SITES WITHIN THE CELL 53 



enzymes are still being synthesized at a normal pace. Some other species of 

 algae, e.g. Spirogyra (Van Wysselingh, 1908) or Elodea (Yoshida, 1959) 

 behave probably very much like Acetabularia; others, like Micrasterias 

 (Waris, 1951), do not stand enucleation as well. 



It remains that enucleation experiments establish that in several of the 

 cell types examined a large part of the cytoplasmic proteins can be made in 

 the absence of the nucleus. The cytoplasm must therefore contain centres 

 of synthesis which can produce perfect protein molecules and are able to 

 operate in the absence of the nucleus for a considerable time period. This 

 conclusion has received new support from recent experiments on isolated 

 mitochondria, which will be examined presently. 



2. Protein Synthesis in Isolated Mitochondria 



Kinetic studies on amino acid incorporation into cellular components 

 in vivo pointed to the microsomes as the most active centres of protein 

 synthesis. In liver, kidney or pancreas, amino acid incorporation into 

 proteins of the mitochondria represents but a minor fraction of total incor- 

 poration and for a long time mitochondria did not retain the attention as 

 sites of protein synthesis. In muscle, however, the picture is different. 

 Mitochondria come close to microsomes in rate of amino acid incorporation 

 in vivo. Indeed, for the first five minutes, incorporation into the mito- 

 chondrial fraction is somewhat higher than in the microsomes (Simpson 

 and McLean, 1955). 



Thanks to studies on oxidative phosphorylation in isolated mitochondria, 

 conditions have been worked out for isolating and preserving mitochondria 

 in vitro in relatively good conditions (e.g. Slater and Holton, 1954). In 

 such preparations from muscle and even from liver, a fairly rapid incorpora- 

 tion of amino acids into protein material could be observed (McLean et al., 

 1958; Campbell and Greengard, 1959; Reiss et al., 1959). Incorporation 

 was later shown to take place in well defined proteins, like cytochrome-c 

 (Bates et al., 1958, 1960) and this line of research culminated in the observa- 

 tion of the net synthesis of cytochrome-c in isolated heart muscle mito- 

 chondria (Bates and Simpson, 1959). A weighable amount of cytochrome-c 

 was actually synthesized in vitro in these experiments, and good evidence 

 was obtained that a synthesis of the protein moiety was taking place in the 

 process. 



This is a very important achievement for several reasons. It shows that 

 the ergastoplasm is not the only centre of cytoplasmic protein synthesis. 

 On the other hand, these experiments, like those on enucleate cytoplasm, 

 show that complete systems for making specific proteins exist in cyto- 

 plasmic organelles and can accomplish their function without the direct 

 participation of the nuclear genetic material. Liver or muscle mitochondria 

 are relatively large elaborate structures and attempts have been made at 



