Synthesis of labile methyl groups by guinea pig tissue in vitro.

The formation of the methyl groups of methionine and choline from C”-labeled formate has been demonstrated in the intact rat (1-3) and in rat liver slices (1). Du Vigneaud and his coworkers (’2) have also shown the incorporation of the carbon of formaldehyde into the methyl groups of choline. It is conceivable that in rat liver the carbon of formate or formaldehyde is first incorporated into the methyl groups of choline or betaine, followed by transfer of the methyl groups to homocysteine. In the present report a study has been made of the possible synthesis of methionine methyl groups by a pathway not involving formation of choline methyl groups. In approaching this problem we have used guinea pig liver which has been reported to lack an active liver choline oxidase system (4), and shown to be unable to methylate homocysteine from choline