Investigating the role of esterified ergosterol in the formation of vitamin D during the UV irradiation of standards and fungi

• Applicant: Professor Dr. Walter Vetter
• Subject Area: Food Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 551381793

Calciferols (vitamin D forms) are low-abundant food compounds that can be formed from precursor sterols (so-called provitamins) by irradiation with UV-B light. The structural prerequisite for this conversion is the presence of a sterol with 5,7-conjugated double bonds. Apart from animals, calciferols are rarely found in other forms of life. Specifically, fungi are the only relevant non-animal vitamin D source due to the presence of the 5,7-sterol ergosterol, shares of which are converted into ergocalciferol (vitamin D2) by exposure to UV-B light. The amounts of ergocalciferol in fungi are rather low but they can be artificially increased through UV-B irradiation. However, the resulting ergocalciferol levels did not correlate with the ergosterol levels. Current explanations in the literature were unsatisfactory and not scientifically sound. Previously, it was shown that fungi not only contain free ergosterols but also ergosterol esterified with a fatty acid (~3-6% ergosteryl esters). Initial UV-B irradiation experiments in our laboratory with one synthesized ergosteryl ester standard indicated a much faster and higher conversion into esterified ergocalciferol compared to free ergosterol. This produced evidence that ergosteryl esters may play a significant role in the formation of ergocalciferol in fungi. This project will therefore investigate the role of ergosteryl esters in the formation of ergocalciferol. For this purpose, purposefully synthesized standards and (thereafter) fungi samples will be UV-B irradiated under standardized conditions and the formation of ergocalciferol and related intermediate compounds will be thoroughly investigated by HPLC and GC/MS methods. Prior to the instrumental analysis free and esterified forms of ergosterol, ergocalciferol, and further isomeric intermediates will be separated be elution into different fractions. Also, abundant precursor sterols will be separated from orders of magnitude less abundant calciferols by SPE. The measurements will be accompanied by digestion model reactions to find indications for the bioavailability of esterified ergocalciferol.

• DFG Programme: Research Grants