CHEMICAL PATHWAYS 111 



acids into microsomal particles. Its effect is especially marked with micro- 

 somes that have been lyophilized or treated with acetone (Sachs, 1957). 

 Microsomes treated under certain conditions by the detergent lubrol or 

 by perfluoro-octanoate can incorporate amino acids in the absence of the 

 amino acid activation enzymes (Cohn, 1959); in this system the S-protein 

 is necessary. A crucial question is the dispensability of the amino acid 

 activation enzymes. If this is well established and if the observed incor- 

 poration reflects protein synthesis, then one must conclude that there are 

 two pathways of amino acid activation leading eventually to protein via 

 s-RNA. This raises questions that the presently available data cannot 

 answer. Are there two ahernative pathways for the synthesis of any protein, 

 or are certain proteins made by one process and others by the other one? 

 Such a duality of proteins certainly does not appeal to anyone, but one 

 should be prepared to the eventuality that e.g. the constitutive proteins of 

 the ribosomes might be made in another way than the enzymes and the 

 other common proteins or that certain cell organelles might not make 

 proteins in exactly the same way as the cytoplasmic ground substance. This 

 is supported by a few observations. Hexetidine disturbs protein synthesis 

 in bacteria in a very strange way : in the presence of this agent the synthesis 

 of all proteins is not uniformly affected. Incorporation of labelled pre- 

 cursors seems to occur in a few protein fractions only, as if the synthesis of 

 most proteins was inhibited, leaving the formation of a few others unaffected 

 (Halvorson and Gorman, 1959). Chloramphenicol at rather high concen- 

 tration inhibits protein synthesis in isolated nuclei and mitochondria, but 

 not into ribonucleoprotein particles (Rendi, 1959). Chloramphenicol and 

 chlortetracyclin inhibit amino acid incorporation into large particles con- 

 tained in a homogenate of Tetrahymena but not into the microsomal 

 fraction (Mager, 1960). 



(c) Lipids. Hendler (1958, 1959) presented evidence for another carrier 

 of amino acids besides soluble RNA. A lipid fraction from hen oviduct was 

 found to incorporate amino acids very rapidly; the amino acids seem to be 

 held by a highly labile bond in this lipid fraction and to be rapidly renewed. 

 Hunter et al (1959) also observed that a lipoprotein fraction isolated from 

 cytoplasmic membranes of B. megaterium takes up labelled amino acids 

 very rapidly. Moreover, they obtained indications that if both soluble RNA 

 and lipids are concerned in protein synthesis, the lipids are involved at a 

 stage subsequent to the action of soluble RNA. A lipoidic substance con- 

 taining carbohydrates was shown to stimulate amino acid incorporation 

 in homogenates (Hradec and Stroufova, 1960). These systems are still very 

 crude, and it is not quite clear at present whether the lipids are involved in 

 protein synthesis or in some other process, e.g. amino acid transport. 

 Further research is certainly needed along this line. 



(d) Incorporation factors. One further point which still awaits clarifica- 



