IMMUNOLOGICAL STUDIES OF MICROSOMAL STRUCTURE AND FUNCTION 213 



Although Other explanations cannot be excluded, the antibody- 

 inducing potencies of the microsomes from various organs may be a 

 reflection of their synthetic activities. More direct indications for such 

 activities have recently been obtained in human patients suffering from 

 ulcerative colitis. In this disease, there occur autoantibodies against a 

 substance of the colon, and the colonic lesions found seem to be of an 

 immunological nature [3]. In regional colonic lymph glands of these 

 patients, antibody-active microsomes could be detected in a remarkably 

 high concentration, whereas microsomes from lymph glands of the ileal 

 region of the same patients were devoid of antibody activity [17]. 



Finally, as the last example, in the chick embryo, one can detect an 

 organ specific lens antigen in the microsomal fraction at a very early age, 

 just before lens formation is initiated [18]. Later during embryogenesis, 

 when lens differentiation is going on, and in the adult, this lens antigen 

 occurs both in the microsomal fraction and in the cell sap. Electron micro- 

 scopy of these microsomal fractions indicates that both ribonucleoprotein 

 particles and membranous elements are present. 



In conclusion, serum proteins, including antibody-active globulins, 

 as well as the lens antigen, are most likely first synthesized in the micro- 

 somes. Assuming these products remain attached to any one of the 

 microsomal substructures for a measurable period of time, it can also be 

 expected that they are available for detection with immunological tools. 

 It has already been shown that this is true in certain cases where proteins 

 are temporarily bound to the endoplasmic membranes before being ex- 

 ported [22]. Definite proof for the immunological recognition of ribonu- 

 cleoprotein particles by means of antibodies against their svnthetic products 

 is so far lacking. It is not at all clear whether or not such transitory products 

 already possess an immunological specificity while being attached to the 

 particles. The fact that ribonucleoprotein particles of the pancreas may 

 carry freshly synthesized chymotrypsin suggests that this could be 

 possible [27]. 



Incorporation of isotopes into antigens of rat liver homogenates 



It should be emphasized once more that the mere presence of these 

 transitory antigens in microsomal fractions only provides indirect evidence 

 for the assumption that they are products of microsomal activity. The 

 microsomal fractions mentioned above are all isolated from heterogenous 

 cellular populations. Moreover, they may contain both the ribosomal and 

 membranous machinery involved in protein synthesis, as well as sub- 

 cellular elements of a completely different functional significance. However, 

 questions related to the problem of microsomal activity in protein syn- 

 thesis can be studied by combining immunological techniques with the 



