Protein Synthesis 233 



The increased P 32 incorporation occurs into two monophosphoin- 

 ositides as well as into phosphatidylcholine, phosphatidylethanol- 

 amine, and phosphatidylserine. Glycerol-C 14 incorporation into 

 the phosphatidic acid was not stimulated but incorporation into 

 the phosphoinositide of high Rf was increased. A modest increase 

 in inositol-H 3 incorporation was also found. Hokin points out that 

 the known pathways of phosphatidic acid synthesis are from oc- 

 glycerophosphate or from ATP. P 32 experiments showed that only 

 the formation of phosphatidic acid derived from ATP was stimu- 

 lated by acetylcholine. The enzymes responsible for this acetyl- 

 choline response are present in the microsome fraction and can be 

 brought into solution by desoxycholate. This behaviour is that 

 expected from an enzyme system located in the lipid membranes 

 either of the cell surface or the endoplasmic reticulum, and has 

 led to the proposal that this effect is concerned with protein secre- 

 tion and perhaps with the transfer of lipid-insoluble substances 

 across the cell membrane by the formation of lipid-soluble phos- 

 phatidic complexes. 



PROTEIN SYNTHESIS 



While no direct study on protein synthesis in vitro has been 

 carried out on the salivary glands, it is quite likely that great simi- 

 larities exist in the synthesis in the pancreas which has been studied 

 in detail by Hokin (1951^ and b) and others. He found that pro- 

 tein synthesis was dependent on normal cell respiration and was 

 abolished by anaerobiosis and metabolic poisons such as cyanide, 

 iodacetate and dinitrophenol. The synthesis was not affected by 

 stimulants of secretion such as feeding or carbamylcholine. Opti- 

 mum synthesis of amylase depended on the presence of ten amino- 

 acids amongst which tryptophane, tyrosine, valine and leucine 

 could be replaced by their corresponding keto-acids. Anrep and 

 Khan (1923) found that restoration of the protein output of the 

 submaxillary gland after physiological exhaustion was very slow 

 so that on the average three days was necessary for complete 

 restitution. Stavraky (1940) arrived at a similar estimate on the 

 basis of the steady state of protein secretion after prolonged stimu- 

 lation of the cat's submaxillary gland. Burgen, Weiss and Seeman 

 (1959) found that the proteins of the dog parotid saliva could be 

 divided into two fractions, one apparently derived from the ducts 

 and the other from the acinar. The acinar protein behaved in a 



