Biochemistry · MU Pleven

36. Conversion of Cholesterol to Specialised Products: Steroid Hormones and Calcitriol. Synthesis and Function. Monooxygenase Systems.

Covers how cholesterol is converted into steroid hormones and into calcitriol, the active form of vitamin D, starting with transport into mitochondria via StAR and the rate-limiting step to pregnenolone. A summary table sets out the steroid hormone groups, their sites and functions. It then traces vitamin D activation through skin, liver and kidney, explains calcitriol's actions, describes monooxygenase (cytochrome P450) systems and their roles, and ends with clinical disorders such as Addison's, Cushing's, Conn's syndrome and rickets.

Inside: 6 sections · 3 labelled pictures · 1 table · 10 flashcards · 3 min

Topic 36 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 6 sections, about 273 words (3 min of reading), with 10 flashcards.

In this note

  1. Conversion of Cholesterol to Specialised Products: Steroid Hormones and Calcitriol.
  2. Conversion of Cholesterol to Specialized Products: Steroid Hormones and Calcitriol
  3. Synthesis of Steroid Hormones from Cholesterol
  4. Synthesis of Calcitriol (Active Vitamin D)
  5. Monooxygenase (Hydroxylase) Systems
  6. Clinical Connections

Conversion of Cholesterol to Specialized Products: Steroid Hormones and Calcitriol

1. Overview

  • Cholesterol is the parent molecule for:
  • Steroid hormones
  • Vitamin D (Calcitriol)
  • Bile acids (already covered previously)
  • Steroid synthesis occurs mainly in:
  • Adrenal cortex
  • Gonads (ovary/testis)
  • Placenta
  • Vitamin D synthesis occurs in:
  • Skin → Liver → Kidney

Synthesis of Steroid Hormones from Cholesterol

2. Transport of Cholesterol Into Mitochondria

  • First regulatory step.
  • Mediated by StAR (Steroidogenic Acute Regulatory Protein).
  • Defect in StAR → Lipoid congenital adrenal hyperplasia (fatal).

3. Rate-Limiting Step

  • Cholesterol → Pregnenolone
  • Enzyme: Desmolase (CYP11A1), a mitochondrial cytochrome P450 enzyme.
  • Requires:
  • O₂
  • NADPH (from pentose phosphate pathway)

4. Pregnenolone → Specialized Steroid Hormones

Pregnenolone serves as common precursor for all steroid hormones:

Synthesis of Calcitriol (Active Vitamin D)

5. Steps

  • In Skin:

Cholesterol derivative 7-dehydrocholesterol → Cholecalciferol (Vitamin D₃)

Trigger: UV light

  • In Liver:

Vitamin D₃ → 25-Hydroxyvitamin D (Calcidiol)

Enzyme: 25-hydroxylase

  • In Kidney:

Calcidiol → 1,25-Dihydroxyvitamin D (Calcitriol)

Enzyme: 1α-hydroxylase

Stimulated by: PTH, low Ca²⁺, low phosphate

6. Function of Calcitriol

  • Increases intestinal absorption of Ca²⁺ and phosphate.
  • Promotes bone mineralization.
  • Acts as hormone, binding to nuclear receptors (like steroids).

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