24-Methylenecyclopropane steroidal inhibitors: A Trojan horse in ergosterol biosynthesis that prevents growth of Trypanosoma brucei.

A new class of steroidal therapeutics based on phylogenetic-guided design of covalent inhibitors that target parasite-specific enzymes of ergosterol biosynthesis is shown to prevent growth of the protozoan-Trypanosoma brucei, responsible for sleeping sickness. In the presence of approximately 15±5µM 26,27-dehydrolanosterol, T. brucei procyclic or blood stream form growth is inhibited by 50%. This compound is actively converted by the parasite to an acceptable substrate of sterol C24-methyl transferase (SMT) that upon position-specific side chain methylation at C26 inactivates the enzyme. Treated cells show dose-dependent depletion of ergosterol and other 24ß-methyl sterols with no accumulation of intermediates in contradistinction to profiles typical of tight binding inhibitor treatments to azoles showing loss of ergosterol accompanied by accumulation of toxic 14-methyl sterols. HEK cells accumulate 26,27-dehydrolanosterol without effect on cholesterol biosynthesis. During exposure of cloned TbSMT to 26,27-dehydrozymosterol, the enzyme is gradually inactivated (kcat/kinact=0.13min-1/0.08min-1; partition ratio of 1.6) while 26,27-dehydrolanosterol binds nonproductively. GC-MS analysis of the turnover product and bound intermediate released as a C26-methylated diol (C3-OH and C24-OH) confirmed substrate recognition and covalent binding to TbSMT. This study has potential implications for design of a novel class of chemotherapeutic leads functioning as mechanism-based inhibitors of ergosterol biosynthesis to treat neglected tropical diseases.

Phytosterol biosynthesis pathway in Mortierella alpina.

The Zygomycetes fungus Mortierella alpina was cultured to growth arrest to assess the phytosterol biosynthesis pathway in a less-advanced fungus. The mycelium was found to produce 13 sterols, but no ergosterol. The sterol fractions were purified to homogeneity by HPLC and their identifies determined by a combination of GC-MS and 1H NMR spectroscopy. The principal sterol of the mycelium was cholesta-5, 24-dienol (desmosterol) (83%), with lesser amounts of 24beta-methyl-cholesta-5,25(27)-dienol (codisterol) (2%), 24-methyldesmosterol (6%), 24(28)-methylene cholesterol (3%) and lanosterol (3%) and several other minor compounds (3%). The total sterol accounted for approximately 0.07% of the mycelial dry wt. Mycelium fed methionine-methyl-2H3 for 6 days, generated 3 2H-24-methyl(ene) sterols, [C28-2H2]24(28)-methylenecholesterol, [C28-2H3]24-methylcholesta-5,24-dienol and [C28-2H3]24beta-methyl-cholesta-5,25(27)-dienol. The formation of the 24-methyl sterols seems to be catalyzed by the direct methylation of a common Delta24-acceptor sterol thereby bypassing the intermediacy of an isomerization step for rearrangement of the Delta24(28)-bond to Delta25(25)-position as operates in Ascomycetes fungi and all plants.