20 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



ergastoplasm. As can be seen in Fig. 7 (p. 15), these structures 

 — which are particularly well developed in gland cells — are essen- 

 tially a system of double membranes, in which small granules (often 

 called Palade's granules) are embedded. There is good evidence for 

 the view that the membranes are made of proteins associated with 

 lipids, while the small granules contain RNA. When the cell is 

 homogenized, the ergastoplasm breaks down into small fragments, 

 which are the microsomes (Fig. 8, p. 16). Palade's granules are still 

 attached to remnants of the double membranes, which now form 

 vesicles of various sizes. It has been found recently that it is possible, 

 by treatment of a homogenate with bile salts, to separate the gra- 

 nules from the rest of the microsomes. Deoxycholate, for instance, 

 dissolves the membranes and, on prolonged ultracentrifugation, 

 the granules can be collected. Since they are very rich in RNA, they 

 are usually called "small ribonucleoprotein particles". 



If RNA really plays an essential part in protein synthesis, one 

 would of course expect the microsomes and ribonucleoprotein 

 particles to be very active sites of protein synthesis. We shall now 

 see that this expectation has been fulfilled. 



Already, in their first papers on the chemical composition of 

 cytoplasmic particles, Brachet and Jeener (1944) pointed out that 

 there is no reason to believe that RNA alone plays a part in protein 

 synthesis : it is quite possible that the whole granule, the microsome, 

 is the active agent. 



They found some support for this hypothesis in the fact that 

 particles obtained in the ultracentrifuge (which were in fact mix- 

 tures of mitochondria and microsomes) always contained an ap- 

 preciable amount of the specific protein synthesized by each organ. 

 This was found to be the case for trypsin and insulin in the pancreas, 

 amylase in salivary glands, hemoglobin in red blood cells, and the 

 melanophore-expanding hormone in the pituitary. More recently, 

 Daly et al. (1955) confirmed that an appreciable proportion of 

 protease and amylase is bound to the microsomes in pancreas. 



Much more direct evidence has come from work done in other 

 laboratories with labeled amino acids. Borsook et al. (1950) were 

 the first to report that incorporation in liver tissue is highest in the 



