Specific activity precursor

Dil = molar specific activity (precursor)/molar specific activity (product)... 

On the other hand, the fragmentation of pyramine obtained from (2 -l3C)AIRs indicated clearly that C-2, in the ribose part, was the precursor of carbon C-7 of the methyl on C-2 of the pyrimidine ring (Scheme 29). This result was confirmed by an experiment with a sample of AIRs labeled with l4C on C-l, C-2, C-3, on the ribose, and C-5 on the imidazole, with an approximate distribution of 1, 1, 3, 3. This precursor produced pyramine with the methyl group almost as radioactive as C-l or C-2, and much less than C-3 of AIRs. Because of the incorporation of C-5 of imidazole into C-4 of pyramine, and the comparable activities of C-3 and C-5 in the precursor AIRs, the specific activity of pyramine... 

The underlying mechanisms involved in the activities of carotenoid oxidation products are due either to a possible role as precursors of retinoids that would be the active species for positive effects or to their own specific activities. This latter case is illustrated by the activity of non-provitamin A carotenoid oxidation products such as those derived from lycopene. However, biological effects of carotenoid oxidation products other than retinoids are only hypothesized in vivo in humans, which hypothesis has been used as the basic principle to justify in vitro studies of these compounds. 

The use of the symbol E in 5.1 for the environment had a double objective. It stands there for general environments, and it also stands for the enzyme considered as a very specific environment to the chemical interconversion step [102, 172], In the theory discussed above catalysis is produced if the energy levels of the quantum precursor and successor states are shifted below the energy value corresponding to the same species in a reference surrounding medium. Both the catalytic environment E and the substrates S are molded into complementary surface states to form the complex between the active precursor complex Si and the enzyme structure adapted to it E-Si. In enzyme catalyzed reactions the special productive binding has been confussed with the possible mechanisms to attain it lock-key represents a static view while the induced fit concept... 

The tritium labelled V.fischeri AHL 31 was prepared with a specific activity of 45-55 Ci/mmol by the tritiation of the corresponding unsaturated precursor, AT-(3-oxo-4-hexenoyl)-L-HSL 30 in the presence of a homogeneous Wilkinson s catalyst, tris(triphenylphosphine)rhodium[I] chloride (Scheme 13) [71]. 

The routes followed by plants in the biosynthesis of hgnin have been widely studied using tracer techniques, principally with radiocarbon. By comparing the specific activities of the precursors fed to plants with the... 

DKPs may be produced without any difficulties and may form without particular activation of the carboxyl function of the dipeptide precursor. However, the cyclization reaction may be a slow process, justifying the need for specific activation of the carboxyl moiety. Kopple stated that there is no single perfect method for the cyclization of all peptides, only guidelines aiding in the judicious choice of combinations of procedures to limit the generation of unwanted by-products of peptide synthesis. 

There may be several distinct PS complexes. Gu et al. (150) proposed the existence of at least three complexes (1) an approx. 150-kDa nicastrin-APHl (anterior pharynx defective 1 protein) complex (which is likely to be a precursor complex) (2) a stable and abundant intermediate complex of 440 kDa that contains APHl, PEN2 (presenilin enhancer 2), nicastrin, and PSl and (3) a high mass (>670-kDa) heteromeric complex (PS i-APHl-NCT-PEN2) associated with the highest y-secretase-specific activity (150) (see Fig. 10.3). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

Micellar catalysis is a broad field (Fendler and Fendler, 1975 Rathman, 1996 Rispens and Engberts, 2001), and caution is needed when using this term. In fact, whereas the broad term catalysis is justihed when referring to an increase of the velocity of reachon, this does not always mean that the velocity constant is increased (namely that there is a decrease of the specific activation energy). Rather, the velocity effect can be due to a concentration effect operated by the surface of the micelles. This is also the case for the autocatalytic self-reproduction of micelles discussed in the previous chapter, where the lipophilic precursor of the surfactant is concentrated on the hydrophobic surface of the fatty acid micelles (Bachmann et al., 1992), a feature that has given rise to some controversy (Mavelli and Luisi, 1996 Buhse etal, 1991 1998 Mavelli, 2004). 

The fluorination of a suitable precursor by a fluorinating agent of specific activity is a method used to synthesize selectively fluorinated molecules. This has been accomplished by various reagents where the source of fluorine is a carbon-fluorine bond (C —F reagents). 

Figure 2. Expanded pathway for the synthesis and use of KDO. The pathway initiates with D-giucose-6-phosphate, shows the branch point whereby D-ribulose-5-phosphate can be used to synthesize either D-arabinose-5-phosphate or D-ribose-5-phosphate and terminates with the transfer of KDO to the lipid A precursor. The numbers in parenthesis below the various enzymes correspond to their specific activities (nmoles per minute per...

In vivo measurements of lipopolysaccharide synthesis in E. coli B have indicated that two nanomoles of KDO must be synthesized per minute per mg of protein in order to meet the cellular requirement for LPS synthesis under the normal conditions of growth on glucose-minimal medium (27). We have measured the specific activities of the enzymes involved in KDO synthesis in crude extracts of E. coli B including those enzymes responsible for the synthesis of D-ribulose-5-phosphate, the precursor of D-arabinose-5-phosphate. D-Ribulose-5-phosphate is a key intermediate in carbohydrate metabolism as shown in Figure 2, since it is the direct precursor of both D-ribose-5-phosphate and D-arabinose-5-phosphate... 

Incorporation of Radioactive Precursors into Lipids. The cultures of surface-adhering cells were exposed for 16 h to either 400 pCi of Hp/C- syo (final specific activity, 50 yCi/ ymol) or 10 pCi of / %/, galactose ( the rate of incorporation was linear during this period). After 16 hr. the radioactive medium was removed and the cultures were washed four times with 0.9 NaCl. The cells were removed from the surface with a rubber policeman and suspended in physiological saline. Lipids were extracted by Bligh and Dyer procedure (26) and analyzed for various lipids according to Neskovic, et al. (27)... 

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