VITAMIN D SYNTHESIS
doc. Ing. Zenóbia Chavková, CSc.
Vitamin D □ Is not a truly a vitamin as it is derived from a steroid that can be synthesized from cholesterol Vitamin D : Is the precursor of a hormone called calcitriol
Calcitriol Is a compound with 3 –OH groups, a triol, that influences Ca metabolism
A hormone is a substance that is Produced and released by synthetic cells Carried by the circulation to target cells where the hormone produces its physiological effects
7-dehydrocholesterol ◊ An endogenous human metabolite, a precursor of calcitriol ◊ A steroid, that contain 4 rings, designated as A, B, C a D
7-dehydrocholesterol Undergoes a photolytic isomeration reaction, that opens the B ring to form cholekalciferol, vitamin D3 (skin) Cholekalciferol
Reacts with O2 and NADPH 25-hydroxycholekalciferol, also 25-hydroxyvitamin D3 (liver) 25-hydroxycholekalciferol The predominant form of vitamin D in circulation and liver stores, undergoes a second hydroxylation at C1 to form calcitriol, the active compound (kidney)
25-hydroxycholekalciferol
Can also undergo hydroxylation at the C-24 position to form an inactive compound
The ratio of the conversion to the active and inactive compounds is physiologically regulated
□ When individuals are exposed to adequate light, there is a little or no dietary requirement for vitamin D □ Dairy products are fortified with vitamin D and deficiency is rare □ Most vitamin D deficiency syndroms result from: ▪ Fat malabsorption ▪ Severe liver or kidney disease ☻ Humans without kidneys cannot form adequate calcitriol. 2+ Serum Ca levels in these individuals can be regulated by hemodialysis
Calcitriol Regulates calcium and phosphate metabolismus Deficiency results in children in rickets characterized by the formation of osteoid matrix, that is improperly mineralized, resulting in soft, pliable bones Vitamin D deficiency in adults Osteomalacia
osteomalacia soft, fragile bones
osteoporosis!!!
Osteoporosis = lack of osteoids, common in the erderly, especially women Osteomalacia results from vitamin D deficiency 1, 25 (OH)2D3 is about 100 x stronger in effect then 25 (OH)D3 is !!!
