Meta-Biol

• Pathways

Vitamin D metabolites such as VD3 are lipophilic and must be transported in the circulation bound to plasma proteins. Vitamin D3 is transported to the liver bound to a plasma protein called vitamin D binding protein (GC aka DBP) (Verboven et al. 2002). GC is a 58 kDa circulating glycoprotein that transports vitamin D metabolites. The vast majority of vitamin D metabolites circulate bound to GC (85–90%), some bound to albumin (10–15%), with the remainder (<1%) circulating in the free form. GC has more than 1000-fold stronger binding affinity for vitamin D metabolites than albumin. Thus, the albumin-bound and free fractions of vitamin D metabolites are considered bioavailable (Denburg et al. 2016).