L-METHIONINE DECARBOXYLASE FROM DRYOPTERIS-FILIX-MAS - PURIFICATION, CHARACTERIZATION, SUBSTRATE-SPECIFICITY, ABORTIVE TRANSAMINATION OF THE COENZYME, AND STEREOCHEMICAL COURSES OF SUBSTRATE DECARBOXYLATION AND COENZYME TRANSAMINATION

L-Methionine decarboxylase from the male fern Dryopteris filix-mas has been purified 256-fold from acetone powder extracts to very near homogeneity. The enzyme is membrane-associated and requires detergent for solubilization during the initial extraction. The enzyme is a homodimer of subunit Mr 57 000 and shows a pH optimum at ~5.0 with 20 mM (2S)-methionine as substrate. The specific activity, kcat, for methionine is ~50 mol s−1 (mol of active site)−1 at pH 4.5 and below. A wide range of straight- and branched-chain (2S)-alkylamino acids are substrates for the enzyme. The values for the rate of decarboxylation, Vmax, and for the apparent Michaelis constant, Km, however, vary with structure and with the chirality at C-3. The pH dependence of V and V/K has been examined for three substrates: (2S)-methionine, valine, and leucine. Pyridoxal 5′-phosphate (PLP) is required for activity, and in the absence of excess PLP, the activity of the enzyme in incubations reduces with respect to time. The addition of PLP fully restores the activity, indicating that an abortive decarboxylation-transamination accompanies the normal decarboxylation reaction. The occurrence of the abortive reaction was confirmed by showing that [35S] methionine is converted to labeled 3-(methylthio)propionaldehyde while [4′-3H]PLP is converted to labeled pyridoxamine 5′-phosphate (PMP). The decarboxylation of (2S)-methionine gave 3-(methylthio)-1-aminopropane. Preparation of the N-camphanamide derivative of the amine allowed the C-1 methylene protons to be distinguished by 1H NMR spectroscopy. Synthetic samples of the camphanamide were prepared in which each of the C-1 methylene protons was replaced by deuterium. When (2S)-methionine and the C-2 deuteriated isotopomer were incubated with the enzyme in deuterium oxide and protium oxide, respectively, and the products were converted to their camphanamide derivatives and analyzed by 1H NMR spectroscopy, it was evident that decarboxylation occurred with retention of configuration at C-2. When the decarboxylation of six other substrates was studied, examination of the N-camphanamide derivatives of the amines indicated that decarboxylation occurred stereospecifically and, by analogy, with retention of configuration at C-2. When tritiated pyridoxal phosphate was incubated with the enzyme, tritiated pyridoxamine phosphate was formed. Analysis of the chirality of the methylene group at C-4′ indicated that, during abortive transamination, protonation occurred from the 4′-si face of the coenzyme, the same stereochemical result as that obtained for several bona fide transaminase enzymes. These results are used to construct possible mechanistic schemes for both reactions, decarboxylation and transamination. The position and possible identities of active-site proton donors are discussed. © 1990, American Chemical Society. All rights reserved.