Malate–aspartate shuttle promotes l-lactate oxidation in mitochondria

Metabolism in cancer cells is rewired to generate sufficient energy equivalents and anabolic precursors to support high proliferative activity. Within the context of these competing drives aerobic glycolysis is inefficient for the cancer cellular energy economy. Therefore, many cancer types, including colon cancer, reprogram mitochondria-dependent processes to fulfill their elevated energy demands. Elevated glycolysis underlying the Warburg effect is an established signature of cancer metabolism. However, there are a growing number of studies that show that mitochondria remain highly oxidative under glycolytic conditions. We hypothesized that activities of glycolysis and oxidative phosphorylation are coordinated to maintain redox compartmentalization. We investigated the role of mitochondria-associated malate–aspartate and lactate shuttles in colon cancer cells as potential regulators that couple aerobic glycolysis and oxidative phosphorylation. We demonstrated that the malate–aspartate shuttle exerts control over NAD⁺/NADH homeostasis to maintain activity of mitochondrial lactate dehydrogenase and to enable aerobic oxidation of glycolytic l -lactate in mitochondria. The elevated glycolysis in cancer cells is proposed to be one of the mechanisms acquired to accelerate oxidative phosphorylation.     

The preparation of crystalline human l-lactate–nicotinamide–adenine dinucleotide oxidoreductase isoenzyme 1 involving preparative polyacrylamide-gel electrophoresis

A method is presented for the preparation of human heart lactate dehydrogenase (l-lactate-NAD(+) oxidoreductase; EC 1.1.1.27) isoenzyme 1; this involves the use of polyacrylamide-gel electrophoresis as a preparative step. The yield was about 10% with a final specific activity of 220 units/mg of protein, one unit being defined as the amount of enzyme catalysing the oxidation of 1mumol of NADH/min at 25 degrees C, in the presence of 0.33mm-pyruvate. The crystalline preparation contained less than 2% of the other isoenzymes, was homogeneous in the ultracentrifuge and showed only a trace of protein contamination on polyacrylamide-gel electrophoresis. Some properties of the crystalline isoenzyme are reported; E(1%) (1cm)=13.2 at 280nm, s(0) (20,w)=7.43S, pI=4.6, and the apparent K(m) for pyruvate=1.02x10(-4)m. The human isoenzyme and the isoenzyme from pig heart differ with respect to amino acid composition, electrophoretic mobility and solubility. It is possible that these differences do not involve the active site, or sites, but are due to changes in amino acid residues elsewhere in the molecule. The importance of purified human LDH-1 isoenzyme with regard to enzyme radioimmunoassay is emphasized.     

R. D. Fitzgerald's F. L. S. „Australian Orchids“

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