Defective photoproduction of cholecalciferol in normal and uremic humans.
The initial step in cholecalciferol (vitamin D3) metabolism is the photo-conversion of 7-dehydrocholesterol to previtamin D3. This reaction occurs in the epidermis and requires ultraviolet light. We measured the circulating concentration of vitamin D (ergocalciferol and cholecalciferol), 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in 14 normal white, 9 normal black subjects, and in 9 white and 17 black hemodialysis patients. The mean plasma vitamin D in normal white subjects was greater than in normal black subjects, 4.01 +/- 1.02 ng/ml versus 0.96 +/- 0.30 ng/ml, respectively (P less than 0.05). Plasma 25-hydroxyvitamin D in normal blacks was also less than in normal whites, 17.7 +/- 1.5 ng/ml versus 31.3 +/- 3.0 ng/ml, respectively (P less than 0.01). In uremic white subjects, plasma vitamin D, 6.7 +/- 2.6 ng/ml, was similar to normal white subjects. However, vitamin D was not detectable in 12 of 17 uremic black subjects and was depressed in the remainder of the group. Following exposure to a single minimal erythema dose of ultraviolet-B irradiation, the maximal increase in plasma vitamin D was depressed in white dialysis patients as compared to healthy white subjects, 6.3 +/- 1.9 ng/ml versus 21.3 +/- 2.8 ng/ml, respectively (P less than 0.02). 7-Dehydrocholesterol content was similar in epidermis from site-matched skin of fresh cadavers and white hemodialysis patients, 131 +/- 23 ng/mg versus 124 +/- 14 ng/mg skin, respectively. It is concluded that chronic hemodialysis patients exhibit defective photoproduction of cholecalciferol, despite normal epidermal content of substrate, 7-dehydrocholesterol.
D000328:Adult; D002762:Cholecalciferol / Q000096:biosynthesis* / Q000097:blood; D003684:Dehydrocholesterols / Q000378:metabolism; D004817:Epidermis / Q000378:metabolism* / Q000528:radiation effects; D004872:Ergocalciferols / Q000097:blood; D006801:Humans; D008297:Male; D006435:Renal Dialysis; D014466:Ultraviolet Rays; D014511:Uremia / Q000378:metabolism*
D003684:Dehydrocholesterols; D004872:Ergocalciferols; D002762:Cholecalciferol; C016705:7-dehydrocholesterol