Glucose Synthesis in Tritiated Water

Abstract

Isolated rat or hamster liver parenchymal cells or kidney cortex segments were incubated in a medium containing 3HHO and the incorporation and partial distribution of tritium in the glucose formed was determined. Gluconeogenesis from lactate or pyruvate caused about 80 to 90% of the possible amount (7 μatoms per glucose molecule) of fixation of tritium from 3HHO in glucose, with a high percentage labelling on C‐6. Glucose formation from glycogen breakdown under anaerobiosis caused less than 20% of the possible fixation of tritium, with most of the labelling on C‐2. This technique may be of eventual use in determining rates and sources of glucose formation.

Sources of hepatic triglyceride accumulation during high‐fat feeding in the healthy rat

“…To our knowledge, there have been no direct measurements of acetyl-CoA enrichment from 2 H-enriched body water. Assuming that pyruvate is the main source of lipogenic acetyl-CoA and given that the exchange of pyruvate methyl protons with those of water is 80-95% complete (23,24), the enrichment of body water is a good approximation for that of the immediate acetyl-CoA precursor.…”

“…However, the measurement relies on some untested assumptions. We considered that pyruvate was the main source of lipogenic acetyl-CoA and that the exchange of pyruvate methyl protons with those of the water is essentially complete (23,24). Given this, the 2 H enrichment of body water is assumed to be equal to that of acetyl-CoA and hence can be used as the precursor enrichment.…”

Quantitation of Gluconeogenesis by 2H Nuclear Magnetic Resonance Analysis of Plasma Glucose Following Ingestion of 2H2O

“…The effective sensitivity of 2 H is comparable to that of 13 (11,12). In the case of plasma glucose, all seven aliphatic hydrogens are labeled to some extent under physiological conditions (3). As a result, the 2 H NMR spectrum of glucose obtained from these studies is highly congested with only the H1␣ and H1␤ signals adequately resolved (11).…”

“…Enrichment of hydrogen 2 (H2) from 2 H 2 O occurs at the level of glucose 6-phosphate (G6P) 1 and thereby reflects the sum of glycogenolytic and gluconeogenic fluxes. Enrichment of H3-H6 occurs at the level of triose phosphates or below, representing the contribution of gluconeogenic precursors to the total glucose output (1)(2)(3). Enrichment of H5 is considered the most representative of total gluconeogenesis since it reflects the contributions of phosphoenolpyruvate (PEP) plus glycerol to hepatic glucose output (2).…”

“…Enrichment of H3 is another potential marker of total gluconeogenesis since this position is enriched by 2 H 2 O at the level of triose phosphate isomerase. In an early study of 3 H 2 O metabolism by hepatocytes, it was concluded that glucose H3 and H5 are labeled to the same extent following gluconeogenic activity and that either site is a good marker of gluconeogenesis (3). Because 2 H 2 O is relatively safe, inexpensive, and easily administered to humans, measurement of the plasma glucose H5/H2 ratio has been applied in the clinical setting to determine the fractional contribution of gluconeogenesis to hepatic glucose output (1, 2, 4 -7).…”