PROTEIN DIFFERENCES 



315 



5.2. Amino Acid Analysis 

 5.2.1. Invertebrate Oxygen Carriers. Detailed analysis of the 

 amino acid composition of proteins is a task which is far more laborious 

 than the determination of an isoelectric point or of the molecular 

 weight of protein. For this reason, the data available for the amino 

 acid composition of the different classes of respiratory pigments refer 

 to a relatively small number of species. 



TABLE VI 



Percentage Amino Acid Content of Oxygen Carriers 

 of Some Invertebrates and the Horse 



' Ec = erythrocruorin, Ch = chlorocruorin. (e) = extracellular, (i) = intracellular. 



So far Roche and co-workers (2313,2318,2319) have published the 

 only amino acid analyses for the invertebrate pigments. These are 

 given in Table VI, and for comparison data from the same laboratory 

 for analyses on the erythrocruorin of Petromyzon and horse hemoglobin 

 are included. 



There is no doubt that as a class the invertebrate pigments differ 

 considerably from horse hemoglobin while the pigment from Petromy- 

 zon stands between. Chlorocruorin has the lowest histidine content 

 of any of the pigments so far examined. Since Fox (935) has shown 

 that this pigment has the same iron content as has mammalian 

 hemoglobin, the molar ratio of histidine to heme is 2.6. If heme- 

 heme interaction takes place in the invertebrate pigments according 

 to the hypothesis described in the previous chapter, and at least two 

 mols of histidine per heme are required, this condition is fulfilled in 

 chlorocruorin. In view of the incompleteness of the amino acid 

 analyses of the invertebrate pigments it is difficult to give a satis- 



