BCII: Vitamin D

Which enzyme is the rate-regulatory enzyme in vitamin D biosynthesis?

1-α-hydroxylase

What is the RDA value for vitamin D?

15 mg

What is the precursor for vitamin D obtained from animal sources?

7-dehydrocholesterol

What are the key functions of vitamin D?

Central role in Ca²⁺ and phosphate metabolism

What is the provitamin form of vitamin D obtained from plant sources?

Ergosterol

What diseases are involved in Vitamin D deficiency conditions?

In children: Rickets Deformed legs, enlarged head, enlarged joints, beaded ribs, deformed pelvis In adult: Osteomalacia

What is the effect of decreased plasma calcium levels on vitamin D biosynthesis?

It activates 1-α-hydroxylase

What is the effect of decreased plasma phosphate levels on vitamin D biosynthesis?

It activates 1-α-hydroxylase

How deficiency of parathyroid hormone can impact vitamin D biosynthesis?

It will decrease the biosynthesis of Vitamin D

Which tissues are involved in the biosynthesis of vitamin D?

Liver and Kidneys

What is hypocalcemia?

Low blood levels of calcium

Which hormone activates the rate-limiting enzyme for vitamin D biosynthesis?

Parathyroid Hormone

Biosynthesis of which vitamin needs exposure of sunlight upon the skin?

Vitamin D

Which dose of vitamin D can produce toxicity?

Vitamin D in large doses is toxic, 100 mg/day or greater for months leading to hyperkalemia and stoppage of the heart.

How is vitamin D biosynthesized in humans from its precursors?

• 7-dehydrocholesterol is present in skin • UV light on skin opens up ring to form vitamin D, cholecalciferol • Two hydroxyl groups are added to form 1,25 dihydroxycholcalciferol (vitamin • D3 or calcitriol). This is the active form of vitamin D. • Both the liver and kidneys are involved in calcitriol synthesis

What is the effect of calcitonin on calcitriol biosynthesis?

Calcitonin hormone inactivates calcitriol production by kidney

What is the effect of heating and general food processing on the activity of vitamin D?

Heating & general food processing does not destroy or reduce vitamin D levels in foods

What can a deficiency in Vitamin D cause?

Hypocalcemia: Low blood levels of calcium Hypophosphatemia: Low blood levels of phosphate Defective skeletal mineralization as follows lead to secretion of parathyroid hormone which attempts to restore the normal conditions: • Calcium & phosphate malabsorption • Decrease in intestinal Ca absorption • Decrease in blood Ca levels

Where is 7-dehydrocholesterol deposited?

In the skin

How does higher amounts of skin pigmentation caused by melanin impact vitamin D biosynthesis?

Higher amounts of skin pigmentation by melanin decreases vitamin D3 synthesis Melanin competes with 7-DHC in the skin for the photons (from UV light)

Compare Vitamin D to a steroid.

Vitamin D is a SECOSTEROID. Notice how similar it is to a steriod. It is a broken structure as seen in section "B".

How is the biosynthesis of vitamin D regulated?

1-α-hydroxylase is the rate-limiting enzyme. Its affects are: • ↑ Parathyroid hormone secretion • ↓ in Ca²⁺ levels • ↓ in phosphate levels

What is the biologically active form of vitamin D in humans?

Calcitriol

What are the major sources of dietary vitamin D?

Meat Fish Milk