THE ENDOPLASMIC RETICULUM 1 33 



system of the cytoplasm and all the data and information that had pre- 

 viously been gathered on liver microsomes became applicable to the intact 

 svstem. The rough form of the ER could be said to incorporate amino 

 acids into proteins, as shown for the microsome by the pioneering experi- 

 ments of Hultin [15] and Borsook et al. [i6]. Then also it became valid to 

 ascribe to the ER glucose-6-phosphatase activity and 40 to 50",, of the 

 RXA in the whole tissue (li\er in this case). These and other properties 

 described by more refined procedures will doubtless be considered in 

 subsequent papers. The sole purpose here is to relate the microsome to 

 the ER. 



As noted above, the term "rough" is applied to this form of the 

 reticulum because of the presence in its surface of numerous particles. 

 Thus the microsome is made up of at least three parts: the particles, the 

 membrane, and the contained space. Interest in the specific functions and 

 properties of these had led to several studies on them as separate entities. 

 With the aid of deoxycholate (DOC), the particles ha\"e been separated 

 from the membranes, and by electron microscopy have been shown to be 

 relati\elv free from contamination [14]. Examination of such preparations 

 have further shown these particles to contain 80 to 90",, of the RNA of 

 the intact or complete microsome and onlv 20",, of the protein nitrogen. 

 The particles are therefore appropriatelv referred to as RNP particles or 

 ribosomes. The other two parts of the microsome, the membrane and 

 content have been freed of particles with versene, likewise examined for 

 purity in the E.AL, and in\"estigated for other properties. As might be 

 expected, they possessed the other properties of the intact microsome 

 before the ribosomes were remo\ed, i.e. most of the protein, nearly all of 

 the phospholipid, and the DPNH-cytochrome c reductase activity. 



Here again, with these fractions of the microsomes, isotope incorpora- 

 tion studies have been fruitful and, as is well known, have described the 

 ribosome as possessing the capacity to combine free labelled amino acids 

 into proteins. Thus, in the case of protein synthesis, the particle rather 

 than the membrane is the important structure and would seem to be 

 responsible for determining the character of the protein synthesized. This 

 conclusion finds strong support in the recent studies of Siekevitz and 

 Palade [17, 18] on the relation of RNP particles to the synthesis of 

 pancreatic enzymes. 



With a specific function assigned to the ribosome, it is of some interest 

 to inquire into the role of the membrane and cavity of the intact system. 

 As previously noted, it ob^"iouslv provides a large surface for the disposi- 

 tion of the particles. There is, moreover, pictorial evidence that this 

 distribution follows certain repeating designs presumablv related to 

 patterns in the membrane [19]. Whether there is some functional purpose 

 in this is not clear. That the membranes are not essential to the svnthetic 



