BIOLOGICAL METHYLATION 181 



spores oxidize ascorbate, but the reaction is believed to be mediated 

 by a new enzyme, probably located on the spore surface (365, 366). 



Two other derivatives of furan — not derivatives, however, of tetronic 

 acid — are known: 2-hydroxy-5-furan carboxylic acid, from Aspergillus 

 spp. (496, 497) and zymonic acid, from yeasts (493). 



The thiophen or thiofuran ring differs from the furan in that a 

 sulphur atom replaces the oxygen. A single compound of this type, 

 junipal, is known in the fungi, from Daedalea juniperina (69). 



6. BIOLOGICAL METHYLATION 



The transfer of intact methyl groups from one molecule to another 

 has been most intensively studied in animal materials. The synthesis 

 of methionine may be taken as an example: 



CH 2 — SH 



I 

 CH 2 



I 

 (GH 3 ) 3 — N— CH 2 — COO- + H 2 N— CH— COOH -+ 



Betaine Homocysteine 



CH 2 — S — CH3 

 CH 2 

 H 2 N— CH— COOH + (CH 3 ) 2 — N— CH 2 — COOH (16) 



Methionine Dimethylglycine 



In this reaction, details of which are neglected in the above over-all 

 formulation, betaine functions as the methyl donor. Methionine 

 can in turn donate a methyl group to other molecules, e.g., in the 

 stepwise methylation of aminoethanol to choline. 



Studies on Neurospora crassa mutants which require methionine 

 show that the final step in methionine synthesis is the conversion of 

 homocysteine to methionine, as in Equation 16 above (285). The 

 methyl donor in the fungus system is not known. 



In a particularly important application of biochemical genetics, 

 Horowitz (284) showed that Neurospora crassa synthesizes choline by 

 the stepwise methylation of aminoethanol, that is: 



Aminoethanol — > monomethylaminoethanol — > 



dimethylaminoethanol -» choline (17) 



Here it is to be presumed that methionine or formate is the source 



