Liver prenylated methylated protein methyl esterase is an organophosphate‐sensitive enzyme

Prenylation and subsequent methylation are essential modifications on a significant proportion of eucaryotic proteins. Proteins such as the G‐γ subunits of G‐protein coupled receptors, nuclear lamins, and guanine nucleotide‐binding proteins such as Ras are prenylated and undergo methylation. Prenylated methylated protein methyl esterase (PMPMEase) readily hydrolyses the prenylated protein methyl esters, thus making this step reversible and possibly regulatory. Benzoyl‐glycyl‐farnesyl‐cysteine methyl ester (BzGFCM) was developed as a specific PMPMEase substrate and characterized by electron spray ionization mass spectrometry (ESI‐MS) to be of the calculated molecular mass. Rat liver and brain PMPMEase hydrolyzed BzGFCM, forming benzoyl‐glycyl‐farnesyl‐cysteine (BzGFC) in a time‐ and concentration‐dependent manner. Both enzymes cleaved BzGFCM with K m values of 4.58 ± 0.30 and 25.57 ± 2.36 μM and V max values of 2.21 ± 0.03 and 0.17 ± 0.003 nmol/min/mg, respectively. The liver enzyme eluted from a gel‐filtration column as a single peak of apparent size, 89 kDa. The brain enzyme eluted as two main peaks of 53 and 890 kDa. Organophosphorus pesticides (OPs), which are suspected to be involved in human disorders such as parkinsonism, neuronal, and retinal degeneration, inhibited the liver enzyme with IC 50 values from 4.77 μM for parathion to 0.04 μM for paraoxon, respectively. Only about 25% of the brain enzyme was inhibited by 0.5–1 mM solutions of mipafox, while 0.1 and 1 mM paraoxon inhibited over 50% and 95% of the enzyme, respectively. Paraoxon is thus about 2250 times less potent against the brain than the liver PMPMEase. BzGFCM was not hydrolyzed by various cholinesterases, indicating its specificity for PMPMEase. Perturbations in prenylated protein metabolism might play a role in noncholinergic OPs‐induced toxicity, since prenylated proteins play such important roles in cell signaling, proliferation, differentiation, and apoptosis. © 2005 Wiley Periodicals, Inc. J Biochem Mol Toxicol 19:347–357, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20100