BARBARA W. LOW AND JOHN T. EDSALL 



/ 



c=o o- 



\ / 



CHCH2O— p=o 



/ \ 



HN O- 



\ 



Of course no cross linkage is involved here. They may, however, 

 be involved in a diester linkage between two such chains, or 

 link different parts of the same chain together into a loop. The 

 cross linkage of two seryl residues is illustrated as an example: 



/ 

 0=G O ^ G=0 



\ II / 



CHCH2OPOCH2CH 



/ I \ 



HN O- NH 



(Chain 1) (Chain 2) 



The linkage may also involve an amino or other nitrogenous 

 group on an amino acid residue of one of the two chains, and a 

 hydroxyl group of the other. Such a linkage may be called an 

 — N — P — O — bond, in distinction from the — O — P — O — 

 bond illustrated above. Finally the cross linkage may involve 

 a pyrophosphate, instead of an orthophosphate, bridge between 

 two chains or chain segments ; such a bridge may be designated 

 as an — O — P — O — P — O — cross link. All of these linkages 

 have been found by Perlmann in proteins, by employing dif- 

 ferent phosphatases specific for each type of linkage in smaller 

 molecules. Thus in a-casein (84) if the molecular weight be 

 taken as 30,000, there are in all ten atoms of phosphorus per 

 molecule. Four of these are found to be present in the form of 

 simple primary phosphate esters, linked presumably to hydroxy- 

 amino acids. Four other phosphorus atoms are held in diester 

 linkages ; the specificity of the enzyme used indicates that these 

 linkages are of the — N — P— O — type discussed above. Finally 

 two of the ten phosphorus atoms are involved in a pyrophosphate 

 linkage of the type — O — P — O — P — O — . The splitting of 



388 



