VitaminD (ergocalciferol) and vitaminD (cholecalciferol) share a similar mechanism of action as outlined above. Metabolites produced by vitamin D are named with an er- or ergo- prefix to differentiate them from the D-based counterparts (sometimes with a chole- prefix).
* Metabolites produced from vitaminD tend to bind less well to the vitamin D-binding protein.
* VitaminD can alternatively be hydroxylated to calcifediol by sterol 27-hydroxylase (CYP27A1), but vitaminD cannot.
* Ergocalciferol can be directly hydroxylated at position 24 by CYP27A1. This hydroxylation also leads to a greater degree of inactivation: the activity of calcitriol decreases to 60% of original after 24-hydroxylation, whereas ercalcitriol undergoes a 10-fold decrease in activity on conversion to ercalcitetrol.
It is disputed whether these differences lead to a measurable drop in efficacy (see ).