Glucose utilization relative rates

Lactate, the end product of anaerobic glucose metabolism, is produced by most tissues of the body, particularly skin, muscle, erythrocytes, brain, and intestinal mucosa. In a normal adult, under basal conditions, these tissues produce 1,300 mM of lactate per day, and the normal serum lactate concentration is less than 1.2 mM/L. During vigorous exercise, the production of lactate can be increased several fold. Lactate is normally removed from the circulation by liver and kidney. Because of its great capacity to use lactate, liver plays an important role in the pathogenesis of lactic acidosis, which may be thought of as an imbalance between the relative rates of production and utilization of lactate (Chapter 39). 

Fig. 17. Representative autoradiographs of the ventral mesencephalon. The relative rates of glucose utilization are related directly to relative optical densities. (A) Conscious, saline treated. The pars compacta can be identified as a well-defined line of relatively increased optical density. (B) Conscious, apomorphine treated. The pars compacta and pars reticulata each display marked increases in relative optical density. (C) Chloral hydrate anesthesia, saline treated. Glucose use in the pars compacta is minimally reduced by anesthesia and remains a well-defined band of optical density. (D) Chloral hydrate anesthesia, apomorphine treated. Glucose use in the pars compacta is reduced, and optical density in the pars compacta is no longer different from that in the adjacent pars reticulata. (From Grome and McCulloch 1981a).

In rat liver, the phosphorylation of fructose by means of ATP occurs at a greater rate than that of glucose. It is inferred that fructokinase predominates in this tissue, and this is supported by the observation that the anaerobic degradation of glucose by rat liver slices is relatively slow. An active fructokinase has been isolated from this tissue, and in contrast to yeast hexokinase, phosphorylates at Ci. The product, fructose-1-phosphate, had previously been isolated after autolysis of liver, during fructose utilization in liver or intestine, or after enzymatic hydrolysis of fructose 1,6-diphosphate by bone phosphatase. It may be noted that both fructose-l-phosphate and fructose-6-phosphate are formed in the latter procedure. The phosphate linkage at Ci is far more labile to acid than is that at Ce. 

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