Figure 1.

Structure and metabolic pathways of steroids. The steroid core structure is typically composed of 17 carbon atoms, bonded in 4 rings: three 6-member cyclohexane rings and one 5-member cyclopentane ring. Steroids vary in the functional group attached to this four-ring core and the oxidation state of the rings. The biosynthesis of steroid hormones begins with cholesterol as the starting material. First, high-density lipoprotein cholesterol in the blood is taken up into the cytoplasm and transported to the inner mitochondrial membrane via the action of steroidogenic acute regulatory protein (StAR), the cholesterol side chain cleavage enzyme, and other enzymes. Subsequently, 3β-hydroxysterone is synthesized, and various steroid hormones are biosynthesized via 3β-hydroxysteroid dehydrogenase, 17α-hydroxylase, 21-hydroxylase, and 11β-hydroxylase. All enzymes except 3β-hydroxysteroid dehydrogenase are cytochrome P-450 enzymes. 11β-Hydroxylase has 18-hydroxylase and 18-dehydrogenase activities, which are the final steps in the aldosterone synthesis system.