Use of Radioisotopes in Steroid Methodology

Abstract

*Quantitative analyses of organic compounds in a mixture of biologic origin are extremely difficult or inconvenient if the com­ pounds to be analyzed are present in trace amounts and are similar to other components of the mixture. Thus, it frequently is necessary to use labeled (radioactive) indications in the analytical assay so that quantitative isolation of the compound of interest is not re­ quired. C14_ and tritium-labeled steroids can be used to measure quantitatively, by the isotope dilution procedure, the amount of a particular steroid present in a mixture when the steroid to be analyzed cannot be quantitatively separated from the mixture, or when the analytical problem cannot be readily solved by other means. With isotope dilution procedures, quantitative isolation of a compound is not necessary for accurate analyses. The only requirement is a sufficiently pure sample for assay of specific activity. Previous publications have described the methods for isotope dilution analyses of various steroids using C14-labeled steroids [1,21. These methods depend on the measured dilution of the isotopically labeled steroid by a nonlabeled variant of the same steroid, and consist of three general types: (a) direct isotope dilution; (b) reverse isotope dilution; and (c) double isotope dilution. Conventional isotope-dilution methods are frequently not ap­ plicable to the determination of certain steroids in biological mixtures, or are subject to a large error, owing to the very small dilution of the isotope carrier. In addition, the quantities of steroid available for the chemical or physical measurement of the purified labeled steroid after dilution and purification may be so small as to make a determination of specific activity impossible. Finally, it may not be possible to obtain a C14_ or tritium-labeled steroid. Dilution methods need not be limited to the addition of labeled *Presented at the Second Symposium on Advances in Tracer Applications of Tritium, 1958. 265