D Vitamini Kimyasal Yapısı ve Metabolizması
Özet
D vitamini, 17. yüzyılda endüstrileşmeyle artan rikets hastalığının tedavisinde güneş ışığı ve morina balığı yağının etkilerinin keşfedilmesiyle tanımlanmıştır. Vücutta endojen üretilebilmesi ve aktif forma dönüşmek için metabolik basamaklar gerektirmesi nedeniyle teknik olarak prohormonal bir yapıdadır. Kimyasal yapısı kolesterole oldukça benzer olup, steroid yapıda yağda çözünen bir hormondur. Temel kaynakları güneş ışığı (UVB), besinler ve takviyelerdir; ihtiyacın %90'ı deride fotokimyasal reaksiyonlarla sentezlenir. Deride enzimatik olmayan süreçlerle üretilen inaktif D vitamini ile besinsel yolla enterositlerden emilen D2 ve D3 formları, kanda D vitamini bağlayıcı protein (VDBP) ve albümin ile taşınır. Aktivasyon için ilk hidroksilasyon karaciğerde gerçekleşerek 25OHD (kalsidiyol) oluşturulur. İkinci hidroksilasyon ise böbrek proksimal tübüllerinde CYP27B1 enzimiyle yapılarak biyolojik olarak aktif form olan 1,25(OH)2D (kalsitriol) sentezlenir. Aktif D vitamini, çekirdekli hücrelerdeki nükleer vitamin D reseptörüne (VDR) bağlanarak transkripsiyonel regülasyon aracılığıyla genomik etkiler gösterir veya membran proteinleri üzerinden hızlı, genomik olmayan yanıtlar oluşturur. Katabolizmasından ise multikatalitik CYP24A1 enzimi sorumludur ve ürünler safra yoluyla atılır. Kemik homeostazının yanı sıra hücresel farklılaşma ve immün yanıtlarda geniş etki çerçevesine sahiptir.
Vitamin D was identified following the discovery of the therapeutic effects of sunlight and cod liver oil on rickets, which became endemic during the industrial revolution in the 17th century. Although technically classified as a prohormone due to endogenous synthesis and the metabolic stages required for its activation, it is a fat-soluble steroid hormone with a chemical structure highly similar to cholesterol. The primary sources are ultraviolet B radiation (UVB), dietary intake, and supplements, with approximately 90% produced in the skin through non-enzymatic photochemical reactions. Inactive vitamin D synthesized in the skin and dietary D2 and D3 forms absorbed via enterocytes are transported in the blood bound to vitamin D binding protein (VDBP) and albumin. The activation process involves a primary hydroxylation in the liver to generate 25OHD (calcidiol), followed by a secondary hydroxylation in the renal proximal tubules mediated by the CYP27B1 enzyme to produce the biologically active 1,25(OH)2D (calcitriol). Active vitamin D exerts genomic effects by binding to nuclear vitamin D receptors (VDR) in nucleated cells to regulate gene transcription, alongside rapid non-genomic responses via membrane proteins, while its catabolism is mediated by the multicatalytic CYP24A1 enzyme for biliary excretion.
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