[Can phosphatidyl sulfocholine, the sulfonium analog of lecithin, efficiently replace lecithin in natural membranes?]

Lecithin does not exist in the marine diatom, Nitzschia alba, being completely replaced by phosphatidylsulfocholine (PSC), the sulfonium analogue of phosphatidylcholine (PC). Thus, the question arises: how can PSC effectively replace lecithin in a natural membrane? We have compared the physical properties of a homologous series of synthetic PSC's (di - 14:0, di- 16:0, di- 18:0, di 18:1) in aqueous dispersion with those of a similar series of PC's. The PSC's formed liposomes having similar properties to those of the PC's. However, the saturated PSC homologues showed main transition temperatures 2-4 degrees C above those of the corresponding PC's as measured by differential scanning calorimetry, fluorescence polarization or electron paramagnetic resonance. Furthermore, there was no significant difference between the two types of membranes with respect to their permeability to urea or 6-carboxyfluorescein either in the presence or absence of cholesterol. We have also shown that yeast cells can grow and survive in spite of a complete replacement of PC by PSC in their membranes. The sulfonium analogue of lecithin would appear to be able to replace PC in natural membranes because of the similarity in their physical properties.