Characterization of a novel, potent, and specific inhibitor of serine palmitoyltransferase.

We have examined the mechanism of action of two natural products identified as broad spectrum antifungal agents (VanMiddlesworth, F., Dufresne, C., Wincott, F. E., Mosley, R. T., and Wilson, K. E. (1992) Tetrahedron Lett., in press; VanMiddlesworth, F., Giacobbe, R. A., Lopez, M. Garrity, G., Bland, J. A., Bartizal, K., Fromtling, R. A., Polishook, J., Zweerink, M. M., Edison, A. M., Rozdilsky, W., Wilson, K. E., and Monaghan, R. L. (1992) J. Antibiot. (Tokyo) 45, 861-867), designated sphingofungin B (2S-amino-3R,4R,5S,14-tetrahydroxyeicos-6-enoic acid) and sphingofungin C (2S-amino-5S-acetoxy-3R,4R,14-trihydroxyeicos-6-enoic acid), and find they are potent specific inhibitors of serine palmitoyltransferase, which catalyze the committed step of sphingolipid biosynthesis. We used Saccharomyces cerevisiae as a model to investigate the mechanism of the antifungal activity of these compounds. Macromolecular synthesis was not immediately affected by either sphingofungin B or C, synthesis continued for 60-90 min following the addition of drug to growing cultures. Significant loss of viability with sphingofungins required growing cultures and began only after several hours, with greater than 99.9% of drug-treated cells non-viable after 24 h. No lysis or other gross changes in cell morphology were observed in drug-treated cells. The structural similarity of sphingofungin B and C to sphingosine and phytosphingosine prompted us to investigate their effects on sphingolipid synthesis. Nanomolar levels of the drugs inhibited the incorporation of [3H]inositol into sphingolipid before incorporation into the sphingolipid precursor, phosphatidylinositol was affected, suggesting specific inhibition of sphingolipid synthesis. This hypothesis was confirmed by experiments in which the growth inhibitory activity of both drugs was completely ablated by the addition of phytosphingosine, dihydrosphingosine, or ketodihydrosphingosine to the culture medium. Reversal of antifungal activity by ketodihydrosphingosine suggested that serine palmitoyltransferase could be the actual target of these compounds. Direct evidence for this hypothesis was the observation of inhibition of serine palmitoyltransferase activity in crude membrane preparations at nanomolar concentrations of each drug. The potent inhibition of serine palmitoyltransferase coupled with the apparent lack of effect of these compounds on other cellular functions suggests that sphingofungin B and C will prove to be important new tools for studying the role of sphingolipids in yeast and perhaps in other organisms.