Brain, sugar metabolism

Several carrier systems have been shown to be present in the brain endothelium, allowing for the selective transport of a group of common substrates (Table 13.1). The most common system is the one that mediates the transport of glucose, which provides the brain with virtually all its energy. Carrier-mediated mechanisms are also responsible for the absorption of two other energy sources ketone bodies, which are derived from lipids, and lactic acid, a by-product of sugar metabolism. Carrier-mediated transport systems are also involved in the uptake of amino acids by the brain. The brain can manufacture its own small neutral and acidic amino acids however, large neutral and basic amino acids are obtained from the bloodstream. 

The authors pointed out that such a metabolic pathway may complicate the results of biosynthetic studies in certain species, as it would provide a route for the oxidation of L-fucose and its eventual provision of carbon atoms to other sugars. This metabolic route does not occur in rats, but it is known215 that humans can oxidize L-[l-,4C]fucose to carbon dioxide. Only pig liver and kidney were found to contain appreciable levels of the major enzymes needed in this pathway, whereas heart, stomach, intestine, submaxillary gland, lung, and brain were deficient in or devoid of them. 

The enzyme has been partially purified (70-fold) from 38,000 X 9 supernatant fluid from sheep brain homogenates by Ipata (55-58). Thq enzyme (MW 140,000) is reported to be specific for 5 -AMP and 5 -IMP although the substrate specificity does not appear to have been examined closely. 2 - and 3 -AMP are not hydrolyzed (56). Unlike the enzyme from many sources the brain enzyme does not require divalent cations and indeed Co2+, which stimulates several other 5 -nucleotidases, was inhibitory at 5 mM. The enzyme is strongly inhibited by very low concentrations of ATP, UTP, and CTP (50% inhibition by 0.3 pM ATP) but not by GTP. 2 -AMP, 3 -AMP, and a variety of other nucleoside monophosphates, nucleosides, and sugar phosphates do not inhibit. A kinetic examination of ATP, UTP, and CTP inhibition (56-58) revealed that inhibition curves were sigmoidal, indicating cooperativity between inhibitor molecules and an allosteric type of interaction between inhibitor and protein. The metabolic significance of ATP inhibition is... 

Neurons synthesize acetylcholine from choline, which is obtained from the diet, and from acetyl groups that originate in mitochondria from the metabolism of sugar. Here is yet another example of the importance of sugar for your brain s normal function. The synthesis of acetylcholine occurs within the cytoplasm of your neurons, and the product is stored in synaptic vesicles, those small round packets that neurons release to communicate with each other. Neurons pay a lot of attention to the shelf life of their neurotransmitters they prefer to release the most recently produced neurotransmitter molecules first. As you can see, neurons do not behave like your local grocer they do... 

Brain function is dependent upon ready availability of energy by aerobic metabolism. This energy is provided by aerobic glycolysis, the breakdown of glucose blood sugar to pyruvic acid with 02 as an electron acceptor. Therefore, brain cells and other nerve cells are highly susceptible to interruptions in the supply of either 02 or blood glucose. 

In addition to growth factors, it has been shown that countless other molecules have multiple functions. Cholecystokinin, for example, is a peptide that acts as a hormone in the intestine, where it increases the bile flow during digestion, whereas in the nervous system it behaves as a neurotransmitter. Encephalins are sedatives in the brain, but in the digestive system are hormones that control the mechanical movements of food. Insulin is universally known for lowering the sugar levels in the blood, but it also controls fat metabolism and in other less known ways it affects almost every cell of the body. 

Molecules convey messages by fitting into appropriate receptor sites in a very specific way, which is determined by their structure. When a molecule occupies a receptor site, chemical processes are stimulated that produce the appropriate response. Sometimes, receptors can be fooled, as in the use of artificial sweeteners—molecules fit the sites on the taste buds that stimulate a sweet response in the brain, but they are not metabolized in the same way as natural sugars. Similar deception is useful in insect... 

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