Researchtopic

Boneandcartilagemetabolism

Summary

VitaminDismainlysynthesizedintheskinfrom7-dehydrocholesterolandispartiallyfromdietaryandsupplementationorigin.Intheliver,VitaminDishydroxylatedoncarbon25 toproducetheintermediate25OHVitaminD.25OHVitaminDisfurthermetabolizedbeforeitcancarryoutthefunctionsofVitaminDonintestine,kidneys,boneandotherorgansandtissues.Thissubsequentreactiontakesplaceinthekidneysandinothertissues.Thus25OHVitaminDisfurtherhydroxylatedinthe1α-positiontoproduce1α,25-dihydroxyvitaminD(1,25(OH)2VitaminD).Inadditiontotheabove-mentionedtissues,placentaofpregnantwomenandmacrophagecellsincaseofsarcoidiscanalsoproducesomeamountof1,25(OH)2VitaminD.1,25(OH)2VitaminDistheactiveformofVitaminDwithregardtotheknownfunctionswhereas25OHVitaminDandVitaminDitselfcanbeexcludedasbeingphysiologicallyfunctional.1,25(OH)2VitaminDstimulatestheintestinalabsorptionofbothcalciumandphosphorus.Italsostimulatesboneresorptionandmineralizationtherebypreventingthedevelopmentofricketsandosteomalacia.1,25(OH)2VitaminDisalsobeactiveinothertissuesresponsIBLeforCalciumtransport(placenta,kidney,mammarygland,…)andendocrineglandssuchasparathyroidglands.1,25(OH)2VitaminDisrapidlymetabolizedanditshalflifeisapproximately12hinplasma.Itsmainmetaboliteiscalcitroicacid,aC-23 carboxylicderivative,essentiallywithoutanyBIOLOGicalactivity.Inadditiontothispathway,1,25(OH)2VitaminDundergoes24-hydroxylationtoproduce1,24,25-trihydroxyvitaminD.Thiscompoundhaslessbiologicalactivitythanitsparentandthismetabolicrouteisconsideredasaminorpathway.Thelevelsof1,25(OH)2VitaminDinplasmaorserumis100 to1000 lessthanthatof25OHVitaminD.Duetoitslowconcentrationsandthepresenceofmanysimilarmetabolites,themeasurementof1,25(OH)2VitaminDrequiresextractionandseparationbychromatography.