IMMUNOLOGICAL STUDIES OF MICROSOMAL STRUCTURE AND FUNCTION 21 5 



small concentrations, against two alpha globulins and one beta globulin. 

 The photograph shows that the lubrol extract of the microsomes gives 

 four distinguishable precipitates with these antibodies. These serum 

 protein-like antigens are immunologically different from other microsomal 

 antigens, reacting with the anti-microsomal serum. Moreover, a con- 

 siderable amount of serum proteins had also accumulated in the cell sap. 

 This also contained additional antigens, some specific for cell sap, and 

 some in common with the microsomal extract. 



From the autoradiograph of Fig. 2{b), it can be seen that an appreciable 

 number of the precipitates were distinctly labelled. As described else- 

 where [10], several lines of evidence, and control experiments such as 

 addition of unrelated precipitin systems to the solutions before agar 

 plating, or dilution with unlabelled amino acids after incubation with 

 i^C amino acids, all suggest that the labelling in the autoradiograph is not 

 due to an unspecific adsorption of radioactive material to the precipitates. 

 Rather, we may assume that the labelling of the precipitates indicates a 

 true incorporation of ^^C amino acids into the antigens. The autoradio- 

 graph shows that the albumin and other serum protein-like antigens 

 which were solubilized from the microsomes were heavily labelled, as well 

 as some additional microsomal antigens of unknown significance. In 

 contrast, in the cell sap, only the serum albumin carried enough of the 

 label to be easily detected. This is in spite of the fact that the concentration 

 of the serum proteins in this fraction probably was higher than in the 

 microsomal extract, as indicated by the relative positions and the appearance 

 of the precipitates. Other cell sap antigens were more or less inactive. 



Experiments of this type make it possible to follow the distribution, 

 under various experimental conditions, of the radioactive label in immuno- 

 logically identical proteins, recovered from different cellular fractions. Thus, 

 the results of the experiment of Fig. 2 confirm other work reporting 

 the first appearance of the radioactive label in the microsomal serum 

 albumin followed by an increasing specific activity of this protein in the 

 cell sap. This is taken to indicate that freshly synthesized molecules are 

 transferred from the microsomes to this fraction [21, 22]. 



A great number of specific precipitates appearing in Fig. 2{a), par- 

 ticularly those formed by the antigens of the cell sap, were hardly labelled 

 at all. Howxver, as we do not know their chemical nature or their antibody 

 combining ratios, we cannot use the degree of labelling of precipitates as 

 the basis for comparison of the metabolism of immunologically different 

 antigens. Problems of this type can be approached in a different way. 

 Figure 3 shows a series of four autoradiographs, made from four 

 experiments with slices of rat liver, incubated with ^^C amino acids. The 

 four experiments were made with aliquots of the same preparation, and 

 the only difference between them was the time of incubation with the 



