to 0.5 mM) of 2,5-anhydromannitol cause a substantial decrease in the cellular content of ATP that is accompanied by decreases in the content of glucose-6-P and fructose-6-P and transient increases in fructose-1,6-P2. In Ehrlich ascites cells, 2,5-anhydromannitol is metabolized to 2,5-anhydromannitol mono- and bisphosphate. The inhibition of glycolysis caused by 2,5-anhydromanitol decreases with time, because the phosphorylated metabolites formed during the preliminary incubation in the absence of glucose are rapidly dephosphorylated during the incubation in the presence of glucose.

Glucose inhibition of epinephrine stimulation of hepatic gluconeogenesis by blockade of the alpha-receptor function.

For isolated rat hepatocytes, glucagon, 3':5'-cyclic AMP, 3':5'-cyclic GMP, and epinephrine stimulate the rate of gluconeogenesis from substrates not involving pathways of mitochondrial metabolism. From estimation of the rates of glucose formation, fructose 6-phosphate phosphorylation, and lactate and pyruvate formation it is concluded that epinephrine and 3':5'-cyclic GMP stimulate gluconeogenesis from either galactose or fructose by influencing the rate of reactions involving fructose 6-phosphate in a manner similar to that already reported for glucagon and 3':5'-cyclic AMP. Each agent acts to inhibit flux through phosphofructokinase (EC 2.7.1.11) and enhance flux through fructose diphosphatase (EC 3.1.3.11), resulting in the re-direction of carbon from lactate and pyruvate formation to glucose synthesis. In addition to 3':5'-cyclic GMP, dibutyryl 3':5'-cyclic GMP, 8-bromo 3':5'-cyclic GMP, 8-benzyl-thio 3':5'-cyclic GMP and 8-(4-chlorophenyl)thio 3':5'-cyclic GMP stimulate glucose formation and inhibit lactate and pyruvate formation from galactose. Guanosine monophosphate and 2':3'-cyclic GMP are inactive. As the stimulatory effect of epinephrine is inhibited by phenoxybenzamine and not by propranolol, and is not simulated by isoproterenol, it is concluded that catecholamine activity is expressed through the alpha-receptor. Increased extracellular glucose concentration (>10 mM) decreases the stimulatory effect of epinephrine, 3':5'-cyclic GMP, and partially that of 3':5'-cyclic AMP but does not alter the efficacy of glucagon.