Labeling pattern

Hot atom reactions have also been used to label organic compounds with T. Irradiation of helium-3 with neutrons according to the nuclear reaction produces very energetic tritium atoms that can displace ordinary hydrogen in organic compounds. This procedure is not very selective, and the labeling pattern must be determined to enable the tritiated product to be used effectively as a tracer (34). 

Describe the labeling pattern that would result from the introduction into the TCA cycle of glutamate labeled at with " C. 

The most plausible explanation of this very unusual labeling pattern is to assume the involvement of succinate, which is synthesized from acetate as shown in Scheme 8a. 

Figure 1. Transverse section of barley leaf epidermal cells taken perpendicular to the long axis of the cells and anticlinal to the leaf surface. The section has been labeled by the EMSIL technique (see Methods) utilizing purified C. sativus endopolygalacturonase and monoclonal antibody EPG-4, which is specific for this enzyme, in order to localize the substrate of the enzyme at the typical site penetrated by the fungal pathogen. Bar = 1 pm. Inset Comparable cell wall region as in Fig. 1 but labeled with monoclonal antibody JIM 5 to localize non-esterified pectin. Bar = 1 pm. Note the identical labeling patterns obtained with either method.

A receptor model was deduced based on the labeling pattern [51]. 

We now have determined the steady-state flux values that yielded the observed labeling pattern. Nevertheless, without statistical analysis these values... 

In reality, the ideal isotope labeling pattern (one component 100% labeled, the other one 0%) is almost never fulfilled first, a 100% isotopic enrichment will be generally very hard to reach (and very costly ) for chemically synthesized as well as for overexpressed compounds. On the other hand, even the nonenriched components will always contain NMR-active isotopes at natural abundance unless they are derived from specially isotope-depleted (and again very expensive) starting material. 

The incorporation of acetate into the monoterpene unit of the indole alkaloids has recently been reexamined (176). Using [l,2- C2]acetate it was established that no intact incorporation occurred, and a similar labeling pattern to that from [2- C2]acetate was observed, i.e., C-3, C-4, C-20, C-22, and C-23. Extensive scrambling of the acetate occurred via the Krebs cycle to label the 1 and 2 positions of acetate prior to incorporation. [2- C]Mevalonate was incorporated equally into C-17 and C-22 of ajmalicine (39), indicating that an equilibration occurs at some point in the pathway, as had been established previously with radiolabeled precursors 176). 

A positional isotopomer has an exactly determined labeling pattern, described by a specific number of atoms in specific positions of the molecule. Generally, 2" isotopomers are possible for a compound with n carbon atoms. The 8 different positional isotopomers of a compound with 3 carbon atoms are shown in Fig. (1). 

In their approach of applying transition matrices for the description of the carbon transfer in metabolic networks, Wiechert et al. [16] defined the labeling pattern of a molecule via cumulated isotopomer (cumomer) fractions, whereby the isotopomers belonging to the same cumomer fraction share labelings at specific carbon positions. For a Cj compound, three 1-cumomer fractions... 

Eichinger D, Bacher A, Zenk MH, Eisenreich, W. Analysis of metabolic pathways via quantitahve prediction of isotope labeling patterns a retrobiosynthehc 13C NMR study on the monoterpene loganin. Phytochemistry 1999 51 223-236. 

A remarkable reaction of the phospho group is its transfer onto carbon in the biosynthesis of the C—P bond (Fig. 6). Incidentally, this leads to a difficulty with nomenclature, because the group -P03H2, which is known to chemists as phosphono, is called phospho only when on a heteroatom (64), so the transfer changes its name Despite much ealier guessing from labeling patterns that phosphoenolpyruvate was the source of the C—P bond, it was only in 1988 (65, 66) that the enzyme responsible was isolated. The difficulty proved to be that the equilibrium favors phosphoenolpyruvate by about 2000-fold (67), so that assays only detected the enzyme in the direction contrary to biosynthesis evidently the biosynthesis takes place because subsequent reac-... 

A biosynthetic study of the 18-carbon chain skeleton of broussonetines was reported [38]. To verify the biosynthetic route of these alkaloids, the plant was grown on an aseptic medium and the enriched l3C of the isolated alkaloids was analyzed by NMR after feeding with [l-13C]glucose. The labeling pattern of broussonetine J (27) obtained... 

---