Lactic glucose metabolism

The mechanism of toxicity of ethylene glycol involves metabolism, but unlike previous examples, this does not involve metabolic activation to a reactive metabolite. Thus, ethylene glycol is metabolized by several oxidation steps eventually to yield oxalic acid (Fig. 7.84). The first step is catalyzed by the enzyme alcohol dehydrogenase, and herein lies the key to treatment of poisoning. The result of each of the metabolic steps is the production of NADH. The imbalance in the level of this in the body is adjusted by oxidation to NAD coupled to the production of lactate. There is thus an increase in the level of lactate, and lactic acidosis may result. Also, the intermediate metabolites of ethylene glycol have metabolic effects such as the inhibition of oxidative phosphorylation, glucose metabolism, Krebs cycle, protein synthesis, RNA synthesis, and DNA replication. 

As a pantothenic acid analogue, hopantenate can affect lactate generation, glucose metabolism, and ammonia disposal, and there have been two fatal cases in elderly people who developed disturbances of consciousness with lactic acidosis, hypoglycemia, and hyperammonemia (903). 

Alcoholic fermentation occurs when the end product is ethanol, as shown in Figure 4.11. In this process the pyruvate is first converted enzymatically to acetaldehyde. The conversion of acetaldehyde to ethanol produces NAD+ from NADH + H+, and the NAD+ is cycled through the glycolysis process. As with lactic acid fermentation, the glycolysis process produces usable energy contained in two molecules of ATP produced for each molecule of glucose metabolized. 

Culture medium needs to be buffered to compensate for CO2 and lactic acid derived from glucose metabolism. Most culture media employed for animal cells are buffered with CO2 originating from the gaseous phase, in equilibrium with sodium bicarbonate (NaHCCF) added to the culture medium, as described by the following equilibrium reaction ... 

Figure 21.1. Pathway of glucose metabolism by homofermentative lactic acid bacteria.

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). 

Lactate exists as two stereoisomers, L-lactate and D-lactate, although L-lactate is the predominant physiologic anion and mainly discussed in the information below. The majority of plasma lactate is derived from glucose metabolism (65 %) and amino acid metabolism through the degradation of alanine (15-20 %) [17]. Lactic acidemia refers to blood lactate levels that are above those typically seen in blood (approximately <2.0 mM). 

Glyoxalase. Before the Embden-Meyerhof scheme was recognized as the major pathway for glucose metabolism, an enzyme was found that could produce lactic acid rapidly. It was reasonable, therefore, that this enzyme, glyoxalase, and its substrate, methyl glyoxal, were used in many speculations about the nature of glycolysis. Today they are not... 

Fig. 10.20. Glucose metabolism in lactic acid bacteria. A homofermentation, B Bifidus pathway, and C heterofermentation (6-phosphogluconate pathway)...

During moderate- to high-intensity exercise, glycogen (a fuel reserve that helps maintain normal body processes) can be depleted within 60 to 90 minutes. Blood sugar levels drop as the glycogen reserves are used up, and lactic add (a by-product of glucose metabolism) builds up in muscle... 

In spite of the number of different structural types lipids share a common biosyn thetic origin m that they are ultimately derived from glucose During one stage of car bohydrate metabolism called glycolysis glucose is converted to lactic acid Pyruvic acid IS an intermediate... 

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

Glycolysis Metabolic pathway involving the conversion of glucose to lactic acid or ethanol. 

The lactic acid (C3H603(flg), AG = —559 kj) produced in muscle cells by vigorous exercise eventually is absorbed into the bloodstream, where it is metabolized back to glucose (AG = —919 kj) in the liver. The reaction is... 

Glycolysis is the process by which glucose is metabolized to lactic acid according to the equation... 

The chemical engineering approach began with an analysis of the biochemistry of platelet metabolism. Like many cells, platelets consume glucose by two pathways, an oxidative pathway and an anaerobic pathway. The oxidative pathway produces carbon dioxide, which makes the solution containing the platelets more acidic (lower pH) and promotes anaerobic metabolism. This second metabolic pathway produces large amounts of lactic acid, further lowering pH. The drop in pH from both pathways kills the platelets. 

During the recovery period from exercise, ATP (newly produced by way of oxidative phosphorylation) is needed to replace the creatine phosphate reserves — a process that may be completed within a few minutes. Next, the lactic acid produced during glycolysis must be metabolized. In the muscle, lactic acid is converted into pyruvic acid, some of which is then used as a substrate in the oxidative phosphorylation pathway to produce ATP. The remainder of the pyruvic acid is converted into glucose in the liver that is then stored in the form of glycogen in the liver and skeletal muscles. These later metabolic processes require several hours for completion. 

Capillaries are the site of exchange between blood and the interstitial fluid surrounding tissue cells. Tissues with a higher metabolic rate have a more extensive capillary network, that is, a greater number of capillaries per unit area. Because of extensive branching of these vessels, the cells of the body are typically within 20 pm of the nearest capillary. Consequently, the distance that substances must travel between blood and the cells is minimized. Capillaries are permeable to water and small water-soluble substances, such as glucose, amino acids, lactic acid, and urea, and impermeable to proteins. 

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