Generation of extramitochondrial reducing power in gluconeogenesis

Abstract

1. Kidney-cortex slices incubated with pyruvate formed glucose and lactate in relatively large and approximately equimolar quantities. The formation of these products involves two exclusively cytoplasmic NADH(2)-requiring reductions, catalysed by lactate dehydrogenase and triose phosphate dehydrogenase. From the rates of glucose and lactate formation it can be calculated that over 1000mu-moles of NADH(2) must have been produced in the cytoplasm/g. dry wt. of tissue/hr. 2. When lactate is a gluconeogenic precursor the required NADH(2) is generated in the cytoplasm, but, when a substrate more highly oxidized than glucose, such as pyruvate, is the precursor, there is no direct cytoplasmic source of NADH(2). Quantitative data on the fate of pyruvate are in accord with the conclusion that the NADH(2) was primarily formed intramitochondrially by the dehydrogenases of cell respiration, with pyruvate as the major substrate. 3. Similar observations and conclusions apply to experiments with mouse-liver slices incubated with pyruvate, serine or aspartate. 4. Addition of ethanol, which increases the formation of NADH(2) in the cytoplasm, increased the formation from pyruvate of lactate but not of glucose. 5. In view of the low permeability of mitochondria for NAD and NADH(2) it must be postulated that special carrier mechanisms transfer the reducing equivalents of intramitochondrially generated NADH(2) to the cytoplasm. Reasons are given in support of the assumption that the malate-oxaloacetate system acts as the carrier. 6. Various aspects of the generation of reducing power and its transfer from mitochondria to cytoplasm are discussed.

Hepatic fructose-1,6-diphosphatase deficiency

“…The markedly reduced state of the NAD-NADH couples lactate-pyruvate and a-glycerophosphate-dihydroxyacetone phosphate in the liver are due to the absence of FDPase, resulting in accumulation of the precursor glyceraldehyde-3-phosphate, a step at which NADH is reoxidized to NAD (20) (Fig. 7).…”

Effect of Oleic Acid on the Oxidation and Gluconeogenesis from [1‐14C]Pyruvate in the Perfused Rat Liver

“…An increase in the NADHiNAD ratio as indicated by the increase in the /I-hydroxybutyratelacetoacetate ratio could also explain the oleic-stimulated gluconeogenesis, when the conversion of 1,345-phospho-glycerate to triose phosphate, which requires NADH, is an important control step for gluconeogenesis [ 15,35,36]. During oleate-stimulated gluconeogenesis from pyruvate, the ratio of pyruvate [331* utilized to glucose formed decreases from a value of approximately 4 in the control to that of 2 in the experiment.…”

“…During oleate-stimulated gluconeogenesis from pyruvate, the ratio of pyruvate [331* utilized to glucose formed decreases from a value of approximately 4 in the control to that of 2 in the experiment. It is suggested that the NADH generated by pyruvate or fatty acid oxidation, which can be effectively trapped as malate in mitochondria, is transported out of mitochondria as a source of NADH for gluconeogenesis [15,35,36]. Thus fatty acid, in this case oleate, seems to provide the hydrogen for gluconeogenesis, which must come from the oxidation of 2 moles of pyruvate for 1 mole of glucose when pyruvate is present alone.…”

The function of energy‐dependent redox reactions in cell metabolism

“…It is known that the shuttle system for hydrogen transfer from mitochondria to cytoplasm involves malate [40,41], but is distinct from that responsible for passage of reducing equivalents into the mitochondria [23,24]. It therefore seems feasible that, within the inner membrane, more than one NAD(H) pool may be involved in poising the [lactate]/ [pyruvate] ratio.…”