Brain tissue, amino acid incorporation

As pointed out earlier in this article, T differs from other G proteins in that it is a peripheral membrane protein. After activation by Rho it seems to undergo subunit dissociation in which both its a subunit and its /3y complex dissociate from the Rho-containing membranes. Purification of brain G-proteins has shown that free a subunits of G0 and Gj are also water soluble, remaining in solution in the absence of detergents [74], The hydrophobicity of the whole ajSy G and Gj complexes was shown to be due to their j8y complexes 189]. Indeed, purified a subunits associate with phospholipid vesicles only if j8y complexes have been incorporated during vesicle formation [189]. Since the amino acid composition of T-/3 is equal to that of other G-j8s, but their ys differ, it follows that the principal role of y subunits should be to anchor non-T G proteins to the plasma membranes. This conclusion assumed, of course, that j8 subunits are not post-translationally modified in a tissue specific manner such that that they become water soluble in retinal photoreceptor cells and... 

Pro-opiomelanocortin. Proopiocortin ACTH-8 lipotropin common precursor precursor to ACTH -LPH-5-endorphin 3lK precursor POMC. A precursor protein oI mol wt about 30,000, synthesized in Che hypothalamus, pituitary gland, brain, and several peripheral tissues that incorporates the amino acid sequences of the pituitary hormones ACTH and (3-lipotropin. These two hormones, in turn, contain biologically active component peptides a-MSH,... 

Krebs cycle has different functions in different tissues. For example, in muscle and brain it oxidises acetyl CoA to form NADH and FADH2, which are used to generate ATP in the respiratory chain (Chapters 11-13). In liver, during fasting, acetyl CoA is not oxidised by Krebs cycle. Instead, sections of Krebs cycle operate to direct amino acid derivatives towards malate for gluconeogenesis (Chapter 46). In liver and adipose tissue, after feeding, the destiny of acetyl CoA is a brief sojourn in Krebs cycle by incorporation into citrate before export to the cytosol for biosynthesis to fatty acids (Chapter 21). 

Similar in vivo labeling studies contributed other important information they demonstrated that the rate of protein synthesis was different in different tissues. Although the proteins of liver, kidney, intestinal mucosa, spleen, pancreas, bone marrow, and testicles are rapidly labeled, labeling is slow in muscle, brain, erythrocyte, and skin. Within a given tissue, some proteins are labeled faster than others. In the pancreas, the hydrolytic enzymes incorporate amino acids more rapidly than other proteins. The label appears sooner in some cell fractions than in others. After in vivo administration of antigens, 50% of the antibodies can be recovered in the microsomal fraction only a few minutes after injection. It was also shown that under conditions of net protein synthesis, labeled amino acids injected in mice are first recovered in the microsomes, then in the supernatant, and later in the zymogen granules of the pancreas. 

The fate of ingested copper, under normal conditions, may be summarized 21s follows copper is probably freed from its organic binding by the gastric HCl as Cu " it is rapidly absorbed in the stomach or upper small gut. It is transported to the liver as albumin bound copper, it is rapidly concentrated in the hepatocytes where between 1 and 2% is incorporated into caeruloplasmin. A very small amount of plasma copper remains bound to albumin, possibly tr2insport copper, whilst an even smaller amount is bound to amino acids Neumann and Sass Kortsak [33] believe that it is the amino acid complexed copper that is aveiilable for crossing cell membranes and/or for renal excretion. Some copper also remains in the red cells. The ultimate fate of the hepatic copper is doubtful but some, probably the major part, is eventually excreted in the bile [40]. Once in the tissues some copper is utilized in the cytochrome oxidase for electron transport, in the bone marrow copper is related to erythropoiesis, possibly mediated via caeruloplasmin. In the brain the role of copper is obscure but one is tempted to speculate that, in the pigmented nuclei, it may be present in any enzyme related to the oxidation of dopamine. 

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