Tritium biotransformations

Pyrrolizidine Alkaloids.—The necic acid component of senecionine (8) derives from two molecules of isoleucine, radioactivity from precursor amino-acid being equally incorporated into both halves of the necic acid fragment, as shown in Scheme 2 (c/. Vol. 9, p. 4). It has now been shown that biotransformation of isoleucine into the necic acid involves loss of half of a tritium label from C-4 in each of the two amino-acid fragments.6 Removal of a proton is, therefore, stereospecific, and oxidation at C-4 does not proceed beyond the two-electron level i.e., a higher intermediate oxidation level, corresponding to a ketone, is excluded. Further results indicate that for each molecule of isoleucine it is the 4-pro-S proton [see (14)] which is lost. 

In contrast to the oxidative reactions discussed above, the only reported biotransformations of reserpine (21) and rescinnamine (23) (42-44) appear to involve hydrolytic processes. Reserpine is readily metabolized by liver homogenates from the mouse (43), rat, guinea pig, dog, and cat (44) to yield methyl reserpate (22) and 3,4,5-trimethoxybenzoic acid in yields of up to 70% (43). The use of reserpine labeled with tritium in the 2 and 6 positions of the trimethoxybenzoate residue indicated that no significant metabolism of reserpine by another route occurred before hydrolysis, reserpine and 3,4,5-trimethoxybenzoic acid being the only detectable radioactive components of the incubation mixture at the conclusion of the reaction (44). An... 

Using chirally tritiated samples of the protoberberine (67) it has been established that hydroxylation of (67) to give (68) occurs with loss of the 13-pro-R hydrogen atom, i.e. normal retention of configuration, and does not involve an enamine intermediate since tritium is not lost from C-14 during the course of this biotransformation.62 It is to be noted that similar results, associated with C-13, have been observed61 for narcotine (63) and chelidonine (62) biosynthesis, except that here the 13-pro-S proton is removed. 

It is known that valine is incorporated into the penicillins with overall retention of configuration at C-3. It has been shown that l- l-14C,3-3H]valine is incorporated into the disulphide dimer of (100), in cultures of a mutant of C. acremonium, with complete retention of tritium at C-3 of the valine moiety.96 This indicates that valine is built into (100) with retention of configuration and that therefore no double inversion occurs during the biotransformation of valine into the penicillins. 

Gault, H. Kalra, J. Ahmed, M. Kepkay, D. Longerich, L. Barrowman, J. Influence of gastric pH on digoxin biotransformation. II. Ebctractable urineuy metabolites. Clin.Pharmacol.Ther, 1981, 29, 181-190 [urine radioactivity detection tritium labeled]... 

Natural attenuation processes are always site-specific, so every site needs to be evaluated individually. The accepted mechanisms for inorganics, including radionuclides, are immobilization and/or biotransformation. For tritium, decay is acceptable. In most of the cases listed in Table 10.2, the attenuation process is only moderately understood. Although recognized as being important. 

A precursor with and labels is normally made by mixing two singly labelled samples. In some cases it has been observed that subsequent biotransformation leads to an increase in the relative amount of tritium-labelled species. This is explained as the result of preferential metabolism of C-labelled material along other paths. ... 

Despite extensive efforts, solid complexes of metal ions with cocaine were not isolated. For the development of a radioimmunoassay for biotransformed cocaine, a tritium-labelled O-benzoylecgonine (30) of high specific activity was... 

Hydrogen atoms in labile positions, for example the hydrogen of caiboxylic acid, undergo rapid exchange and equilibration with tritiated water. These products are not veiy useful for biotransformation studies because the tritium label is veiy susceptible to exchange in aqueous solutions. 

A third and most significant study involves tracer experiments with Papaver rhoeas L. Both 135- and 13/ -tritium labeled scoulerines were incorporated into rhoeadine, but the rhoeadine isolated from plants fed with 135-labeled scoulerine had lost 19% of the tritium present in the precursor, whereas the 13/ -Iabeled scoulerine afforded rhoeadine which had retained 74% of the original tritium. A stereospecific loss of the pro-5 hydrogen must, therefore, have occurred from the C-13 of scoulerine at some specific stage of its biotransformation into rhoeadine. It seems likely that scoulerine is converted first into stylopine which is oxidized to 42. A-Methylation and rearrangement then provide the correct skeleton from which rhoeadine can arise... 

Tritium ( H) and with half-lives of 12.26 years and 5600 years respectively, are the most commonly used radioactive isotopes. They are used extensively to monitor the uptake of precursors into metabolites and to determine the fates of individual hydrogens or carbons in a precursor during biosynthesis. Tritium finds particular application in this latter regard because any number of proton additions/removals and rearrangements may occur in the course of the biotransformation of precursor into metabolite. (Tritium loss is normally measured by reference to a label in the precursor which is known not to be lost during biosynthesis.)... 

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