Purification and kinetic mechanism of a mammalian methionine synthase from pig liver.

Porcine hepatic methionine synthase has been purified to near homogeneity. The enzyme is isolated in two forms which were purified approximately 9,000- and approximately 7,000-fold and were obtained in 0.9 and 2.5% overall yield, respectively. The mammalian enzyme from pig liver is a large monomeric protein with a molecular mass of 151-155 kDa. It is characterized by the absence of any metals other than cobalt which is associated with the cofactor, cobalamin. This enzyme, like the methionine synthase from Escherichia coli is dependent on S-adenosylmethionine for activity. The steady state kinetic studies demonstrate that the reaction operates via an ordered sequential mechanism in which binding of CH3-H4-folate precedes homocysteine, and methionine is released prior to H4-folate.