Which factors control 13C/12 C-ratios of urinary steroids?

There is considerable variation in the stable carbon isotope ratios (δ13C values) of urinary steroids. The total variance necessarily is the sum of biological variability and analytical error. In a reference population of 56 students of physical education, both factors exhibited comparable orders of magnitude. In order to explain the biological variability, several possible factors were investigated by a questionnaire. Sex, physical activity, and travels to the Americas turned out to be significant. Different steroids exhibit different δ13C values values, where females exhibit different patterns than males. Additionally, in females oral contraception results in a significant depletion of δ13C values. The latter phenomenon can possibly be explained by down-regulation of HMG-CoA reductase, the key enzyme in steroid biosynthesis. In fact, in vitro experiments showed that the reaction is associated by a significant 13C isotope effect (12k/13k = 1.0031 ± 0.0004). As this isotope effect finally propagates to androstanes and pregnanes at slightly different proportions, very small differences between the δ13C values of these steroid classes theoretically may be induced. Verf.-Referat