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United States Department of Agriculture/Agriculture Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030
Infant pigs (8.5 kg) were fasted for 16 h and
infused for 6 h with
[U-13C]glucose. The
fractional abundances of all 13C
mass isotopomers of plasma glucose, lactate, and pyruvate and of
plasma, hepatic, and very low density lipoprotein apolipoprotein B-100
(apoB-100) alanine, glutamate, and aspartate were measured. The ratios
of
[13C3]aspartate,
[13C3]glutamate,
and
[13C3]alanine
in apoB-100 were used to estimate the positional equilibrium of
[13C3]oxaloacetate,
the fractional contribution of pyruvate carboxylase to the hepatic
oxaloacetate flux, and the activity of hepatic pyruvate dehydrogenase.
The values were compared with those based on glucose labeling and
previously published equations. The two methods [Katz and Lee
method (J. Katz, P. A. Wals., and W.-N. P. Lee. J. Biol. Chem. 264: 12994-13001, 1989) and apoB
method] gave similar estimates of the positional equilibrium of
[13C3]oxaloacetate
(0.59, Katz and Lee method; 0.61, apoB method) but slightly different
estimates of the contribution of pyruvate carboxylase to the
oxaloacetate flux (0.36, Katz and Lee; 0.31 apoB). Gluconeogenesis
apparently contributed between 71 (Katz and Lee method) and 80% (apoB
method) of the glucose entry rate (25 µmol · kg
1 · min1),
and pyruvate dehydrogenase contributed 20% of the hepatic acetyl-CoA. We conclude that the labeling of aspartate in apoB-100 provides a good
estimate of the isotopomer distribution in hepatic oxaloacetate but may
underestimate the absolute isotopic enrichment by 50%.
hepatic amino acids; stable isotopes
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