steroidogenesis

Steroidogenesis is the assembly-line process by which cells turn cholesterol into steroid hormones. Imagine a single starting block of raw material being passed station to station, with each enzyme snipping, adding, or rearranging a chemical group until a finished hormone rolls off the line. The same starting cholesterol can become cortisol, aldosterone, an androgen, or an estrogen depending on which enzymes are present.

The pathway begins with a rate-limiting step: cholesterol is moved into the mitochondrion (with help from the StAR protein) and converted to pregnenolone, the common precursor for all steroid hormones. From there, a branching series of enzymes — cytochrome P450s and hydroxysteroid dehydrogenases — channels pregnenolone down different routes. The mix of enzymes a cell expresses decides its end product, which is why each adrenal zone, and each gonad, makes its own characteristic hormones.

Because the pathway is a chain, blocking one enzyme backs up everything upstream and starves everything downstream. This is exactly what happens in congenital adrenal hyperplasia, where an inherited enzyme defect (most often 21-hydroxylase deficiency) blocks cortisol production and shunts the accumulating precursors into the androgen branch. Several useful drugs also work by inhibiting specific steroidogenic enzymes.