The sterols and sterol precursors of two amoebae of the genus Naegleria, Naegleria lovaniensis and Naegleria gruberi were investigated. Cycloartenol, the sterol precursor in photosynthetic organisms, is present in both amoebae. In N. lovaniensis, it is accompanied by lanosterol and parkeol, as well as by the 24, 25-dihydro derivatives of these triterpenes. One of the most striking features of these amoebae is the accumulation of 4α-Methylsterols which are present in similar amounts as those of 4,4-desmethylsterols (3–5 mg/g, dry weight). 4α-Methylergosta-7,22-dienol was identified as a new compound. Ergosterol was the major 4,4-desmethylsterol, accompanied by small amounts of C27 and other C28 sterols.

Treatment of N. lovaniensis with fenpropimorph modified the sterol pattern of this amoeba and inhibited its growth. This fungicide, known to inhibit steps of sterol biosynthesis in fungi and plants, induced the disappearance of 4α-methyl-Δ7-sterols and the appearance of the unusual Δ6,8,22-ergostatrienol as in A. polyphaga. These results might be explained by a partial inhibition of the Δ8[RIGHTWARDS ARROW]Δ7 isomerase, the small amounts of Δ7-sterols formed being converted into ergosterol which is still present in fenpropimorph-exposed cells.

De novo sterol biosynthesis in N. lovaniensis was shown by incorporation of [1-14C]acetate into sterols and sterol precursors, especially cycloartenol. Lanosterol and parkeol were not significantly labelled. Furthermore, [3-3H]squalene epoxide was efficiently cyclized by a cell-free system of this amoeba into cycloartenol, and again no significant radioactivity was detected in lanosterol and parkeol. This shows that cycloartenol, the sterol precursor in plants and algae, is also the sterol precursor in Naegleria species, and that these amoebae, like A. polyphaga, are related by some biosynthetic pathways to photosynthetic phyla. Lanosterol, the sterol precursor in non-photosynthetic phyla (animal and fungi) and parkeol are more likely dead-ends of this biosynthetic pathway.

The peculiar phylogenetic position of these protozoa was further emphasized by the action of indole acetic acid and other auxine-like compounds on their growth. Indeed amoebic growth was enhanced in the presence of these higher plant growth hormones. The differences in the sterol composition of the protozoa we have hitherto examined is related to their sensitivity toward polyene macrolide antibiotics. The growth of A. polyphaga containing mainly the C29 stigmasta-5,7,22-trienol was not affected by nystatin, and only slightly by amphotericin. Both antibiotics inhibited however the growth of the ergosterol-containing amoeba N. lovaniensis and trypanosome Crithidia fasciculata.