Metabolism organ specialization

The kidneys are organs specialized to filter the blood. As such, they make an important contribution to the removal of metabolic waste products as well as to maintenance of fluid and electrolyte balance. Specific functions of the kidneys include ... 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

If incorporated into proteins they cause the loss of biological functions. Nonprotein amino acids can therefore be used in the metabolism of specialized organisms only and function as feeding deterrents for most other living beings (E 5.5.3). 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

The varying metabolic activities of bacteria and their response to immediate environmental factors have been exploited in the design of special diagnostic and selective media. Recipes for these run into many hundreds such media are used in hospital and public health laboratories for identifying organisms found in samples believed to be contaminated by them, and as an aid to diagnosis and treatment. In addition they are used to detect contaminants in pharmaceutical products (British Pharmacopoeia 1993). A few examples will be given to illustrate the principle. 

G. Stotzky, Influence of soil mineral colloids on metabolic processes, growth, adhesion and ecology of microbes and virus. Interaction of Soil Minerals and Natural Organics and Microbes (P. M. Huang and M. Schnitzer, eds). Special Publication Number 17, Soil Science Society of America, Madison, Wl, 1986, pp 305-428. 

The free glucose produced by this reaction is supplied to the blood from the tissues. As exemplified by gluconeogenesis, one may easily envision the economical organization of these metabolic routes, since, apart from four special gluconeogenesis enzymes-pyruvate carboxylase, phosphopyruvate carboxylase, fructose bisphosphatase, and glucose 6-phosphatase-individual glycolytic enzymes are also used in the gluconeogenesis. 

The carbohydrate metabolic routes in various tissues of the organism discussed above differ in intensity, which is defined by metabolic features specific of each tissue and organ. However, from the standpoint of activity of the whole organism, certain specializations of the carbohydrate metabolic routes in individual tissues are profitably complementary. For example, strenuous muscular exertion requires energy which is initially supplied by the breakdown of glycogen to lactic acid. The latter compound is excreted into the blood to be supplied... 

No one tissue can survive metabolically without the others. Each of the four major tissue types (liver, muscle, adipose, brain) has a specialized metabolic function. There are some differences in the metabolic pathways in each tissue however, these differences are relatively simple and serve to specialize the metabolic functions of the different tissue types. There is real cooperation between the different organs. Each organ has its own metabolic profile, its own needs, and its own capabilities. 

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