Correct Answer: 3. 1-α-25-dihydroxycholecalciferol

This is the active hormonal form of vitamin D, known as calcitriol, which directly regulates calcium levels in the kidney and bones.

Option Analysis

• 1. 7-dehydrocholesterol: Incorrect. This is the skin precursor converted to cholecalciferol by UV light, not biologically active.

• 2. 24,25-dihydroxycholecalciferol: Incorrect. This is an inactive metabolite produced by 24-hydroxylase for vitamin D degradation, lacking calcium-regulating activity.

• 3. 1-α-25-dihydroxycholecalciferol: Correct. Also called 1,25(OH)₂D or calcitriol, it binds VDR to boost intestinal calcium absorption, renal reabsorption, and bone mineralization.

• 4. Cholecalciferol: Incorrect. This is vitamin D₃, the inactive form from skin or diet, requiring 25- and 1α-hydroxylation to activate.

The biologically active form of vitamin D that regulates calcium levels in the kidney and bones is 1α-25-dihydroxycholecalciferol, essential for students preparing for molecular biology and plant physiology exams.

Vitamin D Metabolism Pathway

Vitamin D starts as cholecalciferol (D₃), hydroxylated in liver to 25(OH)D, then in kidney to active 1,25(OH)₂D by CYP27B1 enzyme under PTH control. Low calcium triggers PTH to activate this final step, while high levels induce CYP24A1 for inactivation.

Calcium Regulation Mechanism

Calcitriol upregulates TRPV6 channels, calbindin, and PMCA1b in intestines for calcium absorption; enhances renal reabsorption; mobilizes bone calcium via osteoclasts. It maintains serum calcium at 8.5-10.5 mg/dL, preventing rickets or osteomalacia.

Biological Relevance

In kidneys, it promotes calcium reabsorption and phosphate handling; in bones, supports mineralization vital for plant science analogs like calcium signaling in physiology. Exam questions test this pathway for biotechnology applications.

Exam Tips

Focus on 1α-hydroxylation as the activation step; distinguish from 24,25(OH)₂D (inactive). Common in GATE Life Sciences biochemistry sections.