Reaction of calcidiol to calcitrol, catalyzed by <a href="https://en.wikipedia.org/wiki/25-Hydroxyvitamin_D3_1-alpha-hydroxylase" class="extiw" title="w:25-Hydroxyvitamin D3 1-alpha-hydroxylase">25-hydroxyvitamin D3 1-alpha-hydroxylase</a>.
<a href="//commons.wikimedia.org/wiki/User:Hbf878" title="User:Hbf878">Hbf878</a>
<span class="int-own-work" lang="en">Own work</span>