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→Active Vitamin D Synthesis
Vitamin D can also be converted from 7-dehydrocholesterol by ultraviolet radiation at wavelength 300nm in the [[Skin - Anatomy & Physiology|skin]]. This is not a major source in animals as the majority of the skin is covered by hair.
Vitamin D can also be converted from 7-dehydrocholesterol by ultraviolet radiation at wavelength 300nm in the [[Skin - Anatomy & Physiology|skin]]. This is not a major source in animals as the majority of the skin is covered by hair.
====Active Vitamin D Synthesis====
====Activated Vitamin D Synthesis====
Vitamin D2 or D3 from dietary sources or skin is transported in the blood bound to ''Vitamin D Binding Globulin''. In the [[Liver - Anatomy & Physiology|liver]] it is converted to '''25-hydroxycholecalciferol''' - 25(OH)Vitamin D3. This is then stored in adipose tissue. Activation requires ''hydroxylation'' with the enzyme '1-a-Hydroxylase' which converts 25-hydroxycholecalciferol into '''1,25-dihydroxycholecalciferol''' = ACTIVE VITAMIN D3.
Regulation of this process occurs within the kidney, regulated by PTH concentration. An antagonistic enzyme to 1-a-hydroxylase is 24-hydroxylase which creates an inactive form of vitamin D3.
PTH increases the activity of 1-a-hydroxylase to increase the amount of active Vitamin D3. This molecule in itself is also responsive to the concentration of PTH.
PTH increases the activity of 1-a-hydroxylase to increase the amount of active Vitamin D3. This molecule in itself is then responsive to the concentration of PTH too.
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