Part A: Structure and Functions of Parathyroid Hormone (PTH)
Structure of Parathyroid Hormone
- Parathyroid hormone (PTH) is a polypeptide hormone composed of 84 amino acids.
- It is synthesized and secreted by the parathyroid glands, which are small glands located behind the thyroid.
- PTH is initially produced as a larger inactive precursor called preproPTH, which is then processed to proPTH and finally to the active 84-amino acid PTH.
Functions of Parathyroid Hormone
PTH plays a key role in regulating calcium and phosphate balance in the blood and bones.
- Bone: Stimulates osteoclast activity, leading to breakdown of bone tissue and release of calcium into the bloodstream.
- Kidneys:
- Reduces calcium excretion in urine (increases calcium reabsorption).
- Increases phosphate excretion in urine (decreases phosphate reabsorption).
- Stimulates production of active vitamin D (calcitriol) in kidneys.
- Intestines (Indirect): Through activation of vitamin D, it promotes absorption of calcium and phosphate from the intestine.
Part B: Synthesis and Functions of Vitamin D
Synthesis of Vitamin D
Vitamin D can be synthesized in the skin or obtained from the diet.
- In Skin:
- 7-dehydrocholesterol (in the skin) is converted to cholecalciferol (Vitamin D3) upon exposure to UVB sunlight.
- In Liver:
- Cholecalciferol is converted into 25-hydroxycholecalciferol (calcidiol).
- In Kidneys:
- Calcidiol is converted into the active form, 1,25-dihydroxycholecalciferol (calcitriol), by the enzyme 1-alpha-hydroxylase.
Functions of Vitamin D
- Calcium and Phosphate Homeostasis:
- Enhances calcium and phosphate absorption from the intestine.
- Supports bone mineralization and prevents rickets and osteomalacia.
- Bone Health:
- Works with PTH to mobilize calcium from bones when needed.
- Immune System:
- Supports immune response and may reduce inflammation.
Conclusion
Parathyroid hormone and vitamin D work together to maintain calcium and phosphate balance in the body, essential for healthy bones and muscle function. Any imbalance in their levels can lead to disorders like osteoporosis, hypocalcemia, and rickets.