Double isotope tracer method

Androsterone and 5)3-androsterone, which are the major metabolites, were thought to be uniquely derived from the plasma androstenedione pool. Korenman et al. [170,388] demonstrated by the double isotope tracer method using carbon-14-labeled testosterone and tritium-labeled androstenedione that neither androsterone nor 5 -androsterone is a unique metabolite of a plasma androstenedione pool. A unique steroid metabolite has been defined by Dorfman [326] as a steroid persisting or formed during metabolism which can be related to one and only one tissue steroid. Korenman et al. obtained different tritium/carbon-14 ratios for androsterone and 5/3-androsterone, suggesting that other pathways of testosterone and androstenedione metabolism also exist. These other pathways, possibly metabolism of testosterone and androstenedione by peripheral tissue, may be responsible for the relative enrichment of either the 5a- or the 5/8-isomer of the urinary metabolites [305]. 

In the estimation of unlabelled metabolites extracted from biological materials, enzymes have been used to convert non-radioactive substrates into labelled derivatives. A labelled co-factor is used in a manner similar to the use of a labelled reagent in isotope derivative analysis. A good example of such methods is the double isotope enzymatic assay for histamine [325]. Samples containing unknown amounts of histamine, tracer amounts of H-histamine and " C-S-adenosylmethionine are incubated with a partially purified preparation of histamine methyl transferase from guinea-pig brain. This enzyme is specific for the methylation of histamine [326]. The product of the reaction, 1,4-methylhistamine, is extracted into chloroform and the ratio of determined by liquid scintillation counting, is directly pro-... 

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