Growth support and metabolism of phytosterols inParamecium tetraurelia

The basis of the growth requirement ofParamecium for one of several structurally similar phytosterols is not known. Previous research has indicated that selective esterification of only growth-promoting sterols may be a key. In this study, it was found that under certain conditions sterols that fail to support growth (e.g., cholesterol) can be esterified in large amounts inParamecium. We found no compelling evidence to support the hypothesis that steryl esters serve a specialized role in the fatty acid metabolism of the cell. Octadecenoic acid, essential for cell growth, was the major fatty acid in both steryl esters and triglycerides. It was also shown thatP. tetraurelia can dehydrogenate Δ⁰ and Δ⁷, as well as Δ⁵-3β-hydroxy sterols, to yield the conjugated 5,7-diene derivative. These results indicate the presence of a Δ⁵, in addition to a Δ⁷, desaturase of the sterol nucleus in this ciliate. Two C₂₄ α-ethyl sterols, Δ²²-stigmasterol (Δ²²) and stigmastanol (Δ⁰), were shown for the first time to promote growth. Finally, we found that non-growth-promoting sterols may compose a high percentage of the free sterols of the surface membrane without adversely affecting cell growth or viability. These data support the conclusion that the growth requirement for select phytosterols inParamecium does not involve the structural or functional role of “bulk” sterols in cell membranes.