Steroids are not only relegated to the animal world; fungi and plants synthesize many steroids as well. One particular example of clinical relevance is ergosterol, found in the cell membranes of fungi where it serves a similar role to cholesterol in animal cell membranes. This can be exploited by antifungal medications: azole drugs such as clotrimazole and miconazole function by inhibiting ergosterol synthesis. Specifically, they inhibit the 14α-demethylase enzyme that converts lanosterol to ergosterol (note the similarities to the cholesterol pathway discussed in this previous post). [1]
Ergosterol can also be converted to ergocalciferol in a UV-light dependent reaction, similarly to the synthesis of Vitamin D3 in animals. In fact, ergocalciferol is also known as Vitamin D2, and like cholecalciferol, ergocalciferol can be hydroxylated twice to 1,25-dihydroxyergocalciferol or ercalcitriol, which binds to the Vitamin D receptor and causes its effects, although the binding of Vitamin D2 may not be as strong. [2]
There are diverse steroids made by plants, some of which have toxic effects. Of note are digoxin and digitoxin produced by the foxglove plant. These two chemicals consisted of a carbohydrate chain attached to a modified steroid, and they can be fatal if ingested. They inhibit the Na+/K+ ATPase responsible for establishing the electrochemical gradient within the cell, which is exploited for the use of digoxin as a drug for arrhythmias and heart failure due to the ability of the medication to increase the contractility of the heart when given at low doses. [3]
These are only some of the steroids occurring in plants and fungi. In the future, maybe more will be discovered with important biological activities!
Sources:
[1] Herrick, E. J., & Hashmi, M. F. (2021). Antifungal Ergosterol Synthesis Inhibitors. In StatPearls. StatPearls Publishing. http://www.ncbi.nlm.nih.gov/books/NBK551581/
[2] Houghton, L. A., & Vieth, R. (2006). The case against ergocalciferol (vitamin D2) as a vitamin supplement. The American Journal of Clinical Nutrition, 84(4), 694–697. https://doi.org/10.1093/ajcn/84.4.694
[3] Hauptman, P. J., & Kelly, R. A. (1999). Digitalis. Circulation, 99(9), 1265–1270. https://doi.org/10.1161/01.cir.99.9.1265