Metabolism of Lumisterol₂ by CYP27A1

Lumisterol₂ (L2) is a photoproduct of UVB action on the fungal membrane sterol, ergosterol. Like vitamin D₂, it is present in edible mushrooms, especially after UV irradiation. Lumisterol₃ is similarly produced in human skin from 7-dehydrocholesterol by UVB and can be converted to hydroxy-metabolites by CYP27A1 and CYP11A1. These products are biologically active on human cells with actions that include photoprotection and inhibition of proliferation. The aim of this study was to test the ability of CYP11A1 and CYP27A1 to metabolise L2. Purified CYP27A1 was found to efficiently metabolise L2 to three major products and several minor products, whilst CYP11A1 did not act appreciably on L2. The three major products of CYP27A1 action on L2 were identified by mass spectrometry and NMR as 24-hydroxyL2, 27-hydroxyL2 and 28-hydroxyL2. Minor products included two dihydroxy L2 species, one which was identified as 24,27(OH)₂L2, and another metabolite with one oxo and one hydroxyl group added. A comparison on the kinetics of the metabolism of L2 by CYP27A1 with that of the structurally similar compounds, L3 and ergosterol, was carried out with substrates incorporated into phospholipid vesicles. CYP27A1 displayed a 12-fold lower K_m with L2 as substrate compared to L3 and a 5-fold lower turnover number (k_cat), resulting in a 2.2 fold higher catalytic efficiency (k_cat/K_m) for L2 metabolism. L2 was a much better substrate for CYP27A1 than its precursor, ergosterol, with a catalytic efficiency 18-fold higher. The major CYP27A1-derived hydroxy-L2 products, 24-hydroxyL2, 27-hydroxyL2 and 28-hydroxyL2, inhibited the proliferation of melanoma and epidermoid cancer cell lines. In conclusion, this study shows that L2 is not metabolized appreciably by CYP11A1, but it is a good substrate for CYP27A1 which hydroxylates its side chain to produce 3 major products that display anti-proliferative activity on skin-cancer cell lines.