Steroid Hormone Synthesis
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Direct answer
Cholesterol side-chain cleavage by mitochondrial CYP11A1 (desmolase), gated by the StAR protein that ferries cholesterol into the mitochondrion, converts cholesterol to pregnenolone — the rate-limiting step every trophic hormone acts on within minutes. From pregnenolone the pathways fan out through 3β-hydroxysteroid dehydrogenase and the cytochrome P450 family: CYP17 sends carbon toward cortisol and androgens, CYP21A2 and CYP11B1 complete cortisol synthesis, CYP11B2 in the zona glomerulosa makes aldosterone, and aromatase (CYP19) converts androgens to oestrogens. Each enzymatic block produces a recognisable congenital adrenal hyperplasia — 21-hydroxylase deficiency (over ninety per cent of cases) starves cortisol and aldosterone while shunting substrate into androgens.
What you must remember
- Rate-limiting pair: StAR delivers cholesterol to the inner mitochondrial membrane; CYP11A1 cleaves it to pregnenolone; ACTH induces StAR within minutes (acute response) and adrenal hypertrophy over weeks (chronic response).
- Zone logic: glomerulosa (aldosterone; angiotensin II and potassium driven; lacks CYP17), fasciculata (cortisol; ACTH), reticularis (androgens; ACTH) — cortical blood drains centripetally so fasciculate cortisol bathes the medulla and induces PNMT, the bridge to catecholamine synthesis.
- Enzyme positions: 3β-HSD converts pregnenolone to progesterone; CYP17 does 17α-hydroxylation and 17,20-lyase; CYP21A2 yields 11-deoxycortisol; CYP11B1 makes cortisol; CYP11B2 (aldosterone synthase) runs 18-hydroxylation; aromatase and 5α-reductase finish sex-steroid metabolism.
- 21-hydroxylase deficiency: 17-hydroxyprogesterone accumulates (the newborn-screening analyte), cortisol and aldosterone fall — salt-wasting crisis in the second or third week of life — while androgen shunting virilises a 46,XX newborn and precociously matures a 46,XY boy; treatment is glucocorticoid with mineralocorticoid replacement and sodium supplementation.
- 11β-hydroxylase deficiency: 11-deoxycortisol accumulates and acts as a mineralocorticoid — hypertension and hypokalaemia with virilisation, the mirror image of 21-block salt wasting.
- 17α-hydroxylase deficiency: hypertension with hypokalaemia but sexual infantilism and amenorrhoea, 46,XY under-masculinisation; weak corticosterone covers glucocorticoid needs, so crisis is rare.
- StAR deficiency (lipoid CAH): all steroidogenesis fails, adrenals and gonads fill with cholesterol esters; 46,XY infants present as phenotypic females.
- Drugs on the pathway: metyrapone (11β blocker, also used as a pituitary reserve test), ketoconazole (multi-CYP), abiraterone (CYP17 inhibitor for castration-resistant prostate cancer, with prednisone cover), spironolactone at the receptor, dexamethasone for suppression testing.
Worked example: the two-week-old who will not feed
A sixteen-day-old infant vomits, loses weight and collapses with sodium 124, potassium 6.1 and hypoglycaemia. The genital exam in retrospect matters: this 46,XX newborn was mildly virilised at birth. The biochemistry completes the story — 17-hydroxyprogesterone many-fold elevated, renin high, aldosterone low: classic salt-wasting 21-hydroxylase deficiency. Resuscitation with saline and glucose, then hydrocortisone and fludrocortisone, turns the corner; parents learn stress dosing for every future fever. Newborn screening by heel-prick 17-OHP exists in many countries and is expanding in India, though programme coverage remains uneven — a fair viva comment. Counselling notes autosomal recessive inheritance with one-in-four recurrence, and that the virilised external genitalia are cosmetic, not gonadal — sex assignment is chromosomal and gonadal, with surgical decisions deferred per current paediatric endocrine practice.
Where students slip
Confusing 11β-hydroxylase deficiency with apparent mineralocorticoid excess (11β-HSD2 failure, the licorice enzyme) — both hypertensive, different enzymes entirely. Forgetting the name's explanation: cortisol low → ACTH high → adrenal hyperplasia and pigmentation. And the inheritance one-liner: all classic CAH forms are autosomal recessive.
Frequently asked questions
Which step is rate-limiting in steroidogenesis?
Cholesterol transport into mitochondria by the StAR protein, followed by CYP11A1 side-chain cleavage to pregnenolone — the step ACTH acutely stimulates.
Why does 21-hydroxylase deficiency virilise a female newborn?
Blocked cortisol and aldosterone synthesis diverts accumulated precursors into androgens under the drive of compensatorily raised ACTH.
What analyte does newborn screening use for congenital adrenal hyperplasia?
17-hydroxyprogesterone, the substrate that accumulates upstream of the 21-hydroxylase block.
Why is 11β-hydroxylase deficiency hypertensive while 21-hydroxylase deficiency wastes salt?
Accumulated 11-deoxycortisol acts as a mineralocorticoid, whereas the 21-block abolishes aldosterone synthesis.
Which steroidogenic enzyme does abiraterone inhibit?
CYP17, cutting androgen synthesis in castration-resistant prostate cancer, with prednisone given to prevent mineralocorticoid excess.