Gluconeogenesis Cori cycle

Lactate as a precursor for gluconeogenesis is mainly derived from muscle (see Cori cycle, p. 338) and erythrocytes. LDH (see p. 98) oxidizes lactate to pyruvate, with NADH+H" formation. 

After a period of intense muscular activity, the individual continues breathing heavily for some time, using much of the extra 02 for oxidative phosphorylation in the liver. The ATP produced is used for gluconeogenesis from lactate that has been carried in the blood from the muscles. The glucose thus formed returns to the muscles to replenish their glycogen, completing the Cori cycle (Fig. 23-18 see also Box 15-1). 

Now let us consider the further conversion of PEP and of the triose phosphates to glucose 1-phosphate, the key intermediate in biosynthesis of other sugars and polysaccharides. The conversion of PEP to glucose 1-P represents a reversal of part of the glycolysis sequence. It is convenient to discuss this along with gluconeogenesis, the reversal of the complete glycolysis sequence from lactic acid. This is an essential part of the Cori cycle (Section F) in our own bodies, and the same process may be used to convert pyruvate derived from deamination of alanine or serine (Chapter 24) into carbohydrates. 

Figure 18.4 The Cori cycle. Glucose is metabolized anaerobically to lactate, as in white muscle fibers or red blood cells. Lactate is exported into the bloodstream, extracted from there by the liver, and reconverted to glucose via gluconeogenesis. Glucose is then returned to the muscle.

Epinephrine increases muscle glycolysis, and therefore blood lactic acid production and an amplified Cori cycle. In the liver, epinephrine stimulates gluconeogenesis and blood glucose will therefore rise. 

The Cori cycle refers to the metabolic pathway in which lactate, produced by anaerobic glycolysis in the muscle, moves to the liver and is converted to glucose, through gluconeogenesis glucose can then return to supply the muscle. 

The liver takes up the alanine and converts it back into pyruvate by transamination. The pyruvate can be used for gluconeogenesis and the amino group eventually appears as urea. This transport is referred to as the alanine cycle. It is reminiscent of the Cori cycle discussed earlier (Section 16.4.2) and again illustrates the ability of the muscle to shift some of its metabolic burden to the liver. 

Lactate is released by red blood cells and other cells that lack mitochondria or have low oxygen concentrations. In the Cori cycle, lactate is released by skeletal muscle during exercise (Figure 8.8). After lactate is transferred to the liver, it is reconverted to pyruvate by lactate dehydrogenase and then to glucose by gluconeogenesis. 

Cori cycle, 252 decarboxylation, 245 electron transport system, 243 fermentation, 243 gluconeogenesis, 235 glucose-alanine cycle, 254... 

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