The Site of Formation

It is now generally recognized that the most intensively studied alkaloids (nicotine in tobacco hyoscyamine in Datura, Atropa, and Hyoscyamus) are produced in quantity in the roots of intact plants, though other sites of formation are not entirely excluded. A fully developed tobacco leaf does not elaborate nicotine, but it has not yet been proved that this almost cosmopolitan alkaloid is produced only in the roots in all other plants. 

The differentiation of the metabolism in the different organs is a problem of the physiological development of that organ. A tobacco leaf resembles a defect-mutant as far as its ability to synthesize nicotine is concerned but since it can generate roots the capacity to synthesize nicotine is still there though latent. 

A variety of methods have played important roles in determining the site of formation of alkaloids, and all are subject to serious experimental and interpretational errors. The culture of isolated organs can lead to erroneous interpretations and the analysis of sap currents (xylem and phloem) is fraught with obvious inherent difficulties. In grafts the shoot often forms adventitious roots which may not be visible when they grow into the stock. Such a graft is not only under the influence of the stock root but under its own, and very short roots can show very great alkaloid synthesis activity (244, 245). 

Bphedrine. Isolated sprouts of Ephedra distachya L. when fed with N H4 increase the absolute quantity of alkaloids and incorporate 

Gramine is elaborated from tryptophan in barley leaves (260). 

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The sites of formation of hot spots may be provided in both liquid and solid expls not only by bubbles of gas but also by bubbles of vapor, if the expl is more or less volatile. The importance of some heterogeneous initiation mechanism is shown by the failure of deton to propagate, in at least some expls, when the density of material exceeds a certain value [See under "Detonation (and Explosion), Resistance to ]. For the type of expls referred to by D. Price as Group 2 (Ref 33), see under "Detonation (and Explosion), Critical or Limiting Charge Densities. Price noted that in other expls, however, the homogeneous process plays the major part in the transmission, if not the initiation, of detonation (Ref 34)... 

On the basis of the likely assumption that the sites of formation of nitriles are different from those reponsible of degradation of the aromatic hydrocarbon (sites of deep oxidation), the evolution of activity and selectivity seems to indicate a change in the same proportion of the two types of sites when the... 

As distinct from the purple bacteria, plants, when photosynthesizing carbohydrates from C02, use water rather than H2S as an oxidized substrate. The oxidation of water requires far more energy than that of hydrogen sulphide. This circumstance plus the necessity for the sites of formation of oxidized and reduced particles to be spatially separated (in order to avoid their rapid recombination) seems to be the reason for the formation in plants... 

The site of formation of renin is not known, although the indirect and circumstantial evidence favors slightly the juxtaglomerular apparatus rather than the tubules as a source (18). Crude renin, however, is extracted readily from renal cortex by saline extraction, acidification, and precipitation with ammonium sulfate and sodium chloride. Other active protein substances are likewise extracted, and their separation from renin is often a matter of considerable difficulty. The renin substrate, an arglobulin, is found in blood serum and is probably formed by the liver. It can be easily salted out of beef serum as a crude preparation. 

The activation of pulmonary toxicants falls into three main categories or mechanisms, depending either on the site of formation of the activated compound or on the nature of the reactive intermediate. 

Fig. 3. Conserved sequences for RNA splicing. The residue marked as A in the branchpoint sequence is the site of formation of the 2 5 branch.

The sites of formation of the various clotting factors, and the metabolic requirements involved are still matters of conjecture. It has long been supposed that the liver is concerned with the production of fibrinogen, but the evidence is not con-... 

The production of highly reactive free radicals leads to primary reactions close to the site of formation. It will not diffuse far before it interacts within its micro-environment which in a membrane would include the initiation of lipid peroxidation. Secondary products of such... 

Figure 11.8. Detection of NOS-dependent S-nitrosylation in vivo, a Immunochemical detection procedure. A primary antibody directed against nitrosothiol groups was used in combination with an enzyme-linked secondary antibody. The dye released by the enzyme is insoluble and thus will precipitate close to the site of formation, b S-nitrosylationinaorticrings. Samplesweie treated with Phenylephrine (PE), acetylcholine (Ach), and the inhibitor N-methylarginine (L-NAME) as indicated. Brown stain deposits indicate S-nitrosylation. The traces above the histology panels illustrate the contraction and relaxation responses evoked by the dmgs.

There is no information on the site of formation of the Fe(III)-0-Fe(III) dimers that have been observed in rHF as well as in horse spleen ferritin (57,112). This may be clarified by Mossbauer spectroscopy on ferritin variants. Possible positions are the putative nucleation center or the double Tb site. In the latter case, either the dimers themselves must move or the iron core nucleus must incorporate iron atoms from the ferroxidase centers. Because ferrihydrite can be deposited both in rLF and in the rHF variant lacking ferroxidase activity, nucleation at this center is not possible for these molecules. Indeed, a specific center may not be required. However, electron micrographs of broken ferritin molecules suggest an attachment of ferrihydrite to the protein shell (113). 

Current density refers to the total current flow in kiloamperes divided by the anode electrode area in square meters, expressed as kA/m. High-current densities are desirable, particularly for electrochemical j rocesses, which yield unstable products. With current densities of 2-3 kA/m", electrolytic products of the diaphragm cell are rapidly moved from the sites of formation, which decreases side reactions and maximizes current efficiencies [14]. High-current densities, however, increase heat generation, anode wear, and the operating voltage so that lower current densities (and more cells) are better if the cells can be made cheaply. 

Since reaction (44) is likely to occur very close to the site of formation of methyl radicals, macroradicals, recombining in reactions (45) and (46), will be formed in close pairs. Crosslinking is thus very probable in the radiolysis of polyvinylacetate. 

The increase in viscosity associated with the formation of either an amorphous solid at the glass transition temperature or a crystalline lattice at the phase transition temperature will significantly reduce the mobility of most radicals. An increase in viscosity in rubbery liquids, due to decreases in temperature down towards the glass transition, will also reduce mobility. Consequently, recombination of primary radicals at the site of formation would be favored, lowering their yield. Other reaction pathways not involving diffusion and migration might also be favored. These effects are discernible and sometimes can be pronounced. 

FIGURE 21.18 A portion of an animal cell, showing the sites of various aspects of fatty-acid metabolism. The cytosol is the site of fatty-acid anabolism. It is also the site of formation of acyl-CoA, which is transported to the mitochondrion for catabolism by the P-oxidation process. Some chainlengthening reactions (beyond Cjg) take place in the mitochondria. Other chain-lengthening reactions take place in the endoplasmic reticulum (ER), as do reactions that introduce double bonds. 

A replication fork is the site of formation of new DNA. The two strands of the original DNA separate, and a new strand is formed on each original strand. 

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