On the Role of S-Adenosylmethionine in the Vitamin B12 Dependent Methionine Biosynthesis

Abstract

Methionine synthetase (methylterahydrofolate-homocysteine methyltransferase) from Escherichia coli B which has been purified by a gentle procedure, shows a spectrum differing from those previously published but resembling the spectrum of the enzyme which had been methylated by prior treatment with S-adenosylmethionine. After cautious purification, the enzyme does not require adenosylmethionine if it is reduced by chemically prepared reduced flavin mononucleotide. The natural methyl group acceptor homocysteine abolishes the ability of the enzyme to react without adenosylmethionine. This adenosylmethionine dependent enzyme preparation can be reactivated by reduction and incubation with adenosylmethionine. It then is greatly independent on the presence of adenosylmethionine. The half-saturation concentration of adenosylmethionine is found to be about 0.1 μM. It is concluded that in the bacterial cell the enzyme is present in its methylated form, that it can be demethylated by homocysteine treatment, and that the methylation can be restored by incubation with adenosylmethionine.