Labels appearance

Assume that acetyl CoA containing a 14C isotopic label in the carboxyl carbon atom is used as starting material for the biosynthesis of mevalonate, as shown in Figure 27.7. At what positions in mevalonate would the isotopic label appear ... 

Salts of aliphatic or aromatic carboxylic acids can be converted to the corresponding nitriles by heating with BrCN or CICN. Despite appearances, this is not a substitution reaction. When R COO was used, the label appeared in the nitrile, not in the C02, and optical activity in R was retained. The acyl isocyanate... 

Fig. 16.37 Formation of parathion hydrolysis product (fraction of initial label appearing in the aqueous extract) in remoistened Gilat soil after application of 10-160p,g parathion per g dry soil. Plotted points are means of three replicates standard error. Continuous curves represent model simulations. Reprinted from Nelson LM, Yaron B, Nye PH (1982) Biologically-induced hydrolysis of parathion in soil. Soil Biol Biochem 14 223-227. Copyright 1982 with permission of Elsevier...

Hamilton marked the carbonyl group of acetoacetic acid with ieO, and then carried out the enzymic decarboxylation (Hamilton and Westheimer, 1959). The product of the decarboxylation, acetone, contained none of the label. This result is demanded by the ketimine mechanism, whereas the mechanism of uncatalyzed decarboxylation would have required that the label appear intact in the product. Of course, in order to make these statements we had to carry out an elaborate set of control experiments, since 180 is washed out of both acetone and acetoacetic acid by buffers and even more... 

The proposed mechanism agrees with the general picture presented in Eqs. (5) and (6). The electronic reorganization of the adduct allows the release of N20 and NH3, with [Fen(CN)5H20]3- formation [Eq. (11)]. Strong evidence for the mechanism was obtained by using 15N in NP, with the result that the label appeared only in N15NO, as measured by mass spectrometry, but not in NH3 or in the amines. 

C]acetyl-CoA is added to a rat liver homogenate that is synthesizing cholesterol, where will the 14C label appear in A3-isopentenyl pyrophosphate, the activated form of an isoprene unit ... 

In another set of experiments, Schlesinger and Anderson (44) showed that the isozymes are formed in vivo by alteration of the dimer. Using an E. coli mutant that makes an altered subunit, which will only dimerize (in vitro or in vivo) in the presence of phosphate or zinc, they found that the monomers produced by the cell growing in the absence of phosphate and zinc produced only one electrophoretic form when the monomer was converted to the dimer in vitro. However, if the medium is made 2 vaM in phosphate and 10 /iM in zinc, in the exponential phase of growth, three isozymes are formed. Additional support for the conclusion that isozymes are made by alteration of the dimer comes from the fact that independent of when 14C-labeled amino acids are added to the growing culture, label appears first in isozyme I. It thus appears that a mechanism is available in the periplasmic space for the conversion of isozyme I to the other isozymes. 

Liver cells in culture are given 2-[l4C]-acetate. Where does this label appear in HMG-CoA ... 

The thiazole formed under the conditions used by Cemy and Davidek250 was only singly labelled, the label appearing at C-2, suggesting that the compound had been formed from cysteamine and formaldehyde derived from ribose (cf. ref. 258). 

Answer Figures 14-2, 14-6, and 16-7 and Box 16-3 outline the fate of all the carbon atoms of glucose. In one pass through the pathways, the label appears at ... 

The A piperideine trimer hypothesis is supported initially by the equal incorporation of lysine and cadaverine into all three alkaloid fragments13,14 but more significantly by the incorporation of three molecules of A piperideine (1) into lupanine (10) and by the manner of this incorporation label from C-6 appeared at C-2, C-15, and, by inference, C-10, whereas C-2 label appeared at C-17, C-ll, and, by inference, C-6, consistent with the hypothesis (see Scheme 3).19 Further, approximately a third of the label was located at each of the determined sites. 

Before approval of the label, the FDA negotiates with the pharmaceutical company concerning its contents. After approval, the label appears in package inserts. It is published by the drug companies in the Physicians Desk Reference (PDR), a commercial book sent free to all practicing physicians and found in most treatment facilities and doctors offices. A shortened form of the label with emphasis on adverse effects must be included in advertising and promotional materials. 

Remember that the 180 label appears in both oxygens of the acetic acid starting material. 

The new USDA organic label appears on all certified products in the U.S. 

These mechanisms were finally discounted by Loftfield [3047, who used the isotopically labelled compound 15) and showed that only half of the label appeared at the carboxyl-bearing carbon in the product. A mechanism involving the reactive cyclopropanone intermediate (16) accounts for the isotopic distribution, and has gained general acceptance... 

The biosynthesis of 5-aminolevulinic acid (ALA, a chlorophyll precursor) was studied in greening barley leaves. Preliminary long-term experiments showed that radioactivity from glycine-2-C (or-carbon labeled) and from glutamate-2,S.-C " (methylene carbons labeled) appeared in ALA. The specific activity data of a more detailed study is shown in Figure 6-6. What conclusions can be drawn ... 

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