S-Adenosylmethionine-dependent Protein Methylation in Mammalian Cytosol via Tyrphostin Modification by Catechol-O-methyltransferase

Abstract

It has previously been shown that incubation of mammalian cell cytosolic extracts with the protein kinase inhibitor tyrphostin A25 results in enhanced transfer of methyl groups from S-adenosyl-[methyl-3 H]methionine to proteins. These findings were interpreted as demonstrating tyrphostin stimulation of a novel type of protein carboxyl methyltransferase. We find here, however, that tyrphostin A25 addition to mouse heart cytosol incubated with S-adenosyl-[methyl- H]methionine or S-adenosyl-[methyl-14 C]methionine stimulates the labeling of small molecules in addition to proteins. Base treatment of both protein and small molecule fractions releases volatile radioactivity, suggesting labile ester-like linkages of the labeled methyl group. Production of both the base-volatile product and labeled protein occurs with tyrphostins A25, A47, and A51, but not with thirteen other tyrphostin family members. These active tyrphostins all contain a catechol moiety and are good substrates for recombinant and endogenous catechol-O-methyltransferase. Inhibition of catechol-O-methyltransferase activity with tyrphostin AG1288 prevents both base-volatile product formation and protein labeling from methyl-labeled S-adenosylmethionine in heart, kidney, and liver, but not in testes or brain extracts. These results suggest that the incorporation of methyl groups into protein follows a complex pathway initiated by the methylation of select tyrphostins by endogenous catechol-Omethyltransferase. We suggest that the methylated tyrphostins are further modified in the cell extract and covalently attached to cellular proteins. The presence of endogenous catechols in cells suggests that similar reactions can also occur in vivo.S-Adenosylmethionine-dependent protein methylation is a common post-translational modification, occurring at N-terminal amino, arginine, lysine, modified aspartyl, and C-terminal carboxyl residues (1). These reactions not only create additional diversity in protein amino acids but also provide opportunities for biological regulation by reversible modifications (2-9).We have been especially interested in identifying novel types of protein methylation reactions. In 1999, Bilodeau and Béliveau (10) -dependent protein methylation in rat kidney cytosolic extracts is greatly stimulated by the addition of the protein-tyrosine kinase inhibitor tyrphostin A25 and the protein phosphatase inhibitor sodium vanadate. These results suggested a regulatory interplay between protein phosphorylation and methylation systems. We were able to confirm these results in mouse kidney cytosol (11). We were also able to show that tyrphostin A25, in the presence or absence of vanadate, stimulates protein methylation in mouse heart cytosol, but not in testes or brain cytosol (11). Of a variety of tyrphostins tested only the A25 and A47 derivatives were active in stimulating protein methylation in kidney cytosol. Analysis of a 3 H-methylated 15-kDa polypeptide indicated that the radioactivity was not due to protein lysine or arginine methylation. The...

The enzymatic activities of brain catechol‐O‐methyltransferase (COMT) and methionine sulphoxide reductase are correlated in a COMT Val/Met allele‐dependent fashion

“…Post‐mortem human brain sections were homogenized as previously described, and their COMT activity was monitored according to established procedures [26,44,45] with minor modifications. Briefly, the 3,4‐dihydroxybenzoic acid was used as a catechol substrate to determine the level of COMT‐dependent methylation activity.…”