30 Philip Siekevitz 



previously (Siekevitz and Watson, 1956) were concentrated 

 in phospholipid over the original mitochondria, the same 

 cannot be said definitely about the microsomal membrane 

 preparation, for we could not find any concentration in 

 phospholipid in the microsomal membranes over the original 

 microsomes. In the case of the liver, the isolated microsomes 

 have a higher concentration of phospholipid than the original 

 homogenate (Palade and Siekevitz, 1956a), but this is not the 

 case with the pancreas (Palade and Siekevitz, 19566). For 

 these reasons, the mitochondrial membranes may be regarded 

 as being lipoprotein in nature, and the microsomal membranes, 

 the membranes of the ER, as being protein membranes. 

 These differences in composition might explain the differences 

 in manipulation which are necessary to obtain the membranes 

 from mitochondria (Watson and Siekevitz, 1956) and from 

 microsomes (cf. above, p. 25). Also, as can be seen from 

 Table III, the mitochondrial DPNH-cytochrome c reductase 

 was solubilized, like the microsomal enzyme, by DOC. But 

 when we tried diluting this supernatant with sucrose, in the 

 same way as we did for the DOC-treated microsomes, nothing 

 appeared upon recentrifugation. Since most of the mitochon- 

 drial cytochrome oxidase and succinoxidase had already been 

 centrifuged down as membranes, we are at a loss to decide 

 whether the mitochondrial DPNH-cytochrome c reductase 

 is not a membrane enzyme, or whether it is and is so damaged 

 that it cannot be reformed. 



Mention having been made of the identification of certain 

 enzymes with membranes, a phenomenon to which we have 

 alluded before (Siekevitz and Potter, 1955) is now described 

 in more detail. First, Table IV shows the peculiar results of 

 what at first glance should be a rather straightforward experi- 

 ment. As an increasing amount of hexokinase is added to a 

 medium containing adenosine triphosphate (ATP) and sub- 

 strate together with mitochondria, we obtain the familiar 

 picture of the increasing inorganic phosphate (P^) uptake into 

 glucose-6-phosphate (G-6-P) and an increasing oxygen uptake 

 due to the well known phosphate acceptor effect. However, 



