Lactate oxidation

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. 

Lactate consumption The direction of the lactate dehydrogenase reaction depends on the relative intracellular concentrations of pyruvate and lactate, and on the ratio of NADH/NAD+ in the cell. For example, in liver and heart, the ratio of NADH/NAD+ is lower than in exercising muscle. These tissues oxidize lactate (obtained from the blood) to pyruvate. In the liver, pyruvate is either converted to glucose by gluconeogenesis or oxidized in the TCA cycle. Heart muscle exclusively oxidizes lactate to CO2 and H20 via the citric acid cycle. 

Solution The fumarate-succinate system has a more positive AEq than the pyruvate-lactate system, and hence the fumarate will oxidize lactate ... 

Now the NAdVNADH half-reaction has the more positive reduction potential. Thus, this half-reaction goes as written (as a reduction), driving the pyruvate/lactate half-reaction backwards (as an oxidation). Lactate is now the reducing agent NAD" is the oxidizing agent. The overall spontaneous reaction is ... 

NADH), lactate oxidase (oxidizes lactate to pyruvate), and catalase (figure... 

Other examples of amperometric enzyme electrodes based on the measurement of oxygen or hydrogen peroxide include electrodes for the measurement of galactose in blood (galactose oxidase,enzyme), oxalate in urine (oxalate oxidase), and cholesterol in blood serum (cholesterol oxidase). Ethanol is determined by reacting with a cofactor, nicotinamide adenine dinucleotide (NAD" ) in the presence of the enzyme alcohol dehydrogenase to produce the reduced form of NAD", NADH, which is electrochemically oxidized. Lactate in blood is similarly determined (lactate dehydrogenase enzyme). 

Pymvate is produced in the hver from the gluconeogenic precursors lactate and alanine. Lactate dehydrogenase oxidizes lactate to pymvate, generating NADH (Fig. 31.4A), and alanine aminotransferase converts alanine to pyruvate (see Fig. 31.4B). 

The lactate produced mainly in muscle diffuses in blood and reaches the heart where the principal function of LDH-H is oxidizing lactate to pyruvate, which can then be utilized through the Krebs cycle. In fact, if blood concentrations of lactate are increased, the uptake of that metabolite by the heart is also increased. Thus, the H-type is the true lactic dehydrogenase, while the M-type is really a pyruvic reductase. 

Note that the partial oxidation of lactate to acetate yields 66.7% fewer eleetrons than the complete oxidation to carbon dioxide and that while S. oneidensisMR-l appears to be incapable of oxidizing lactate to completion under normal anaerobic conditions, other strains of 5. oneidensis appear capable of the complete oxidation of lactate [60]. 

The addition of fumarate to the suspension of P. freudenreichii oxidizing lactate was shown to result in a partial oxidation of cytochromes b and 02 (de Vries et al., 1977). In the presence of 2H-heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of cytochrome b, lactate dehydrogenase activity was reduced by 85% if fumarate served as an electron acceptor, but only by 25% in the presence of methylene blue (in the latter case H2 is transferred to the dye without participation of cytochrome b). Keeping in mind the evidence presented above, the propionic acid fermentation can be viewed as a harmonious combination of the soluble and membrane-bound redox enzymes. The involvement of cytochrome b in anaerobic electron transport to fiimarate confirms the suggestion (Bauchop and Elsden, 1960) that there is an oxidative phosphorylation site between fumarate and succinate in propionibacteria. 

Gluconobacter and Acetobacter are differentiated by their ability to oxidize lactate 20 g of lactate per liter is added to the medium, already constituted of yeast extract at 5 g/1. Acetobacter... 

It is evident that the metabolic potentialities of intact organisms make difficult the interpretation of the role of pantothenate and that examination of cell-free systems concerned with pyruvate from deficient organisms is necessary (40). Little use has been made of inhibitors, but dried P. morganii has been observed to have lost its ability to oxidize pyruvate. The ability to oxidize lactate remained, but whereas in intact cells it was stimulated some 80% by pantothenate, little or no stimulation was found in the dried preparation. The effect of pantothenate on lactate w as presumably secondary to its effect on pyruvate (12). 

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