THE CYTOPLASM 239 



in any other fraction. He has also shown that incorporation does not occur 

 when microsomes or mitochrondria are incubated separately with the 

 substrate, but it does occur when the two fractions are combined. A re- 

 quirement for the reaction is the addition of substrates and cofactors per- 

 mitting the simultaneous occurrence of oxidative phosphorylation. During 

 this oxidative reaction, the mitochondria apparently produce a soluble 

 factor, possibly a derivative of ATP, that enables the microsomes to in- 

 corporate alanine. These very interesting experiments thus suggest that 

 microsomes contain the enzyme system responsible for the incorporation 

 of the amino acid but in order to carry out the reaction require a source of 

 energy which in this case is provided by the respiratory enzyme systems of 

 the mitochondrion. These findings may well represent another example of 

 the integrated activities of several cell structures in carrying out a com- 

 plicated metabolic process. There are, in addition, data indicating that 

 microsomes play some role in Krebs cycle reactions,^"- perhaps through their 

 content of the cytochrome c reductases, and in glycolysis.^^^ 



There is, in addition to the cytological evidence discussed earlier, a con- 

 siderable amount of biochemical data indicating that microsomes are a 

 group of distinct cellular entities. In this respect, the conclusion of Har- 

 mon" and Green^^ that microsomes are fragments of mitochondria is not 

 supported by data obtained in a number of other laboratories. There is good 

 reason to believe, in fact, that qualitative biochemical differences exist be- 

 tween these two particulate elements of the cell. When adequate fractiona- 

 tion of cytoplasmic components is achieved, glucose-6-phosphatase is re- 

 covered almost completely in microsomes.^" Conversely, the specific cyto- 

 chrome oxidase activity of the mitochondrial fraction is almost 20 times 

 that of the microsomes,^* and the low activity of the latter fraction can be 

 readily explained on the basis of the presence of contaminating mito- 

 chondria. Furthermore, mitochondrial fragments, although similar in sedi- 

 mentation characteristics to microsomes, are entirely different in certain 

 biochemical properties. Thus, when isolated mitochond.ria are disrupted 

 by means of sonic vibrations to yield optically empty suspensions, the 

 mitochondrial fragments isolated by high-speed centrifugation show an 

 even higher specific cytochrome oxidase activity than do the original 

 preparations of intact mitochondria.^^ '^^ It seems likely that the inability 

 of Green^* to distinguish biochemically between microsomes and mito- 

 chondria results from his use of the Waring Blendor in the preparation of 

 homogenates. Since it is well known that this method of homogenization 

 results in the disruption of a significant proportion of the mitochon- 

 (jj.j^ 8,63.136 ^]^g microsomal fraction isolated from such homogenates would 

 be expected to contain mitochondrial fragments and thus to exhibit quali- 



1" G. A. LePage and W. C. Schneider, /. Biol. Chem. 176, 1021 (1948). 



