Isotope effects in the metabolism of labeled steroids

Although considerable data exist on differences in physico-chemical properties of labeled and unlabeled compounds, data on isotopic effects on the metabolism of labeled compounds are limited to conflicting studies by Gold and Few. The present study shows that significant isotope effects may occur in the ³H/¹⁴C ratio of hormone metabolites in some subjects though not in all. The use of doubly labeled precursors in establishing metabolic patterns is not precluded provided that only deviations beyond the range of these effects are used in drawing metabolic conclusions. Alternatively misinterpretations due to isotope effects may be avoided by repeating the experiments with the isotope labels on the two precursors reversed (¹⁴C compound A and ³H compound B in some studies and ³H-compound A and ¹⁴C-compound B in repeat studies). Under these conditions real metabolic differences and isotope effects can be readily distinguished.