Purines transmethylation

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

Polyamines are formed from ornithine and SAM, thus aflEording an alternative fate for this purine derivative (P3). The polyamines, as well as adenosine, regulate transmethylation (Hll, L3). Ornithine decarboxylase is produced in greater quantity in rapidly dividing cells (B17, D7, Rl, RIO). Levels of S-adenyosylmethionine decarboxylase are also increased in such tissues (H2, R9). There is evidence that this increase is cyclic AMP mediated (B7), This ties these compounds although indirectly to adenosine concentrations. 

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