(;ii.i5i:rt ii. mudge 113 



aiul eleclroU-le cxchangeabiliiy raises ihe question of whellier or nol such intra- 

 cellular potassium fractions may nol be associated with the mitochondria. 



By the use of electron microscopy, (he line structure of the mitochondria has 

 been greatly clarified. In mammalian kidney and liver an internal system of 

 membrane-like ridges subdivides the amorphous central matrix into a number of 

 compartments. Of particular interest to problems of permeability and of trans- 

 port is the demonstration that the mitochondria have a clearly defined external 

 membrane. Pallade (14) estimates this to be 7 to 8 m^u in thickness and believes 

 that it is continuous with the internal ridges which protrude from its inner 

 surface. Most preparations of mitochondria are found to have a water content 

 of 80% or higher (7). The dried solids are about 60% protein and 25% lipid 

 (2). Since these particles are cai)able of catalyzing an enormous number of re- 

 actions, they must contain a comparable number of specific enzymes, as well as 

 a variety of co-enzymes and metabolic intermediates. 



POTASSIUM MET.\BOLISM 



In mitochondria isolated from homogenates of rabbit liver by differential cen- 

 trifugation, Stanbury and Mudge (17) found about 0.8 mEq of potassium per 

 gram of nitrogen- and observed that this amount of potassium could not be re- 

 moved by repeated washings in cold 0.15 m/1. NaCl.'' When the mitochondria 

 were washed with cold 0.15 m/1. KCl, they were found to contain about 0.3 

 mEq of sodium per gram of nitrogen. These values are very similar to those 

 obtained by Spector (15) who used sucrose solutions for washing the mito- 

 chondria for the determination of both sodium and potassium. The constancy of 

 these values following repeated washings suggests that a rather fi.xed amount 

 of electrolyte adheres to the mitochondria when prepared in this manner. 



E.xperiments designed to study the metabolism of sodium and potassium in 

 isolated mitochondria have in general been planned as follows: the mitochondria 

 are suspended in an incubation medium in Warburg flasks; they are then in- 

 cubated under the desired conditions; and subsequently they are promptly 

 separated from the suspending medium by centrifugation. When the rate of 

 potassium exchange between liver mitochondria and the suspending medium 

 was determined with isotope techniques, Stanbury and Mudge (17) found that 



- Nitrogen constitutes almost exactly 10% of the dry weight of mitochondria, and this 

 value is constant following incubation under different conditions. Electrolyte values are cal- 

 culated on the basis of nitrogen, even though in some of the experiments reported here the 

 dry weight alone was determined. 



^Biochemically distinct tyjies of mitochondria have been described (13), but there is no 

 evidence that this degree of heterogeneity is of signiticanpe for the type of problems discussed 

 here. Slight contamination of the mitochondrial fraction by nuclei cannot be excluded. How- 

 ever, the electrolyte metabolism of nuclei, or of sub-cellular particles other than the mito- 

 chondria, has not been examined. 



