Purification and chemical characterization of the vitamin-B12-dependent 5-methyltetrahydrofolate: homocysteine methyltransferase from Escherichia coli B.

The transferase was isolated by means of hydrophobic chromatography and combination of ion-exchange and gel filtration at different pH values and ionic strengths. As judged by disc electrophoresis, the enzyme is homogeneous. Electrophoresis in the presence of sodium dodecylsulfate reveals only one band with Mr = 49500 +/- 10%. In gel filtration the native enzyme has a Mr of 200,000. The subunits can be crosslinked by iminothiolane followed by hydrogen peroxide oxidation. In sodium dodecylsulfate electrophoresis this results in a band pattern of integer multiples of 50,000 up to 20,000 but not higher. The high-Mr aggregates disappear on splitting the crosslinks by reduction. Thus the enzyme appears to be composed of four subunits identical or nearly identical in Mr. By the dansyl method, only phenylalanine and methionine were found as the amino-terminal residues, suggesting the existence of two different types of subunits.