Figure 1.

Noncanonical pathways of vitamin D3 and lumisterol (L3) activation. D3, L3 and 7DHC are substrates for CYP11A1 activity that by itself or in cooperation with other CYPs enzymes produces the corresponding hydroxyderivatives. In the case of L3 and 7HDC, the side chain can be cleaved by CYP11A1 to produce 7DHP or pL that can be further metabolized by steroidogenic enzymes (ES). In the skin UVB acting on 5,7-dienes can lead to production of D3, L3 and T3 derivatives with a full-length side chain, and pD, pL and pT derivatives with a shortened side chain. While the cut-off for UVC/UVB is 280 nm, we show the range 290–315 nm because wavelengths below 290 nm are filtered by the ozone layer and no additional pre-D3 is produced above 315 nm ¹⁴³.