In order to directly determine the amount of label exchange that occurs in the tricarboxylic cycle from labeled alanine and lactate after the ingestion of a glucose load 11-'3Cjglucose was administered by continuous intraduodenal infusion to awake catheterized rats to achieve steady state jugular venous glycemia (160 mg/dl) for 180 min. Liver was freeze-clamped at 90 and 180 min, and perchloric acid extracts of the liver were subjected to '3C and 'H nuclear magnetic resonance analysis. Dilution in the oxaloacetate pool was determined by comparing the intrahepatic 13C enrichments of C2, C3 positions of glutamate with the C2, C3 positions of alanine and lactate. In addition steady state flux equations were derived for calculation of relative fluxes through pyruvate dehydrogenase/TCA cycle flux and pyruvate kinase flux/total pyruvate utilization.After glucose ingestion in a 24-h fasted rat direct conversion ofglucose was responsible for 34% ofglycogen. The intrahepatic dilution factor for labeled pyruvate in the oxaloacetate pool was 2.4. Using this factor, alanine and lactate contributed -55% to glycogen formation. Pyruvate dehydrogenase flux ranged between 24 and 35% of total acetyl-coenzyme A (CoA) production and pyruvate kinase flux relative to total pyruvate utilization was 40%.

Substrate cycling between pyruvate and oxaloacetate was assessed in awake 24-h fasted normal and triiodothyronine (T3)-treated rats. After a 20- or 60-min infusion of [3-13C]alanine (99% enriched, 12 mg/min) the 13C enrichments of liver glucose and alanine carbons were analyzed by 13C and 1H nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. Substrate cycling from phosphoenolpyruvate to pyruvate [via pyruvate kinase (PK)] and from oxaloacetate to pyruvate [via malic enzyme (ME)] relative to the pyruvate carboxylase (PC) flux [i.e., (PK+ME)/PC] was assessed by the ratio of the 13C enrichment of C-2 alanine relative to that in C-5 glucose. In the normal rats (PK+ME)/PC was 0.26 +/- 0.07 (n = 7, t = 20 min) and 0.37 +/- 0.08 (n = 4, t = 60 min). In the T3-treated rats the (PK+ME)/PC increased four- to fivefold to 1.03 +/- 0.19 (n = 8, t = 20 min) and to 1.83 +/- 0.19 (n = 3, t = 60 min) (P < 0.05 vs. normal rats). The liver enzyme activity of PK did not change with T3 treatment (normal 14.22 +/- 5.25 U/g liver vs. T3 treated 13.40 +/- 1.10 U/g liver), whereas both the enzyme activity ratio of PK (normal 0.47 +/- 0.15 vs. T3 treated 0.77 +/- 0.03, P < 0.05) and the activity of ME (normal 0.89 +/- 0.30 U/g liver vs. T3 treated 4.25 +/- 0.60 U/g liver, P < 0.05) increased with T3 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

In order to quantitate the pathways by which liver glycogen is repleted, we administered 11-'3Cjglucose by gavage into awake 24-h fasted rats and examined the labeling pattern of 'IC in hepatic glycogen. Two doses of i1-'3Cjglucose, 1 and 6 mg/g body wt, were given to examine whether differences in the plasma glucose concentration altered the metabolic pathways via which liver glycogen was replenished. After 1 and 3 h (high-dose group) and after 1 and 2 h (low-dose group), the animals were anesthetized and the liver was quickly freeze-clamped. Liver glycogen was extracted and the purified glycogen hydrolyzed to glucose with amyloglucosidase. The distribution of the "3C-label was subsequently determined by "C-nuclear magnetic resonance spectroscopy. The percent 'IC enrichment of the glucosyl units in glycogen was: 15.1±0.8%(C-1), 1.5±0.1%(C-2), 1.2±0.1%(C-3), 1.1±0.1%(C-4), 1.6±0.1%(C-5), and 2.2±0.1%(C-6) for the high-dose study (n = 4, at 3 h); 16.5±0.5%(C-1), 2.0±0.1%(C-2), 1.3±0.1%(C-3), 1.1±0.1%(C4), 2.2±0.1%(C-5), and 2.4±0.1%(C-6) in the low-dose study (n = 4, at 2 h). The average "3C-enrichment of C-1 glucose in the portal vein was found to be 43±1 and 40±2% in the high-and low-dose groups, respectively. Therefore, the amount of glycogen that was synthesized from the direct pathway (i.e., glucose -glucose-6-phosphate -glucose-l-phosphate -UDP-glucose -glycogen) was calculated to be 31 and 36% in the high-and low-dose groups, respectively. The "C-enrichments of portal vein lactate and alanine were 14 and 14%, respectively, in the high-dose group and 11 and 8%, respectively, in the low-dose group. From these enrichments, the minimum contribution of these gluconeogenic precursors to glycogen repletion can be calculated to be 7 and 20% in the high-and low-dose groups, respectively. The maximum contribution of glucose recycling at the triose isomerase step to glycogen synthesis (i.e., glucose -triose-phosphatesglycogen) was estimated to be 3 and 1% in the high-and lowdose groups, respectively.In conclusion, our results demonstrate that (a) only one-third of liver glycogen repletion occurs via the direct conversion of glucose to glycogen, and that (b) only a very small amount of glycogen synthesis can be accounted for by the conversion of glucose to triose phosphates and back to glycogen; this suggests that futile cycling between fructose-6-phosphate and fructose-1,6-diphosphate under these conditions is minimal. Our results also show that (c) alanine and lactate account for a minimum of Receivedfor publication 22 February 1985. between 7 and 20% of the glycogen synthesized, and that (d) the three pathways through which the labeled flux is measured account for a total of only 50% of the total glycogen synthesized. These results suggest that either there is a sizeable amount of glycogen synthesis via pathway(s) that were not examined in the present experiment or that there is a much greater dilution of labeled alanine/lactate in the oxaloacetate pool than previously appreciated, or some combination of...

Although the amino acid sequences and the structures of pyruvate kinase (PYK) isozymes are highly conserved, allosteric regulations differ. This suggests that amino acids with low conservation play important roles in the allosteric mechanism. The current work exploits a 'natural screen'‐ the 122 point mutations identified in the human gene encoding the erythrocyte PYK isozyme and associated with nonspherocytic hemolytic anemia ‐ to learn what amino acid positions in PYK may be important for allosteric regulations. In addition to the mutations, we consider the conservation of each amino acid position across 241 PYK sequences. Three groups of residue positions have been created, those with: (1) no disease causing mutation identified; (2) a disease causing mutation identified and high conservation across isozymes; and (3) a disease causing mutation identified and low conservation. Mutations at positions not identified in the natural screen are likely to be tolerated with minimal loss of function. Mutations at highly conserved positions are more likely to disrupt properties common to all PYK isozymes (e.g., structure, catalysis). Residues in the third group are likely to be involved in roles that are necessary for function but not common to all isozymes (e.g., allostery). Many of the Group 3 residues are located in the C‐domain and to a lesser extent the A domain. IUBMB Life, 58: 31 ‐ 38, 2006

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