Glycoconjugates as noninvasive probes of intrahepatic metabolism: I. Kinetics of label incorporation with evidence of a common precursor UDP-glucose pool for secreted glycoconjugates.
Conditions are described under which hepatic metabolism of administered labeled sugars can be monitored in the intact rat by measuring output of label from the liver in two secreted glycoconjugates derived from intracellular UDP-glucose, namely glucuronic acid linked to an administered drug (acetaminophen), and galactose secreted in glycoproteins. Rats with indwelling intrajugular catheters were given constant infusions of acetaminophen at nontoxic doses, and acetaminophen-glucuronide was isolated from rat urine by HPLC. When an unprimed continuous infusion of [U-14C]glucose or [1-3H]galactose is begun during acetaminophen infusion, labeling of urinary acetaminophen-glucuronide attains a plateau with an apparent half-life of 2.1 h from labeled glucose as the 14C-donor, and 1.2 h from labeled galactose. The times required to attain steady-state are attributable either to formation or to hepatic secretion of the glucuronyl conjugate rather than to mixing in extracellular pools or to delays in renal excretion since the half-time of urinary excretion of intravenously injected acetaminophen-glucuronide is only 21 minutes. Consequently, the excreted glucuronide provides noninvasive evidence of the recent specific activity in the precursor pool of hepatic UDP-glucose feeding glucuronic acid. The second glycoconjugate used for comparison was galactose, isolated from the acute-phase plasma protein alpha 1-acid glycoprotein. This represents hepatic UDP-galactose labeling. Comparison of the labeling patterns of secreted galactose and glucuronic acid demonstrates that they are derived from the same intracellular pool of UDP-glucose. This is in contrast to the patterns of labeling of hepatic glycogen from (1-3H) and [U-14C]glucose which have been shown to diverge from those of secreted glucuronic acid.