A 68-year-old woman with a history of stage 4 chronic kidney disease presents to the clinic for a routine follow-up. Laboratory evaluation reveals a serum calcium level of 7.8 mg/dL and an elevated intact parathyroid hormone level. The physician notes that her hypocalcemia is primarily driven by the loss of functional renal parenchyma, which impairs a critical step in a major endocrine pathway. To review the pathophysiology of her condition, arrange the following biochemical and cellular events involved in the synthesis and mechanism of action of the deficient hormone in the correct chronological sequence, beginning with the initial precursor event.
- 1Photochemical cleavage of the B-ring of 7-dehydrocholesterol
- 2Cytochrome P450-mediated 25-hydroxylation in hepatocytes
- 3Parathyroid hormone-stimulated 1-alpha-hydroxylation
- 4Heterodimerization of the bound ligand-receptor complex with the retinoid X receptor
- 5Upregulation of calbindin-D9k transcription in enterocytes
Cevap
The correct sequence begins with the photochemical cleavage of 7-dehydrocholesterol in the skin, followed by hepatic 25-hydroxylation, then renal 1-alpha-hydroxylation, followed by intracellular heterodimerization with the retinoid X receptor, and finally the upregulation of calbindin-D9k transcription in enterocytes.
The correct sequence accurately reflects the biological pathway of vitamin D. It begins in the skin with the UV-mediated formation of cholecalciferol from 7-dehydrocholesterol. This is followed by hepatic 25-hydroxylation to calcidiol. The rate-limiting and strictly regulated step, 1-alpha-hydroxylation, occurs in the renal proximal tubule to form active calcitriol. Once secreted into the circulation, calcitriol enters target cells (such as enterocytes) to bind the intracellular Vitamin D Receptor (VDR). The VDR-ligand complex heterodimerizes with the Retinoid X Receptor (RXR), binds to DNA response elements, and upregulates the transcription of calcium-binding proteins such as calbindin, which are necessary for the intestinal absorption of calcium.
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Anahtar Kavram
Vitamin D synthesis and intracellular mechanism of action