Site of biosynthesis

The mode, and subeellular site, of biosynthesis of Neu5Gc is similar in bovine, submandibular glands, although the activity is less pronounced.216 Neu5Ac-mono-oxygenase activity has also been determined in liver and serum of the pig.216 Because of the wide occurrence of Neu5Gc in Nature, the mono-oxygenase is, presumably, also widespread. 

Amino Acids Amino acids that enter the liver follow several important metabolic routes (Fig. 23-14). (1) They are precursors for protein synthesis, a process discussed in Chapter 27. The liver constantly renews its own proteins, which have a relatively high turnover rate (average half-life of only a few days), and is also the site of biosynthesis of most plasma proteins. (2) Alternatively amino acids pass in the bloodstream to other organs, to be used in the synthesis of tissue proteins. (3) Other amino acids are precursors in the biosynthesis of nucleotides, hormones, and other nitrogenous compounds in the liver and other tissues. 

Battershill, C. N., Pomponi, S. A., Willoughby, R., Northcote, L., McClean M. L., Munro, M. H. G., Blunt, J. W., and Garson, M. J., Sponge cells or symbionts The site of biosynthesis of biologically active metabolites in two species of sponges and the feasibility of cell culture as a production option, in 9th International Symposium on Marine Natural Products, 1998, Symposium Proc., Tonnville, Australia. 

Extensive studies to quantitate the production of indole alkaloids in Catharanthus roseus hairy root cultures have revealed that they accumulate several compounds including ajmalicine, serpentine, catharanthine, tabersonine, horhammericine, and lochnericine.27, 28 The presence of tabersonine in hairy roots has raised speculations that this intermediate in vindoline biosynthesis, together with catharanthine, is transported from this potential site of biosynthesis through the vasculature to the stem and to the leaves where tabersonine is further elaborated into vindoline within laticifers and/or idioblasts.26 However, oxidized derivatives of tabersonine, such as horhammericine and lochnericine, are present at 5 to 15 times the levels of tabersonine in hairy roots,27 and presumably this prevents their transport and/or use for vindoline biosynthesis. In this context, it would be interesting to... 

From a biogenetic standpoint, capsaicin is an acylated degraded phenylpropanoid. Both its aromatic and its acyl moiety are the result of unique metabolic processes that, though simple in principle, are still poorly characterized in terms of enzymology and regulation. Since vanillamine (23) is abundant in placental tissues of Capsicum, the only site of biosynthesis of capsaicinoids, the limiting factor for the synthesis of... 

Sites of biosynthesis are compartmentalized in the plant cell. While most biosynthetic pathways proceed (as least partially) in the cytoplasm, there is evidence that some alkaloids (such as coniine, quinolizidines and caffeine), furanocoumarins and some terpenes (such as monoterpenes, diterpenes. 

In many instances, the site of biosynthesis is restricted to a single organ, such as roots, leaves or fruits, but an accumulation of the corresponding products can be detected in several other plant tissues. Long-distance transport must take place in these instances. The xylem or phloem are likely transport routes, but an apoplastic transport can also be involved. 

The major site of steroid synthesis in animals has been considered to be the liver, but recently (1975) this has been disputed and detailed studies now show that sterol synthesis in guinea pig occurs more readily in ileum and lung than in liver under a variety of conditions" " and a brief study indicates similar sites of biosynthesis in swine." All tissues of guinea pig studied had an active feedback system controlling cholesterol biosynthesis and the results of feeding cholesterol and cholestyramine" showed that both the former and possibly bile acids suppress cholesterol synthesis in the liver to a far lesser extent than in the small intestine. 

The negative incorporations must be interpreted with caution, since solubility aspects and transport of the radioactive precursors to the site of biosynthesis may be extremely important 45, 47, 166). The last compound cited in Table III was incorporated into galantha-mine with no randomization (presumably via isovanillin), yet isovanillin was not utilized for alkaloid formation in either the Narcissus Deanna Durbin or in Nerine bowdenii 45, 47). Phenylalanine, an established precursor of the C-6—C-1 fragment of the alkaloids when injected into the leaves, flower stems, or bulbs, is not incorporated into alkaloids when introduced via root absorption from a hydroponic solution containing the radioactive amino acid 166). 

The multiplication of the common enzymatic steps, through isoenzymes, could provide a separate supply of the same prec irsor to competing pathways However, the efficiency of the majority of such systems also requires the separation of the isoforms at different levels of organization Additionnally, the sites of accumulation may frequently be spatially separated from the sites of biosynthesis (Ref 2) and these findings suggest the importance of passage of reactants from one compartment to emother in the realization of the complete sequence ... 

Fig.2. A model of the PSI complex explaining the relationships among the extent of sequence conservation, the site of biosynthesis, and the function of the subunits.

There is a growing interest in sterols and their derivatives in plants. The subject has been reviewed by Grunwald (1975a) Mudd and Garcia (1975) and by Eichenbeiger (1977). Much has been done on identification and biosynthesis, but some aspects of these studies require further elucidation, such as the subcellular sites of biosynthesis. In other respects the study of sterol derivatives in plants is in its infancy very little is known about function and metabolic turnover. 

Although much is known of the enzymatic steps in the biosynthesis of SE, SG, and ASG, much remains to be learned about the physiological importance of the reactions studied in vitro. The sites of biosynthesis of the sterol derivatives need to be studied at the same time as the accumulation of the compounds. Are sterol derivatives formed in the Golgi and then incorporated into the plasma membrane and tonoplast ... 

The specific incorporation rates in this type of experiments are usually low. In higher plants they are frequently in the range of O.OI %. The low rates are mainly due to loss of precursors to competing reactions during transport to the sites of biosynthesis and to dilution of labeled precursors by large pools of unlabeled precursors and end products. 

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