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].
The rate of turnover in a metabolic pathway, otherwise known as the metabolic flux , is regulated based on the stoichiometric reaction model, the utilization rate of metabolites, and the translocation pace of molecules across the lipid bilayer .  The regulation methods are based on experiments involving 13C-labeling , which is then analyzed by Nuclear Magnetic Resonance (NMR) or gas chromatography-mass spectrometry (GC-MS) -derived mass compositions. The aforementioned techniques synthesize a statistical interpretation of mass distribution in proteinogenic amino acids to the catalytic activities of enzymes in a cell.