HUGO THEORELL 



exceptional for a hemochromogen, was tentatively explained by 

 the assumption that the hematin is built into a crevice of the 

 protein molecule in such a way as to prevent oxygen from re- 

 acting with the iron. It was further pointed out that if imida- 

 zoles were attached to the iron, electrons from suitable sub- 

 strates could be led to or from the iron by radical reactions in the 

 imidazole rings (20). However, obviously much more ex- 

 perimental evidence was needed before it could be regarded as 

 conclusively proved that imidazole groups are attached to the 

 iron. Some recent experiments have shed some more light 

 upon this question. 



The thioether bonds are strong enough to keep the cysteine 

 residues attached to the hematin even during acid or enzymatic 

 degradation of the protein. Under such circumstances the 

 hematin part may be regarded as a colored label on the heme- 

 linked peptide fragment produced by a partial degradation. 

 Furthermore, since the molecule contains none but the two 

 heme-linked cysteine residues, these serve as indicators to tell 

 where the hematin is attached to the peptide chain. Tsou (29) 

 obtained a brown-red split-product of cytochrome c by peptic 

 digestion. This "hemopeptide" contained the hematin part of 

 cytochrome c linked to a peptide fragment of the native molecule. 

 The iron content (2.21%) indicated a molecular weight of 2500. 

 The peptide was insoluble in weakly acidic solution, and the 

 enzymatic properties of cytochrome c had been profoundly al- 

 tered by the digestion procedure. This was also the case with 

 the hemopeptide obtained by digestion with papain, trypsin, 

 and chymotrypsin. 



Tuppy and Bodo (30) very recently published some in- 

 teresting papers on the amino acid sequence in hemopeptides 

 produced by partial hydrolysis of cytochrome c with mineral 

 acids, or by digestion with trypsin or subtilisin. The Sanger 

 DNP technique was used after removing the hematin part by 

 Paul's silver-splitting procedure. The sequence of ten constitu- 

 ent amino acids was established as a result of this work (Nos. 3- 

 12, see below) : 



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