Metabolic pathways explanation

An explanation of the nutritive aspects of D-glucuronic acid may be related to a metabolic pathway of D-glucuronic-acid degradation which results in the production of pentoses, possibly through the aid of the intestinal flora (see above). 

Figure 2.17. How matching of flux of substrate to product at each step in a complex metabolic pathway such as glycolysis with the overall maximum flux through the pathway (Jnlax) is achieved may depend upon the kind of enzyme functioning at any specified locus. Traditional explanations for three kinds of enzymes are summarized above. Panels (A), (B), and (C) summarize processes thought to allow for large changes in flux with more moderate changes in substrate and product concentrations for three different kinds of enzymes. See Hochachka et al. (1998) and text for further details.

Most reports of aminophylline hypersensitivity reactions in the English language literature were delayed reactions. However, most of the Japanese cases were immediate reactions. Acetylation is the main metabolic pathway of ethylenediamine. Most Japanese are rapid or intermediate acetylators, while 50% of Caucasians are slow acety-lators. This difference suggests an explanation for the different incidences of immediate and delayed reactions to ethylenediamine in Japanese and Caucasians. 

There are small but significant alterations in the plasma levels of two of the amino acids involved in the urea cycle. Both citrulline and arginine levels are low, about one-third to one-half the normal levels (Lll) sometimes even lower (Table 6). These changes would be expected since these acids are in the metabolic pathway beyond the block. What appears surprising is that the level of ornithine, the amino acid whose further metabolism is blocked, which would be expected to rise, is actually usually within normal limits. There are two possible explanations for this. The biosynthesis of urea forms a cycle in which each intermediate is re-formed in the process. The block in the synthesis of citrulline from ornithine results in a decreased formation of arginine which in turn will... 

The main metabolic pathway for esomeprazole, lansoprazole, omeprazole, pantoprazole, and to a lesser extent rabeprazole, is through the cytochrome P450 isoenzyme CYP2C19. This isoenzyme is subjeetto genetic polymorphism, (see Genetic factors , (p.4), for a further explanation of polymorphism). The poor metaboliser phenotype for CYP2C19 is found in approximately 1 to 6% of Caucasians, 1 to 7.5% of Blacks and 12 to 23% of Oriental and Indian Asians. ... 

Examination of metabolic pathways in extreme thermophiles has provided interesting insights into the necessary adaptations for life at high temperatures and possible explanations for the mechanisms used by early life. These pathways include enzymes not found in mesophiles and that can perform unique reactions which often bypass steps found in conventional pathways characterized in mesophilic organisms (e.g., [33]). These shortened metabolic pathways presumably compensate for changes in equilibrium or instability of intermediates at higher temperatures. Thus, these shortcuts may be of some use in construction of synthetic metabolic pathways. 

The initiation step consists in the formation of IPP (12), and its isomer DMAPP (13). The conventional metabolic pathway to form these two molecules is called the acetate/mevalonate pathway (MVA pathway) in which three molecules of acetyl-CoA (3) condense successively to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) (5), which leads to a key intermediate molecule, namely mevalonic acid (MVA) (6). The latter is further phosphorylated and decarboxylated to form the IPP molecule (12). In Hevea brasiliensis, this cytosolic pathway was described by Lynen and Lebras" " in the early 1960s and reviewed more recently by Kekwick and Ohya." Most experimental validations were obtained by observing the incorporation of radioactive tracers, such as [2- C] MVA and [3- C]HMG-CoA. The incorporation of [ ClIPP into rubber was found to be much faster than that of [2- C] MVA. This was assumed to be due to slow conversion of MVA into IPP." Another explanation might be that the MVA pathway was not exclusive for IPP biosynthesis. Indeed less than 10 years ago, a new, mevalonate-independent, IPP biosynthesis pathway was discovered by Rohmer." This plastidic DXP-MEP pathway initiates with a... 

A number of explanations can be advanced which may explain the apparent dichotomy of f-element binding to transferrin and the imperfect participation in the iron metabolic pathways. 

Having many active synergistic constituents (a multicomponent character) interacting with multiple receptors in the human body is seen as a major advantage of TCMs and pivotal for the overall therapeutic effect [2]. However, the multicomponent character is also a major obstacle for the quantitation of components, identification of structures, explanation of the mode of action, and elucidating metabolic pathways. To date, himdreds of papers report on the simultaneous determination of multiple components in crude materia medica or finished TCMs [10,12—20]. 

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