S-OXIDATIVE CLEAVAGE OF FARNESYLCYSTEINE AND FARNESYLCYSTEINE METHYL-ESTER BY THE FLAVIN-CONTAINING MONOOXYGENASE

Posttranslational modification of proteins with a farnesyl or geranylgeranyl group appears to be crucial in the signal transduction of eukaryotic cells. For example, farnesylation of ras-encoded proteins is a key process that apparently leads to membrane association of proteins that perform a function in cell growth-promoting activity. Although it has been suggested that prenylation of proteins may be an important regulatory mechanism, little is known about the mechanism whereby prenylated proteins are removed from the membrane. In our previous report [(1992) Chem. Res. Toxicol. 5, 193-201], we showed that S-alkenylated cysteines and mercapturates of xenobiotics were S-oxygenated by the flavin-containing monooxygenase. The S-oxides were not indefinitely stable and rearranged or underwent elimination reactions that cleaved the C-S(O) bond. As a model for farnesylated proteins and peptides, the biotransformation of farnesylcysteine methyl ester was examined in the presence of pig liver microsomes. Two prominent products were formed: farnesyl methyl ester sulfoxide and farnesylcysteine, arising from the action of the flavin-containing monooxygenase and esterase of pig liver, respectively. Formation of farnesylcysteine methyl ester sulfoxide by the flavin-containing monooxygenase was stereoselective (i.e., 71.5%:28.5%, major to minor diastereomer) in good agreement with previously reported stereoselectivity studies of other related S-alkylcysteine-containing compounds. That the stereoselectivity observed was due to S-oxygenation of the sulfur atom was verified in parallel chemical oxidation studies by using micellar electrokinetic capillary chromatography. Farnesylcysteine methyl ester was an excellent substrate for the flavin-containing monooxygenase, and the S-oxide product was confirmed by HPLC electrospray mass spectrometry. Also, farnesylcysteine was efficiently S-oxygenated in the presence of pig liver microsomes in a flavin-containing monooxygenase-dependent process. Farnesylcysteine methyl ester S-oxide was not indefinitely stable and decomposed presumably via an elimination reaction to produce carbon-sulfur bond cleavage products. It is possible that S-oxidative cleavage of farnesylcysteine methyl ester derivatives may have important implications for predicting regulation and homeostasis of prenylated proteins.