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 (PDH(a)) 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 PDH(a) 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 over-night-fasted refed group), although [¹⁴C]glucose contribution to acetyl-CoA increased disproportionately compared with PDH(a) 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 PDH(a). 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.