2l8 PETER PERLMANN AND WINFIELD S. MORGAN 



Summary 



The microsomal fraction isolated from rat liver homogenates by 

 differential centrifugation contains a number of antigens which are 

 typical for this fraction. Extracts of microsomes of other organs of the rat 

 contain antigens which are typical for these fractions but are different 

 from the microsomal liver antigens. In addition to such "microsomal" 

 antigens, microsomes sometimes contain transitory antigens which are 

 only temporarily bound to microsomal particles or membranes. Examples 

 of such antigens are serum proteins in liver microsomes, antibody-active 

 proteins in microsomes of spleen or lymph nodes, and lens antigen in 

 microsomes of early chick embryos. The presence of these transitory 

 antigens in the microsomes may be an expression of their synthetic function. 

 This function can be studied by combining immunological techniques 

 with isotope incorporation. Examples are given, where the in vitro in- 

 corporation of ^^C amino acids into various antigens of rat liver homo- 

 genates was studied. After 30 min. of incubation of liver slices, serum 

 albumin and other serum protein-like antigens of the microsomes were 

 more strongly labelled than similar antigens in the cell sap. After longer 

 periods of incubation, strongly labelled serum proteins also appeared in 

 the cell sap. Under the same conditions the labelling of most other liver 

 antigens was weaker than that of the serum proteins. Incubation of cell- 

 free systems also led to the labelling of the different antigens. 



Acknowledgments 



This work has been supported by grants from the Swedish Cancer 

 Society. The technical assistance of Miss Margareta Engdahl is gratefully 

 acknowledged. 



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