Figure - PMC

Skip to main content

An official website of the United States government

Here's how you know

Here's how you know

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

View full-text article in PMC

. 2020 Sep 26;11:2040622320924159. doi: 10.1177/2040622320924159

Search in PMC
Search in PubMed
View in NLM Catalog
Add to search

© The Author(s), 2020

This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).

PMC Copyright notice

Figure 1. — A simplified schematic representation of the synthesis and metabolism of vitamin D.

7-dehydrocholesterol in the skin is converted to previtamin D₃ by UV-B and then is thermally isomerized to vitamin D₃. Transport of vitamin D₃ from the skin to the liver is mediated by vitamin D binding protein (DBP) where vitamin D₃ is hydroxylated at position 25 to 25(OH)D₃, DBP then transports 25(OH)D₃ to the kidney. 25(OH)D₃/DBP is filtered by the glomeruli and 25(OH)D₃ is taken up into the tubular cells, following DBP binding to megalin, a transmembrane protein. 25(OH)D₃ undergoes a second hydroxylation step by the 1-alpha-hydroxylase Cyp27B1, converting to the active 1α,25 (OH)₂D₃, while 24 hydroxylase Cyp27A1 converts to 24,25(OH)₂D_3. Keratinocytes contribute to the 3-epimerase activity, but the exact sites of activity remain unknown. 3epi25(OH)D₃ is converted by Cyp27A1 to 24,25(OH)D₃ and Cyp27B1 to 1,25(OH)D₃, in equal measure. Ergosterol (provitamin D₂) is plant-based and is converted to ergocalciferol by UV-B light that then follows the same pathway as vitamin D₃ that is depicted with the dotted line * to the liver to form 25(OH)D₂ and then transported to the kidney and epimerized to the D₂ metabolites.