A PEPTIDE INHIBITOR FOR S-ADENOSYL-L-METHIONINE-DEPENDENT TRANSMETHYLATION REACTIONS - PURIFICATION AND CHARACTERIZATION

1. A proteinaceous inhibitor for S-adenosyl-L-methionine (AdoMet)-dependent transmethylation reactions has been purified to apparent homogeneity from mt liver cytosolic fraction. 2. The peptide was made up of 29 amino acid residues with a molecular weight of 2,584. Glycine accounted for 52% of the total amino acids. 3. Employing AdoMet: protein-carboxyl O-methyltransferase (Protein methylase 11) and bovine serum gamma-globulin as in vitro substrate, the mode of inhibition was found to be non-competitive with K(i) value of 1.9 x 10(-8) M. 4. When the inhibitor was present in the reaction mixture together with S-adenosyl-L-homocysteine (AdoHcy), which is a competitive inhibitor for AdoMet, the extent of inhibition exceeded that exerted by each individual inhibitor alone, suggesting that the sites of the inhibitors on the enzyme molecule are different. 5. Almost a stoichiometric relationship exists between the enzyme and the inhibitor molecule, the ratio being approx one.