Pathway of Betaine and Ch.oline Synthesis in Beta vuEp-is

During the investigation of the formation of choline and betame in leaf disks of Beta vulgaris it was found that formate contributed its carbon essentially exclusively to the N-methyl groups of choline and betaine, while the a-carbon of glycine was distributed equally between the N-methyl groups and the &-moiety of these compounds (1). This distribution of radioactivity raised the question as to whether choline and betaine were formed in this system via serine and aminoethanol, a pathway already established in animals and Neurospora crasaa (2, 3,4), or by direct methylation of glycine to bctaine (5). Whereas the latter pathway appeared unlikely from considerations of the specific activities of the isolated choline and betaine, the former reaction sequence seemed irreconcilable with the findings of Sakami (6) and of Siekevitz and Greenberg (7)) which indicated that formate is a precursor of the P-carbon of serine and therefore should contribute its carbon to the C&-moiety of choline and betaine. In the meantime, evidence has accumulated that in some systems formaldehyde and formate are not incorporated into serine (8) and that the a-carbon of glycine seems to have a different synthetic potential from other conventional one-carbon precursors (9). In the present paper our previous investigations have been extended, and evidence has been obtained that serine and aminoethanol are indeed intermediate in the formation of choline and betaine in B. vulgaris, and that formaldehyde and formate, although efficient precursors of the N-methyl carbon of choline and betaine, are relativeIy poorly utilized for the formation of the P-carbon of serine.