Liver cells carbohydrate metabolism

The calorific capacity of amino acids is comparable to that of carbohydrates so despite their prime importance in maintaining structural integrity of cells as proteins, amino acids may be used as fuels especially during times when carbohydrate metabolism is compromised, for example, starvation or prolonged vigorous exercise. Muscle and liver are particularly important in the metabolism of amino acids as both have transaminase enzymes (see Figures 6.2 and 6.3 and Section 6.4.2) which convert the carbon skeletons of several different amino acids into intermediates of glycolysis (e.g. pyruvate) or the TCA cycle (e.g. oxaloacetate). Not all amino acids are catabolized to the same extent... 

The physiological relevance together with chnical importance of transamination and deamination is wide-ranging. As an aid to understanding the somewhat complex nature of amino acid metabolism, it can be considered (or imagined) as a metabolic box (represented in Figure 8.13). Some pathways feed oxoacids into the box whereas others remove oxoacids and the ammonia that is released is removed to form urea. The box illustrates the role of transdeamination as central to a considerable amount of the overall metabolism in the liver cell (i.e. protein, carbohydrate and fat metabohsm, see below). 

Mechanism of Action Acts as a coenzyme for various metabolic functions, including fat and carbohydrate metabolism and protein synthesis. Therapeutic Effect Necessary for cell growth and replication, hematopoiesis, and myelin synthesis. Pharmacokinetics In the presence of calcium, absorbed systemically in lower half of ileum. Initially, bound to intrinsicfactor this complex passes down intestine, binding to receptor sites on ileal mucosa. Protein binding High, Metabolized in the liver. Primarily eliminated unchanged in urine. Half-life 6 days. 

Rich in both phase I (principally the cytochromes P450, catalyzing hydrolysis, reduction, and oxidation reactions) and phase II (catalyzing conjugation of xenobiotic molecules with hydrophilic moieties) biotransforming enzymes, the liver is the metabolic center of the body. In fact, most of the field of biochemistry is concerned with its metabolic reactions. The liver essentially converts ingested food into a balanced cell culture medium via metabolic interconversion of amino acids, carbohydrates, and lipids and synthesizes many substances that are subsequently exported for use in other areas of... 

Lactic acid, present in blood entirely as lactate ion (pK = 3.86), is an intermediate of carbohydrate metabolism and is derived mainly from muscle cells and erythrocytes (see Chapter 25). It represents the end product of anaerobic metabolism and is normally metabolized by the liver. The blood lactate concentration is, therefore, affected by the rate of production and the rate of metabolism, both of which are dependent on adequate tissue perfusion. An increase in the concentration of lactate to >2 mmol/L and the associated increased is considered lactic acidosis. 

Thyroid hormone effects on metabolism arc diverse. The rates of protein and carbohydrate synthesis and catabolism are inlluenced. An example of the effect of thyroid hormones on lipid metabolism is the observation of a high serum cholesterol in some hypothyroid patients. This is a consequence of a reduction in cholesterol metabolism due to down regulation of low-density lipoprotein (LDL) receptors on liver cell membranes, with a subsequent failure of sterol excretion via the gut. 

P. are particularly abundant in liver and kidney cells, e.g. a single rat liver cell contains 350-400 P. The half-life of P. is 1.5-2 days. Other cell types usually contain smaller P. with homogeneous contents (microperoxisomes). In addition to catalase, P. contain, inter alia, o-amino acid oxidase, a-hydroxyacid oxidase and urate oxidase. The latter enzyme is often present as a large crystal in the otherwise homogeneous matrix. These enzymes are particularly important in the oxidative degradation of metabolic intermediates (e.g. purine bases) and in the formation of carbohydrates from amino acids and other materials. [L.J. Olsen J.J.Harada Peroxisomes and Their Assembly in Higher Plants Annu. Rev. Plant Physiol. Plant Mol. BioL 46 (1995) 123-146)... 

Transketolase (EC 2.2.1.1) an enzyme that catalyses transketolation, an important process of carbohydrate metabolism, especially in the Pentose phosphate cycle (see) and Calvin cycle (see). T. has been found in a wide variety of cells and tissues, including mammalian liver, green plants and many bacterial species. The enzyme contains divalent metal cations and the coenzyme, thiamin pyrophosphate. Transketolation involves transfer of a C2-unit (often called active glycolaldehyde or a ketol moiety) from a ketose to Cl of an aldose. Only ketoses with L-configuration at C3 and preferably irons configuration on the next carbon (i.e. Cl, 2, 3 and preferably 4 as in fructose) can serve as donors of the C2-unit. The acceptor is always an aldose. Thins-ketolation is reversible. Details of the reaction in which xylulose S-phosphate serves as the donor of... 

Vitamin Bs consists of six forms in nature, i.e. pyridoxine (PN), pyridoxal (PL), pyridoxamine (PM), pyridoxine 5 -phosphate (PNP), pyridoxal 5 -phosphate (PLP), and pyridoxamine 5 -phosphate (PMP) (Figure 22.1). PLP is a coenzyme for many enzymes involved in amino acid and carbohydrate metabolism, and plays a key role in the nutritional function of vitamin Bg. The other forms show the same nutritional efficiency because they are converted into PLP in cells. The free forms of vitamin Bg are adsorbed through the intestinal mucosa, and then are phosphorylated and converted into PLP in the liver. Some PLP exits the liver and travels in the blood on albumin, being turned over slowly. PL is the form most actively transported to other cells from the liver. The cells adsorb PL and then phosphorylate it to yield PLP. The final metabolite derived from vitamin Bg is 4-pyridoxic acid (4-PA, Figure 22.1), which is excreted into the urine. Plants contain a storage form of vitamin Bg, pyridoxine-p-glucoside... 

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