Sterol C24-methylation activity in Acanthamoeba. (A) Proposed sterol C24-methylation pathway in Acanthamoeba showing incorporation of SAM and [methyl-2H3]SAM = D3-SAM at C24 and C28 of the sterol side chain. (B) Mass spectral analysis of sterols (ergosterol, erg, poriferasterol, por, and 7-dhydroporiferasterol, DHP) recovered from trophozoites incubated in the presence and absence of [methyl-2H3]methionine.