182 



CARBON METABOLISM III 



of methyl groups. The ultimate source for those fungi — perhaps all 

 naturally occurring fungi— which do not require preformed methyl 

 groups in the medium is, of course, carbohydrate; that is, most or all 

 fungi can synthesize methyl groups from the products of glucose 

 metabolism. 



Early in the nineteenth century, it was recognized that an arsenical 

 poisoning of humans by volatile compounds is associated with the 

 action of fungi on wallpaper or other building materials containing 

 arsenical pigments. The history of this problem is reviewed by Chal- 

 lenger (117); the toxic material was finally identified as trimethylar- 

 sine (121). The ability of certain fungi to form methylated com- 

 pounds of arsenic and other elements has since been investigated vigor- 

 ously by the group at the University of Leeds. Some examples of the 

 products identified are listed in Table 5; these and others are reviewed 



Table 5. Methylated Products of Fungi 



Substrate 

 Na methylarsenate 

 Na cacodylate 

 Arsenous oxide 

 Diethylarsonic acid 

 K tellurite 

 Na selenate 



Na selenite 

 Ethyl disulfide 

 Inorganic sulfate 



Product 



Trimethylarsine 

 Trimethylarsine 

 Trimethylarsine 

 Methyldiethylarsine 

 Dimethyl telluride 

 Dimethyl selenide 



Dimethyl selenide 

 Methyl ethyl sulfide 

 Methyl mercaptan, 

 dimethyl sulfide 



Organism 



Scopulariopsis brevicaulis (121) 



S. brevicaulis (121) 



S. brevicaulis (121) 



S. brevicaulis (120) 



S. brevicaulis, Penicillium spp. (62) 



Aspergillus niger (170) 



Penicillium spp. (62) 



Scopulariopsis brevicaulis (123) 



S. brevicaulis (123) 



S. brevicaulis (124) 



Schizophyllum commune (76, 119) 



by Challenger (118). It should be noted that the methods used permit 

 only identification of the final product. Obviously, many steps in 

 addition to methylation must be assumed in the conversion of the 

 highly oxidized selenate ion, for example, to the reduced dimethyl 

 selenide. 



Evidence is now at hand that these products do in fact arise by 

 transmethylation. Isotope experiments (122, 170) show that the methyl 

 groups of dimethyl selenide and trimethylarsine originate from 

 methionine in Scopulariopsis brevicaulis and Aspergillus niger. 

 Betaine does not serve as a methyl donor; carbon from formate and 

 choline enters the system, probably through methionine. 



