Cell Structure and Metabolic Regulation 25 



on the intracellular membranes, the mitochondrial membrane 

 and the membranes of the ER, for it is evident that con- 

 comitantly with the divisions which they produce in the cell 

 these membranes can be important regulators of cellular 

 metabolism. 



After a good many years, it was finally shown (Watson and 

 Siekevitz, 1956; Siekevitz and Watson, 1956) what a good 

 many biochemists had long suspected (cf. references in 

 Siekevitz and Watson, 1956): that the succinoxidase system 

 and most, if not all, of the electron transport system of the 

 mitochondria constituted a structural part of the membrane. 

 Fig. 10 shows such a mitochondrial membrane preparation 

 and Table I gives the relevant data as to the concentrations of 

 phospholipid, succinoxidase, and cytochrome oxidase in this 

 membrane fraction. Together with previous results, these 

 latest ones should leave no doubt concerning the intimate 

 localization of these enzymes. 



Table I 



Some chemical and enzymic properties of 

 mitochondria and mitochondrial membranes 



mg. Protein/g. mg. Phospholipid fg. mg. Phospholipidlmg. 

 liver liver protein 



Mitochondria 9-50 2-12 0-22 



Mitochondrial 0-56 0-33 0-59 



membrane 



Succinoxidase Cytochr. oxid. 



DPNH-cyt. Adenyl. kinase 



I V ^ 



Act, Sp. act. Act. Sp. act. c red. Act. Act. 



Mitochondria 1320 868 233 153 492 90 



Mitochondrial 498 5550 79 878 



membrane 



Data from Sieke\atz and Watson (1956). 



Now in the last several months. Dr. Ernster of the Wenner- 

 Grens Institute, Stockholm, working in our laboratory, has 

 succeeded in isolating microsomal membranes, or rather, in 

 reforming membranes. Palade and the present author 

 (Palade and Siekevitz, 1956a) had previously found that liver 



