Methylation of guanidoacetic acid by homocystine plus choline with rat liver slices

The methylation of guanidoacetic acid by liver slices is accelerated by methionine; choline, under these conditions, exerts no significant accelerating effect (1). In view of the fact that homocystine plus choline can replace methionine for growth (2), and of the isotope experiments which proved the transfer in viva of the methyl groups of choline to creatine,(3) it has been suggested, from indirect evidence, that the pathway of the methyl group to creatine is more direct from methionine than from choline.(4) More specific evidence is desirable, especially as neither homocystine nor homocysteine has been identified in animal tissues.