Polyisoprenylation Potentiates the Inhibition of Polyisoprenylated Methylated Protein Methyl Esterase and the Cell Degenerative Effects of Sulfonyl Fluorides

The polyisoprenylation pathway incorporates a reversible step that metabolizes polyisoprenylated methylated proteins from the ester to the carboxylate form. Polyisoprenylated protein methyl transferase (PPMTase) catalyses the esterification whereas polyisoprenylated methylated protein methyl esterase (PMPMEase) hydrolyzes them. Significant changes in the balance between the two enzymes may alter polyisoprenylated protein function possibly resulting in disease. Previous studies show that PMPMEase is the serine hydrolase, Sus scrofa carboxylesterase. Its susceptibility to the nonspecific serine hydrolase inhibitor, phenylmethylsulfonyl fluoride (PMSF) paved the way for its use as a prototypical compound to design and synthesize a series of putative high affinity specific inhibitors of PMPMEase. Pseudo first-order kinetics revealed an over 680-fold increase in kobs/[I] values from PMSF (6 M-1s-1), S-phenyl (L-50, 180 M-1s-1), S-benzyl (L-51, 350 M-1s-1), S-trans, trans-farnesyl (L-28, 2000 M-1s-1), to S-trans-geranylated (L-23, 4100 M-1s-1) 2-thioethanesulfonyl fluorides. C10 S-alkyl substitution revealed a kobs/[I] value (1800 M-1s-1) that was 298 times greater than that for PMSF. The compounds induced the degeneration of human neuroblastoma SH-SY5Y cells with EC50 values of 49, 130 and <1000 μM for L-28, L-23 and PMSF, respectively. The increased affinity with the polyisoprenyl derivatization is consistent with the observed substrate specificity and the reported hydrophobic nature of the active site. These results suggest that (1) PMPMEase is a key enzyme for polyisoprenylated protein metabolism, (2) regulation of its activity is essential for maintaining normal cell viability, (3) abnormal activities may be involved in degenerative diseases and cancers and (4) its specific inhibitors may be useful in combating cancers.