Generalities of Metabolism

Synthesis and degradative pathways don t happen at the same time. 

Low glucose levels turn on gluconeogenesis and protein degradation. 

Protein phosphorylation in response to rising cAMP levels activates enzymes involved in maintaining blood glucose levels and in m ng ATP. 

There are a few generalities of metabolism that will help you understand metabolism and why a lot of things work the way they do. 

A FUTILE CYCLE is set up when both synthetic and degradative pathways are operating at the same time. The net reaction of a futile cycle is just ATP hydrolysis. 

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Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Practically all toxicokinetic properties reported are based on the results from acute exposure studies. Generally, no information was available regarding intermediate or chronic exposure to methyl parathion. Because methyl parathion is an enzyme inhibitor, the kinetics of metabolism during chronic exposure could differ from those seen during acute exposure. Similarly, excretion kinetics may differ with time. Thus, additional studies on the distribution, metabolism, and excretion of methyl parathion and its toxic metabolite, methyl paraoxon, during intermediate and chronic exposure are needed to assess the potential for toxicity following longer-duration exposures. 

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthesis—and also in the pentose phosphate pathway. 

The results presented above indicate that the previously unknown head-to-tail polymerization is the major reaction product of the iminium methide species. To investigate the generality of this reaction, we next studied a neutral ene-imine species shown in Scheme 7.9.48 As illustrated in this scheme, the generation of this reactive species requires quinone reduction followed by elimination of acetic acid. The ene-imine is structurally related to the methyleneindolenine reactive species that is a metabolic oxidation product of 3-methylindole (Scheme 7.9).57 59... 

Serum electrolytes should be monitored in patients with CKD for the development of metabolic acidosis. Metabolic acidosis in patients with CKD is generally characterized by an elevated anion gap greater than 17 mEq/L (17 mmol/L), due to the accumulation of phosphate, sulfate, and other organic anions. 

In general, contributing factors such as diuretics, nasogastric suction, and corticosteroids should be discontinued if possible. Any fluid deficits should be treated with IV normal saline. Recognize that patients with varieties of metabolic... 

He describes molecular populations mathematically in the way physicists calculate classical dynamic systems. Very exact dynamic equations are devised, while the laws of interaction are left very general. This leads to a general theory of molecular systems, which makes it possible to define what is understood by the origin of metabolism (Dyson, 1999). 

Schuster S et al. A general definition of metabolic pathways useful for systematic organization and analysis of complex metabolic networks. Nature Biotechnol 2000 18 326-332. 

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