Adrenocortical Hormone Synthesis and Zone Function

The adrenal cortex converts cholesterol into steroid hormones through an ordered series of enzymatic reactions. Its three histological zones express different combinations of steroidogenic enzymes, so each zone makes a characteristic class of hormone: mineralocorticoids in the outer zone, glucocorticoids in the middle zone, and androgens in the inner zone.

Definition

Adrenocortical hormone synthesis is the enzymatic conversion of cholesterol into mineralocorticoids, glucocorticoids, and adrenal androgens by the cortex, with each of its three zones (glomerulosa, fasciculata, reticularis) producing a distinct steroid class according to the enzymes it expresses.

Scope

This topic covers the biochemistry of cortical steroidogenesis and the principle of functional zonation. It addresses the cholesterol substrate, the rate-limiting side-chain cleavage step, the cytochrome P450 and hydroxysteroid dehydrogenase enzymes, and how zone-specific enzyme expression directs the pathway toward aldosterone, cortisol, or DHEA. Regulation of glucocorticoids and mineralocorticoids and the medulla are treated in sibling topics.

Core questions

• How does cholesterol enter the steroidogenic pathway, and what makes side-chain cleavage the rate-limiting step?
• Which enzymes are expressed in each cortical zone, and how does that determine the hormone produced?
• Why does the zona glomerulosa make aldosterone rather than cortisol despite sharing most of the pathway?

Key concepts

• Cholesterol as steroid precursor
• StAR protein and cholesterol import
• CYP11A1 (cholesterol side-chain cleavage)
• Functional zonation
• Zona glomerulosa, fasciculata, reticularis
• CYP11B1 (11-beta-hydroxylase) versus CYP11B2 (aldosterone synthase)
• CYP17A1 and the cortisol/androgen branch
• Pregnenolone and progesterone intermediates

Mechanisms

Steroidogenesis begins with the regulated import of cholesterol into the mitochondrion, facilitated by the steroidogenic acute regulatory (StAR) protein; there the side-chain cleavage enzyme CYP11A1 converts cholesterol to pregnenolone, the committed and rate-limiting step shared by all cortical zones. From pregnenolone, the path taken depends on which enzymes a zone expresses. The zona glomerulosa lacks CYP17A1 and uniquely expresses aldosterone synthase (CYP11B2), channeling steroids toward aldosterone. The zona fasciculata expresses CYP17A1 with 17-alpha-hydroxylase activity and CYP11B1, producing cortisol. The zona reticularis expresses CYP17A1 with strong 17,20-lyase activity, favoring the androgens DHEA and DHEA-sulfate. This zone-specific enzyme distribution is the molecular basis of functional zonation.

Clinical relevance

Defects in specific steroidogenic enzymes produce recognizable patterns of hormone excess and deficiency, the congenital adrenal hyperplasias being the classic example, in which a block at one step diverts precursors and alters glucocorticoid, mineralocorticoid, and androgen output. Knowing the pathway explains why these patterns arise. This entry describes normal and disordered biochemistry for reference and is not a basis for diagnosing or managing any individual.

Evidence & guidelines

The pathway and its enzymology are well-established and consolidated in comprehensive reviews (Miller & Auchus, 2011; Payne & Hales, 2004) and standard endocrinology and physiology textbooks. There is broad consensus on the enzymatic steps and zonation; this topic reports settled physiology rather than contested findings.

History

The cortical steroids were isolated and their structures determined in the 1930s and 1940s, work for which Kendall, Reichstein, and Hench shared a Nobel Prize. The enzymology was clarified across the later twentieth century as individual cytochrome P450 enzymes were purified and cloned, and the discovery of the StAR protein in the 1990s explained the regulated, rate-limiting import of cholesterol that initiates the pathway.

Key figures

• Walter Miller
• Richard Auchus
• Edward Kendall
• Tadeusz Reichstein

Seminal works

• miller-2011
• payne-2004

Frequently asked questions

What is the rate-limiting step in adrenal steroidogenesis?

The delivery of cholesterol into the mitochondrion, mediated by the StAR protein, followed by its conversion to pregnenolone by the side-chain cleavage enzyme CYP11A1. This is the committed step shared by all three cortical zones.

Why does only the zona glomerulosa make aldosterone?

Because the glomerulosa uniquely expresses aldosterone synthase (CYP11B2) and lacks CYP17A1; the absence of 17-alpha-hydroxylation prevents cortisol synthesis and channels the pathway toward aldosterone.