RECOMBINANT RAT-LIVER S-ADENOSYL-L-METHIONINE SYNTHETASE TETRAMERS AND DIMERS ARE IN EQUILIBRIUM

Rat liver S-adenosyl-L-methionine synthetase is present in two oligome ric forms, tetramers and dimers, with different substrate kinetics and regulation. III vivo the relative amounts of both forms may change in some instances. The basis of this regulatory mechanism is not known. When rat Liver cDNA was used to express the protein in Escherichia col i the two oligomeric forms were found. Gel filtration chromatography o f the purified recombinant enzyme suggested that these two isoforms mi ght be in equilibrium. This was confirmed by kinetic experiments which showed that the specific activity of the enzyme was dependent on the protein concentration. From these experiments, apparent equilibrium co nstants of (5.6 +/- 0.4) x 10(5) M-1 and (3.5 +/- 0.9) x 10(5) M-1 wer e obtained at 2 mM and 60 mu M methionine concentrations, respectively . Using hydrophobic chromatography on phenyl-Sepharose to separate the tetrameric and dimeric forms, an equilibrium constant of (4.9 +/- 0.7 ) x 10(5) M-1 was calculated. A rate constant for the dissociation of the tetramer of k(-1) = (8.1 +/- 0.4) x 10(-4) s(-1) at 4 degrees C wa s also calculated using the same approach. In summary, we have shown t hat the rat liver S-adenosyl-L-methionine synthetase produced in bacte rial cells is present in two oligomeric forms, tetramers and dimers, w hich are in equilibrium. This system might be useful for studying the dynamics and the regulation of the distribution of oligomeric forms in the mammalian liver. (C) 1997 Elsevier Science Ltd.