Which of the following is not true regarding Vitamin D? (AIIMS MAY 2008)
|A||1 hydroxylation occurs in kidney.|
|B||25-hydroxylation takes place in the liver.|
|C||In absence of sunlight 200-400 IU of vitamin D is the daily required.|
|D||William’s syndrome is characterized by precocious puberty, obesity and mental retardation.|
I. The active transport of Ca2+ and PO43– from the intestine is increased by a metabolite of vitamin D.
II. The term "vitamin D" is used to refer to a group of closely related sterols produced by the action of ultraviolet light on certain provitamins.
III. Vitamin D3, which is also called cholecalciferol, is produced in the skin of mammals from 7-dehydrocholesterol by the action of sunlight.
IV. The reaction involves the rapid formation of previtamin D3, which is then converted more slowly to vitamin D3. Vitamin D3 and its hydroxylated derivatives are transported in the plasma bound to a globulin vitamin D-binding protein (DBP). Vitamin D3 is also ingested in the diet.
V. Vitamin D3 is metabolized by enzymes that are members of the cytochrome P450 (CYP) superfamily. In the liver, vitamin D3 is converted to 25-hydroxycholecalciferol (calcidiol, 25-OHD3). The 25-hydroxycholecalciferol is converted in the cells of the proximal tubules of the kidneys to the more active metabolite 1,25-dihydroxycholecalciferol,which is also called calcitriol or 1,25-(OH)2D3. 1,25 Dihydroxycholecalciferol is also made in the placenta, in keratinocytes in the skin, and in macrophages.
VI. In patients with sarcoidosis, pulmonary alveolar macrophages also produce 1,25-dihydroxycholecalciferol, apparently upon stimulation by -interferon.
VII. The normal plasma level of 25-hydroxycholecalciferol is about 30 ng/mL, and that of 1,25-dihydroxycholecalciferol is about 0.03 ng/mL (approximately 100 pmol/L).
VIII. The less active metabolite 24,25-dihydroxycholecalciferol is also formed in the kidneys.
IX. Williams syndrome is a rare genetic disorder, occurring in about 1 in 7,500 live births.
It is characterized by a distinctive, "elfin" facies, along with a low nasal bridge; an unusually cheerful demeanor and ease with strangers, coupled with unpredictably occurring negative outbursts; mental retardation coupled with unusual language skills; a love for music; and cardiovascular problems, such as supravalvular aortic stenosis and transient hypercalcinemia.