Molds as Metabolic Models 91 



discovered. Worth mentioning here are a few mold models 

 that many years ago augured developments which today 

 we regard as fundamental throughout the biological king- 

 dom. For example, how much earlier might have been de- 

 veloped our knowledge of transmethylation and one-carbon 

 metabolism in biosynthesis had the remarkable discovery 

 made by the Italian, Gosio, in 1891, in relation to death 

 of humans by arsenic poisoning, been systematically in- 

 vestigated. Gosio showed that in the rooms where poison- 

 ing recurred, volatile compounds of arsenic were liberated 

 into the atmosphere by fungi growing on the wallpaper 

 when moisture conditions were suitable. The arsenic com- 

 pound, Gosio gas, originated from arsenic-containing pig- 

 ments used for the designs on the wallpaper. Numerous 

 pure cultures of filamentous fungi were proved capable 

 of volatilizing inorganic arsenic, the outstanding organ- 

 ism in this respect being Scopulariopsis hrevicaulis. 

 Thought originally to be diethylarsine, the garlic-odored 

 Gosio gas later was shown to be trimethylarsine by Chal- 

 lenger (2) and his colleagues at Leeds. The extensive work 

 of Challenger on the methylation of arsenic, as well as 

 selenium and tellurium compounds, has recently cul- 

 minated in the demonstration (3) that the process is in 

 fact a one-carbon transfer. The methyl group of methio- 

 nine, and probably of choline and betaine, is donated to 

 the inorganic arsenious acid to produce trimethylarsine, 

 via monomethyl and dimethyl intermediate stages. Radio- 

 active formate carbon also is a precursor of the methyl 

 carbons of trimethylarsine, dimethylsulfide, and dimethyl- 

 selenide, as shown by the Challenger gioup. Thereby the 

 evidence was completed that methylation of the metals is 

 indeed a special case of the universal biochemical pattern 

 of one-carbon transfer in which the usual methyl donors 

 transfer the one-carbon group to an unusual methyl ac- 

 ceptor — arsenic. Functionally, however, the arsenic is like 

 the usual methyl acceptors. Fantastic as it seems, the curi- 



