Storage intermediaries

FIGURE 23.7 Dopamine (DA) is synthesized within neuronal terminals from the precursor tyrosine by the sequential actions of the enzymes tyrosine hydroxylase, producing the intermediary L-dihydroxyphenylalanine (Dopa), and aromatic L-amino acid decarboxylase. In the terminal, dopamine is transported into storage vesicles by a transporter protein (T) associated with the vesicular membrane. Release, triggered by depolarization and entry of Ca2+, allows dopamine to act on postsynaptic dopamine receptors (DAR). Several distinct types of dopamine receptors are present in the brain, and the differential actions of dopamine on postsynaptic targets bearing different types of dopamine receptors have important implications for the function of neural circuits. The actions of dopamine are terminated by the sequential actions of the enzymes catechol-O-methyl-transferase (COMT) and monoamine oxidase (MAO), or by reuptake of dopamine into the terminal. 

While this multiplication and regrouping task is a trivial matter for a small number of electrons, the procedure becomes more complicated for larger systems, owing to the number of AO-based determinants that can be generated. To keep the method tractable by hand, and/or to render it efficient in terms of memory storage by a computer program, it is convenient to use a mathematical intermediary that we call half-determinant (2). 

Intralysosomal substrate storage represents the initial insult to cells by-products of intermediary metabolism (e.g., psychosine in globoid cell leukodytrorphy), a disruption of normal lysosome function, and/or the consequent deficiency in recycling of certain substrates are putative disease events (Ballabio and Gieselmann,... 

Most aspects of intermediary metabolism require essential trace elements in the form of metalloenzymes that have a range of catalytic properties. Specific metalloproteins are required for the transport and safe storage of very reactive metal ions, such as Fe or Cu. Examples are metaUo-thionein (Cu, Zn), transferrin, ferritin and hemosiderin (Fe), and ceruloplasmin (Cu). 

Lipid oxidation in muscle foods is one of the major deteriorative reactions causing losses in quality during processing and storage. The oxidation of unsaturated fatty acids leads to formation of free radicals and hydroperoxide. These intermediary compounds are unstable and cause the oxidation of pigments, flavors, and vitamins. Oxidized unsaturated lipids bind to protein and form insoluble lipid-protein complexes. This accounts for toughened texture and poor flavor of frozen seafoods (Khayat and Schwell, 1983). 

Muscle converts chemical free energy from ATP hydrolysis into mechanical work with an efficiency approaching 80% under optimal circumstances. ATP is only an intermediary in energy storage in red muscles. Its concentration remains relatively constant during long exercise, as shown in the NMR studies of Figure 12,14. 

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