64 INTERMEDIARY METABOLISM AND GROWTH I 



Cystineless mutants of the mold, Aspergillus nidulans were grown in the presence 

 of various inorganic sulfur compounds. Some strains grew on sulfite but not on 

 sulfate. Others grew on thiosulfate but not on sulfate or sulfite (Fig. 25) : 



HjSO^ ^^"^^ ''^'^ ^ H2S03 Genes 4.5.6 . ^ ^^^^^ ^ ^^^^^ 



7 



OH 



I " 



HS-CHj-CH-COOH— OjS-S-CHa-CH— COOH 



NH2 NHg 



Cysteine Cysteine - S - sulfonate 



Fig. 25. Postulated pathway of cysteine formation from sulfate. 



It is not clear at what level of reduction inorganic sulfur is converted to organic 

 sulfur. The thiosulfate of serine (cysteine-S-sulfonate) has been suggested as an 

 intermediate in cysteine synthesis. Cysteine-S-sulfonate supports good growth in 

 the "thiosulfateless" strains o^ Aspergillus ^NhWt growth is luxuriant on thiosulfate 

 plus serine. All known mutants o^ Aspergillus can utilize thiosulfate for growth. 



Animal organisms are also capable of fixing sulfate into organic linkage. Cysteine 

 sulfinic acid and taurine rather than cysteine are however the principal products of 

 these reactions (Machlin^irt/., 1953, i955;Dziewiatkowski, 1954; Lowe and Roberts, 

 1955; Block et al., 1951). Acetone powders of rabbit kidney catalyze the synthesis of 

 cysteine sulfinic acid from sulfite, pyruvate, and glutamic acid (Chapeville and 

 Fromageot, 1954; Chapeville et al., 1956). The reduction of cysteine sulfinic acid to 

 cysteine has also been demonstrated recently. Thus, sulfite-35 was incorporated 

 into the cysteine, cysteine sulfinic acid, and taurine of eviscerated rabbit tissues. 

 The specific activity measurements of the three compounds suggested that cysteine 

 sulfinic acid was the precursor of the other two. Under the same conditions, labelled 

 sulfate was not incorporated into cysteine. The enzymes concerned in these trans- 

 formations are probably the same as those which also participate in the catabolism 

 of cysteine to sulfite and sulfate. The conversion of cysteine to sulfate has been 

 shown in liver and kidney slices. Cysteine sulfinic acid is itself converted to sulfate 

 and alanine by liver mitochondria or Proteus vulgaris enzymes (Fig. 26; Froma- 



Cysteine 



a-Ketoglutaric 



Cysteine sulfinic 



^ -Sulfinylpyruvic 



Mn"^"^^ Pyruvate + SO2 ^ SO3 



'Alanine 

 Fig. 26. Metabolism of cysteine sulfinic acid. 



