H-2 ANTIGENS OF THE MOUSE 57 



Gill, Pro, Ser, Thr, Gly, Ala, Val, Leu, Tyr, Plie, CysSH, Lys, 

 Arg, and one unidentified). 



Homogeneity 



Evidence for homogeneity proves very difficult to obtain. 

 The products are essentially free from all ascitic fluid components 

 v^hich are soluble in saline or v^ater and have only one serological 

 specificity (mouse species specificity) other than that of histo- 

 compatibility (see below). They behave as single components by 

 density. Further evidence is limited to experiments in v^hich 

 impurities cannot be found under circumstances where little 

 information can be obtained about the behaviour of the bulk of the 

 material. Analytical ultracentrifugation of SP fractions in water 

 shows a rapid clearing of opalescence between 20,000 and 30,000 

 rev./min. (Spinco Model E) followed by a single peak representing 

 about 10 per cent of the material, which sediments with rapid 

 diffusion. The TM2 and SM fractions examined in the same way 

 show a similar rapid clearing of opalescence to give a gel on the 

 bottom of the cell, but there is no indication of a second com- 

 ponent. At pH 9-0 (tris-phosphate buffer) a similar picture is 

 found, showing that the second component in the SP fraction is 

 not a degradation product of the H-2 antigen. Electrophoresis on 

 cellulose-acetate paper at pH 9 gives no movement of protein or 

 lipid-staining material from the starting point, showing that at 

 this pH no material w^as present which might have failed to 

 separate from the meniscus in the ultracentrifuge run at the same 

 pH, and that at pH 7 to 9 the material behaves as a single substance. 



Ultracentrifuge runs have also been carried out under conditions 

 where degradation might be expected to have occurred. Solu- 

 tions in 0-2 per cent sodium dodecyl sulphate at pH 8 showed a 

 single sedimenting component but at pH 6- 5 there was a separa- 

 tion of two peaks having proportions of about 4:1. In deoxy- 

 cholate at pH 9 some material is present which fails to break away 

 from the meniscus. Runs in solvents which give relatively clear 



