Isotope incorporation studies

Recent isotope incorporation studies by a number of workers have indicated increased turnover of muscle membrane lipids following denervation. These include increased incorporation of labeled phosphate and glycerol into glycerophosphatides (B17), increased turnover of... 

A biosynthetic relationship between purines and flavin had been initially conjectured on the basis of fermentation experiments where the addition of certain purine derivatives increased the yield of riboflavin produced by flavinogenic microorganisms. Ample confirmation for that hypothesis was obtained by isotope incorporation studies performed in parallel by various research groups using different microorganisms. Specifically, it was shown that the pyrimidine ring of purines can be contributed to biosynthetic riboflavin in its entirety, whereas C-8 of the imidazole ring of purine precursors is lost. The early work has been reviewed repeatedly and will not be discussed in detail. ... 

Isotope incorporation studies showed that the dimethylbenzimidazole moiety of vitamin Bj2 can be biosynthesized under aerobic conditions from riboflavin (10). Specifically, all carbon atoms of dimethylbenzimidazole are derived from the xylene ring and the 1 -carbon of riboflavin. Recently, this transformation was shown to be catalyzed by the BluB protein. Surprisingly, the reaction requires only FMNH (46) and molecular oxygen. The structure of BluB protein of Sinorhizobium meliloti) s been reported (Figure 26). ... 

A series of early biochemical studies have identified the primary precursors of methanofuran as phosphoe-nolpyruvate, dihydroxyacetone phosphate, tyrosine, glutamate, 2-ketoglutarate, acetate, and CO2 as illustrated in Figure 2. Isotope incorporation studies with H- and C-labeled precursors have shown that the fiiran moiety of methanofuran is formed from phosphoenolpyruvate and dihydroxyacetone phosphate. Following dehydration, the next step in the biosynthesis of the fiiran would be a reduction of the carboxylic acid to an aldehyde. The final step in the biosynthesis of the fiiran moiety of methanofuran is expected to be a transamination reaction to form the 2-aminomethyI subunit. 

Lam and coworkers conducted oxygen isotope incorporation studies to validate Evans hypotheses [12]. By using 02, it could be shown that the phenols were not formed from atmospheric oxygen, which in turn suggests that the phenol is formed from water produced during the reaction. Indeed, when H2 0 was used as additive, the isotope label was incorporated into the biphenylphenol product, as shown in Scheme 4.4. 

Swart PK (1980) The enviromnental geochemistry of scleractinian corals. An experimental study of trace element and stable isotope incorporation. PhD Dissertation, University of Londort, Londort, United... 

One of the most attractive features of borohydride reductions is that under micro-wave-enhanced conditions they can be performed in the solid state, and rapidly. We were attracted by the work of Loupy [57], and in particular Varma [58, 59] who has shown that irradiation of a number of aldehydes and ketones in a microwave oven in the presence of alumina doped NaBH4 for short periods of time led to rapid reduction (0.5-2 min) in good yields (62-93%). In our study [60] seven aldehydes and four ketones were reduced (Tab. 13.3). Again reduction was complete within 1 min, the products were of high purity (>95%), of high isotopic incorporation (95%, same as the NaBD4) and the reactions completely selective. 

It is difficult to reconcile the unique chemical structure of tetrodotoxin with that of an animal product. Its structure is not related to that of other animal products by any readily recognized biosynthetic scheme. It is not a terpenoid, not obviously formed from amino acid or carbohydrate units, and apparently not constructed from acetate or propionate units. Nor does it resemble any of the various plant alkaloid patterns. It thus appears to be a very unlikely animal product to result from known biogenetic pathways. In this connection the metabolic incorporation of radioactive precursors using torosa and ]C. granulosa salamanders was studied by Shimizu et al. (47). They observed significant isotopic incorporation into amino acids and steroid metabolites, but they found no such incorporation associated with tetrodotoxin. 

The condensation of a 2-aminobenzoxazole 141 with a-bromo ketones 142 gives 2(3/f)-oxazolones 148 on heating in ethanol. Isotope labeling studies with have shown that the additional oxygen that is incorporated into the ring is derived from the solvent, ethanol (Fig. 5.36). ... 

Incorporation studies with isotopes showed that when anthranijate was converted to tryptophan, the carboxyl group df anthranilate was lost as carbon dioxide, but the nitrogen was retained. Because the enzymes in the tryptophan biosynthetic pathway have only a limited specificity, it was possible to substitute 4-methyl-anthranilate in E. coli extracts that could convert anthranilate to indole. This nonisotope label was conserved during the conversion to yield 6-methyl indole. 

Drugs that have a radioactive isotope incorporated into their structures are invariably used in studies to establish safe withdrawal periods. The presence of the radioactive isotope is harmless to the animal, but offers a highly sensitive means of determining the biological half-life of the drug and the total residue concentrations in different tissues in different animals. 

Attempts to use molecular oxygen as the oxidant failed except in solvents that undergo efficient autoxidation to the corresponding hydroperoxide (e.g., THF). Mechanistic studies, including isotopic labeling studies, indicate that fBuOOH is the source of the oxygen atom incorporated into the product, and the reaction proceeds via a hydride-shift pathway that avoids formation of an enol intermediate (Scheme 12). 

Only preliminary studies of the homogeneous technique have been performed and the procedure has been used to label only relatively simple molecules. However, extrapolation to some heterocyclic systems has been achieved. In general, the orientation of isotope incorporated by both homogeneous and heterogeneous procedures is similar, ortho deactivation effects being observed for positions adjacent to bulky substituents such as alkyl groups. 

---