Cori cycle

Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to synthesize glucose, which can then return to muscle (the Cori cycle). 

Each mole of glucose that goes through the Cori cycle costs the liver 6 ATP equivalents. If the Cori cycle were perfect and there were an endless supply of ATP, liver could supply glucose equivalents forever just using the same carbon atoms (6 in as lactate, 6 out as glucose). However, some of the lactate (pyruvate) in the nonliver tissues is burned to C02 by the TCA cycle in die muscle and other tissues. It s this pool that must be replaced by fresh glucose from liver. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

Glycogenolysis and glycogen synthesis P-oxidation of fatty acids transamination and deamination of amino acids Cori cycle and glucose-alanine cycle, which recycles substrates between muscle and liver. 

Figure 6.10 The extended Cori cycle. Tumour cells convert glucose to lactate at a high rate and could be incorporated into this figure (Chapter 21).

Lactic acid The normal concentration of lactate in the blood is about 1 mmol/L which is the balance between production and utilisation. Various tissues produce lactic acid and release it into the blood, from where it is taken up by liver and converted to glucose (or glycogen) (the Cori cycle, see above). 

Substrate cycling (e.g. the Cori cycle and the intra- and inter-cellular triacylglycerol/fatty acid cycles (Chapter 3)) in which there is no net metabolic change so that the energy from ATP hydrolysis is released as heat. 

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. 

---