Two flavoproteins, both of which are required for the TPNH- and adenosylmethionine-dependent activation of the Bll-containing methionine synthetase, have been purified to homogeneity from Escherichia coli K-12. The larger flavoprotein (mol wt 27,000) contains 1 mole of noncovalently bound FAD per mole of protein and has an atypical spectrum (X,,, at 400 and 456 nm). The smaller flavoprotein (mol wt 19,400) is acidic, contains 1 mole of noncovalently bound FMN per mole of protein, and has absorbance maxima at 369 and 465 nm. The methionine synthetase, which was also purified to homogeneity from E. coli K-12, contains 1 mole of B12 per mole of protein (mol wt 186,000) and has an absorbance maximum at 474 nm. In the presence of TPNH, both flavoproteins, and adenosylmethionine, the synthetase has a specific activity at 37” of 3.8 pmoles per rnin per mg of protein with respect to formation of methionine from 5- methyltetrahydrofolate and homocysteine.
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