On the Role of the Oxidation in the Methylation of Guanidoacetic Acid*

There are two, at least, methyl transfer reactions promoted by liver slices in vitro (2). The fundamental distinction between them is that one is dependent on oxygen and the other is not. Transmethylation reactions found to be independent of oxygen are methionine formation by the methylation of homocysteine, homocystine, or homocysteine thiolactone by either choline or betaine. They are not inhibited by oxidation inhibitors. Homogenized tissue and slices are equally effective without any modification of the reaction ,mixture. Transmethylations found to be dependent on oxygen are the methylation of guanidoacetic acid to form creatine and of nicotinamide to N1methylnicotinamide (3). The methyl donor is methionine; choline and betaine are ineffective. Neither reaction in this category proceeds when the tissue (liver) is homogenized or finely minced and the reaction mixture consists only of methyl donor and acceptor,. After many unsuccessful experiments to gain further insight into the r81e of the oxidation in the methylation of guanidoacetic acid with rat liver slices we turned to the use of liver homogenates. We found that guanidoacetic acid is methylated by methionine in rat and guine& pig liver homogenates if sufficient adenosine triphosphate (ATP) is provided. The yields with rat liver homogenate were low and irregular. Gu@a pig liver homogenates were better; they gave, on the average, more than twice the creatine obtained with rat liver slices (on the basis of dry weight’of tissue used), and the results were consistent. The general picture d’the reaction was the same as with rat liver slices. Accordingly we have confined ourselves in the study of the r61e of the oxidation in the methylation of guanidoacetic acid to homogenates of the livers of guinea pigs.