FIGURE 2. Major Pathways of Steroid Biosynthesis.
The pathways outlined here are common to the adrenals, the gonads and, to some extent, to the fetoplacental unit. The first committed step is the conversion of cholesterol to pregnenolone, catalysed by the P-450scc enzyme, which is under pituitary hormone control (ACTH or LH depending on the tissue). Cholesterol side-chain removal is blocked specifically by aminoglutethimide , a steroid biosynthesis inhibitor. From pregnenolone, steroid biosynthesis can proceed either through the so-called "delta-5" pathway (17α-hydroxypregnenolone, dehydroepiandrosterone, testosterone), or through the "delta-4" pathway (progesterone onwards). Progesterone is the starting point for mineralocorticoid synthesis, whereas glucocorticoids are derived from its metabolite, 17α-hydroxyprogesterone. Estrogens are formed from androgens (androstenedione and/or testosterone). Most reactions are irreversible (as denoted by a single arrow). Reversible reactions (double arrows) depend on cofactor availability (. the NADP/NADPH ratio). [Abbreviations used here for the various enzymes are listed in the figure].
Sex hormone-binding globulin (SHBG) is thought to mainly function as a transporter and reservoir for the estradiol and testosterone sex hormones. However it has also been demonstrated that SHBG can bind to a cell surface receptor (SHBG-R). The SHBG-R has not been completely characterized. A subset of steroids are able to bind to the SHBG/SHBG-R complex resulting in an activation of adenylyl cyclase and synthesis of the cAMP second messenger.  Hence the SHBG/SHBG-R complex appears to act as a transmembrane steroid receptor that is capable of transmitting signals to the interior of cells.
In order to overcome many limitations of immunoassays, high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) has the potential to find its place in the clinical laboratory medicine for quantification of steroid hormones. A prerequisite for the application of a new analytical procedure in clinical diagnostics is standardization to minimize analytical intra- and interlaboratory variability and inaccuracy. We evaluate a newly standardized HPLC-MS/MS assay in kit-format, developed for routine determination of 16 steroid hormones in human serum samples. Fifteen metabolites can be measured quantitatively, which include aldosterone, androstenedione, androsterone, corticosterone, cortisol, cortisone, 11-deoxycortisol, dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEAS), 17β-estradiol (E2), estrone (E1), etiocholanolone, 17α-hydroxyprogesterone (17OHP), progesterone, and testosterone. 11-Deoxycorticosterone is the only compound rated as semi-quantitative in this kit. The sample preparation is performed by solid phase extraction (SPE) on a 96-well plate. The standardized assay has been validated for human serum in terms of lower and upper limit of quantification (LLOQ -32 ng/mL, ULOQ 5-8000 ng/mL), linear correlation coefficient of calibration (R(2)>), intra- and inter-day precision (intra-day -%, inter-day -% and -% for 11-deoxycorticosterone), accuracy (intra-day -% and -% for 11-deoxycorticosterone, inter-day -% and -% for 11-deoxycorticosterone), analytical total error (-%), proficiency test accuracy (-%), recovery (68-99%), and metabolite stability (freeze/thaw stability -%, short term stability -%). Inter-assay comparison with a routine reference HPLC-MS/MS assay and seven immunoassays demonstrates the outstanding high performance of this HPLC-MS/MS based kit by improvements in accuracy for progesterone, androstenedione, and 17OHP. Finally, results of two metyrapone tests demonstrate the potential of the standardized HPLC-MS/MS assay for the analysis of a comprehensive steroid hormone profile in clinical diagnostics.