The rate of gluconeogenesis from various precursors in the perfused rat liver

Abstract

1. The rates of gluconeogenesis from many precursors have been measured in the perfused rat liver and, for comparison, in rat liver slices. All livers were from rats starved for 48hr. Under optimum conditions the rates in perfused liver were three to five times those found under optimum conditions in slices. 2. Rapid gluconeogenesis (rates of above 0-5,.umole/g./min.) were found with lactate, pyruvate, alanine, serine, proline, fructose, dihydroxyacetone, sorbitol, xylitol. Unexpectedly other amino acids, notably glutamate and aspartate, and the intermediates of the tricarboxylic acid cycle (with the exception of oxaloacetate), reacted very slowly and were not readily removed from the perfusion medium, presumably because of permeability barriers which prevent the passage of highly charged negative ions. Glutamine and asparagine formed glucose more readily than the corresponding amino acids. 3. Glucagon increased the rate of gluconeogenesis from lactate and pyruvate but not from any other precursor tested. This occurred when the liver was virtually completely depleted ofglycogen. Two sites of action of glucagon must therefore be postulated: one concerned with mobilization of liver glycogen, the other with the promotion of gluconeogenesis. Sliced liver did not respond to glucagon. 4. Pyruvate and oxaloacetate formed substantial quantities of lactate on perfusion, which indicates that the reducing power provided in the cytoplasm was in excess of the needs of gluconeogenesis. 5. Values for the content of intermediary metabolites of gluconeogenesis in the perfused liver are reported. The values for most intermediates rose on addition of lactate. 6. The rates of gluconeogenesis from lactate and pyruvate were not affected by wide variations of the lactate/pyruvate ratio in the perfusion medium.

Restricted permeability of rat liver for glutamate and succinate

“…The data for the perfused liver are taken from Table 1 of Ross et al (1967). The homogenates were prepared from 1 part of liver (48hr.-starved) and 9vol.…”

“…Experiments on the rate of gluconeogenesis in rat liver from glutamate, aspartate and the intermediates of the tricarboxylic acid cycle indicate that these precursors do not readily penetrate to the site where they are metabolized (Ross, Hems & Krebs, 1967). If added to the perfused liver or to liver slices their rate of removal is slight compared with that of many other metabolites.…”

“…in the presence of 2N-H2S04 followed by heating to 1000 in the presence of 1N-KOH for 45min. oe-Oxoglutarate was determined by the method of Bergmeyer & Bernt (1963); succinate, the sum of L-malate plus fumarate, lactate, pyruvate and glucose were determined as described by Ross et al (1967). Alanine was determined by the method of Williamson, Lopes-Vieira & Walker (1967).…”

Mitochondrial aspartate glutamate carrier (citrin) deficiency as the cause of adult-onset type II citrullinemia (CTLN2) and idiopathic neonatal hepatitis (NICCD)

“…Lack of aspartate for the ASS reaction probably causes accumulation of citrulline. Aspartate is low in the plasma and is much less taken up by the liver, as judged from the rate of gluconeogenesis (Ross et al 1967), suggesting that the requirement of aspartate in the cytosol largely depends upon the supply from the mitochondria. Therefore, citrin deficiency probably causes a deficiency of aspartate in the cytosol of the liver, followed by inhibition of protein and nucleotide synthesis, resulting in hypoproteinemia in NICCD.…”

Requirement for alanine in the amino acid control of deprivation-induced protein degradation in liver.

“…But because there is little relevant information on the question of location, it is more appropriate to ask whether alanine itself or its metabolic products is the active regulator, independent of locus. Lactate, pyruvate, and alanine all produce glucose as their major end product (26)(27)(28). The first two, though, are more effective substrates even at physiological concentrations (28).…”