Fraction of hepatic cytosolic acetyl-CoA derived from glucose in vivo: relation to PDH phosphorylation state.
We measured the contribution of glucose to hepatic cytosolic acetyl-CoA in vivo in rats and compared it with the phosphorylation state of a potentially regulatory enzyme complex [pyruvate dehydrogenase (PDH)]. Xenobiotic probes were used to sample hepatic cytosolic acetyl-CoA [acetylated sulfamethoxazole (SMX)] and UDP-glucose (glucuronidated acetaminophen) in vivo during [U-14C]glucose infusions. Percent active (dephosphorylated) form of PDH (PDHa) was determined on freeze-clamped liver. First, we confirmed using liver cell elutriation that acetylation of SMX occurs in parenchymal hepatocytes. Next, the fraction of cytosolic acetyl-CoA derived from [14C]glucose in vivo was shown to depend on dietary state. Specific activity of acetyl-CoA relative to plasma glucose or hepatic UDP-glucose was lower after 48 h fasting than after overnight fasting, and glucose refeeding (25 mg.kg-1.min-1 iv) maximally increased [14C]-glucose fractional contribution to acetyl-CoA within 2 h in the overnight-fasted but not in the prolonged fasted group. Hepatic PDHa demonstrated a similar but not identical pattern. The isotopic and enzymatic parameters showed significant correlations (r2 = 0.61 in 48-h fasted-refed group, r2 = 0.28 in overnight-fasted refed group), although [14C]glucose contribution to acetyl-CoA increased disproportionately compared with PDHa as refeeding progressed. The indirect pathway of UDP-glucose synthesis correlated inversely with the fractional contribution of glucose to acetyl-CoA. In summary, the fraction of hepatic acetyl-CoA derived from glucose in vivo is influenced by acute and chronic dietary factors and is only partially explained by PDHa. Regulation of the carbon source of hepatic acetyl-CoA in vivo and interactions suggested by these results (e.g., glucose-fatty acid cycle; branch-point regulation of glucose recycling) can be addressed in a quantitative fashion using this experimental framework.
Kaempfer
S
S
Department of Nutritional Sciences, University of California, Berkeley 94720.
Blackham
M
M
Christiansen
M
M
Wu
K
K
Cesar
D
D
Vary
T
T
Hellerstein
M K
MK
D000105:Acetyl Coenzyme A / Q000378:metabolism*; D000818:Animals; D002250:Carbon Radioisotopes; D003600:Cytosol / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D008954:Models, Biological; D010766:Phosphorylation; D011768:Pyruvate Dehydrogenase Complex / Q000378:metabolism*; D011865:Radioisotope Dilution Technique; D051381:Rats; D011919:Rats, Inbred Strains; D013420:Sulfamethoxazole / Q000378:metabolism; D014316:Tritium; D014532:Uridine Diphosphate Glucose / Q000378:metabolism
D002250:Carbon Radioisotopes; D011768:Pyruvate Dehydrogenase Complex; D014316:Tritium; D000105:Acetyl Coenzyme A; D005947:Glucose; D013420:Sulfamethoxazole; D014532:Uridine Diphosphate Glucose