Tritium exchange labelling

Deuterium/tritium exchange-labeling at C2 of amino acids via imine/iminium... 

Catalysts for tritium exchange labeling of compounds in solution utilizing liquid-phase tritium sources (usually tritiated water), aside from acids and bases or organic Lewis acids, are all metal based. Soluble metal catalysts are either metal salts or organometallic complexes. A recent review covers many aspects of this chemistry ". ... 

Figure 3.56 Example of amino group-mediated tritium exchange labeling catalyzed by [(cod) lr(PCy3)(py)]PFe...

Binding assays for the saxitoxins were conducted with homogenized rabbit brain and saxitoxin exchange-labelled with tritium at C-11 (92, 93). If the various saxitoxins were available with suitably intense radiolabels, then the equilibrium dissociation constant, K, could be measured directly for each. Since only saxitoxin is currently available with the necessary label, the binding experiments instead measure the ability of a compound to compete with radiolabelled saxitoxin for the binding site. The value obtained, Kj, corresponds to the uilibrium dissociation constant, K, that would be observed for the compound if it were measured directly. Affinity is defined for this assay as the reciprocal of Kj. The affinities of several of the saxitoxins (94) are summarized in Figure 11, expressed relative to saxitoxin and plotted on a logarithmic scale. 

Once the dissociative mechanism is established, it is possible to apply Gutmann s theoretical treatment (40) to the elucidation of the rate-determining step of the exchange reaction. For deuterium-tritium double labeling procedures, i.e., D O 100%, TgO 1%, it may be shown that the following normalized equations apply under initial exchange conditions ... 

This rate law, for example, applies to the tritium exchange from labeled dihydroxyacetone phosphate to water in the aldolase reaction or the exchange from labeled malate to water in the fumarase reaction. 

Laszlo Heredy We did make experiments with labeled anthracene, but the results were inconclusive as to whether any tritium exchange took place at any rate it is very small. It seems that the linear compound is simply less efficient both as solvent and reactant than an angular hydrocarbon like phenanthrene. 

Klein, P. D., and J. C. Knight The Exchange Labeling of Keto Steroids with Tritium by Adsorption Chromatography on Basic Alumina. J. Amer. chem. Soc. 87, 2657 (1965). 

Enoate reductase exclusively splits off the (4S)-hydrogen atom from NADH. There is no direct hydrogen transfer from NADH to the products. If (45)-[ H]-labelled NADH with a total tritium activity of 4.67.10 decays per minute was dehydrogenated with an excess of enoate, the isolated product showed a tritium content of less than 0.005 % of that of the NADH. Almost all the tritium was in the water. In the absence of an enoate as acceptor, the tritium exchange from (45)-[4- H]-NADH catalysed by enoate reductase is very slow. Depending on the substrate concentration, the isotope effect of the reduction of ( )-2-methyl-2-butenoate with (4S)-[4- H]-NADH varies from 6.8 to 1.3. The presence of NAD" decreases the isotope effect (17). 

Kirby, G. W., and L. Ogunkoya Deuterium and Tritium Exchange Reactions of Phenols and the Synthesis of Labelled 3,4-Dihydrophenylalanines. J. Chem. Soc. 1965, 6914. 

Bile acid synthesis can also be measured isotopically. Of these procedures Lindstedt s method is widely used because it permits the measurement of the pool size, turnover and synthesis rates of the two primary bile acids separately provided that the two differently labeled bile acids are administered and that the H-label is tightly bound and is not subjected to losses due to tritium exchange. The method may not be valid in gross bile acid malabsorption due to incomplete mixing of the labels within the bile acid pool. Comparison of the quantitative data obtained with this and GLC methods have given similar and even markedly different results (cf. 7). Usually the values of the isotopic procedure are somewhat higher than those of the GLC method, a discrepancy which has no explanation at the moment even when the nonspecific losses of tritium is taken into consideration. Hypertriglyceridemic subjects in particular produce much more bile acids on the basis of the isotopic than chemical balance. 

The total amount of labeled drug to prepare, in terms of weight, will depend on the specific activity and the total amount of radioactivity required. This will vary widely but usually does not fall much below 0.5 mM for a drug prepared by a synthetic sequence. In the case of tritium exchange and reduction reactions it may be considerably lower, however. 

The tritium gas-exposure method for radiation-induced exchange labeling of organic compounds was introduced by Wilzbach in 1957 and has come to be known as the "Wilzbach method" j . In this method the compound to be labeled is exposed to curie amounts of carrier-free tritium gas in a sealed flask for a period of several days to several weeks. The activation energy necessary for exchange of stable hydrogen atoms in the compound with tritium gas is derived from the 6-decay of tritium. Tritium... 

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