i io THE CHEMICAL CONSTITUTION OF THE PROTEINS 



The Sulphur in Proteins. 



Although cystine is the only compound containing sulphur now 

 known to be present in the protein molecule, yet numerous data have 

 been collected which point to the presence of the element sulphur in 

 some other compound than cystine. These data have been neglected 

 since cystine was isolated and its constitution established, for its pro- 

 perties answered most of the questions which arose from the earlier 

 work. Johnson, in 1911, drew attention to certain anomalies, and has 

 made the suggestion that their explanation may be the presence of 

 thioamide groups. 



The presence of sulphur in proteins was a subject of very early- 

 investigation. It was commenced by Mulder, who was the first to 

 observe that albumin, caseinogen, etc., when heated with alkali gave 

 off hydrogen sulphide ; in consequence of this he regarded these com- 

 pounds as composed of sulphur and protein in various proportions. 

 Fleitmann, a pupil of Liebig's, in 1847 then showed that this view of 

 the constitution of albumin, etc., was erroneous, for he found that only 

 a portion of the sulphur was split off by alkali, and that a portion still 

 remained combined with the protein. The later investigators upon this 

 question Nasse, Danilewsky, Kriiger, Suter, Malerba, Schulz con- 

 firmed Fleitmann's results, and in addition they determined the ratio of 

 total sulphur to loosely bound sulphur, as this, which was easily split 

 off by alkali, was called. Their results varied considerably, which 

 was due to the different methods they employed. In some proteins, 

 e.g., serumalbumin, the ratio of loosely bound sulphur to total sulphur 

 was as 2 : 3, in others I : 2 or 3 : 5. Osborne found similar ratios for 

 the large number of vegetable proteins which he examined. From 

 these values determinations were made of the molecular weight : thus 

 serumalbumin was given a molecular weight of 5,100, egg-albumin of 

 4>9OO, globulin of 4,600, edestin of 7,300. 



If cystine be the only compound containing sulphur, the number of 

 sulphur atoms in the protein molecule must be two or a multiple of 

 two, then those proteins containing about O'4 per cent, of sulphur 

 must have a molecular weight of 1 5,000. The ratio of the sulphur to 

 the iron in haemoglobin and the data obtained by the depression of 

 the freezing-point also pointed to a molecular weight of (15,000),,. 

 These conclusions could not be accepted, and Morner's explanation of 

 other sulphur-containing complexes in certain proteins, which yield 

 hydrogen sulphide on boiling with alkali, was overlooked. 



