THE CYTOPLASM 



225 



TABLE V 

 Cytochrome Oxidase Activity of Liver Fractions^^ 



° 23.7 mg. of nitrogen per g. of whole tissue. 



*" Specific activity = 4.15 /iM. of reduced cytochrome c oxidized per min. per mg. of total nitrogen at 24°. 



' 9.4 Mg. of DNA phosphorus per mg. of total nitrogen. 



"^ 11.2 X IQi" mitochondria per g. of whole tissue. 



and Ochoa^'^ is carried out within the mitochondria. Thus, if fluoroacetate 

 is injected into female rats, a procedure resulting in the accumulation in 

 the livers of relatively large amounts of citric acid, and the livers are then 

 homogenized in sucrose solutions and fractionated in the centrifuge, most 

 of the citric acid is recovered in the mitochondrial fraction (Table IV). 

 The synthesis of citrate by isolated kidney mitochondria has been demon- 

 strated by Kalnitsky.^'^ Glutamic dehydrogenase, which is closely related 

 to the Krebs cycle and requires DPN or TPN as a coenzyme, is largely 

 recovered in the mitochondrial fraction (Table II). By means of experi- 

 ments similar to those described for cytochrome oxidase (Table V), it was 

 possible to demonstrate that glutamic dehydrogenase is solely confined to 

 mitochondria.*^ Although complete data are not available on the intra- 

 cellular distribution of /3-hydroxybutyric dehydrogenase, this DPN-re- 

 quiring enzyme, according to Lehninger,^"'^ is also localized in mitochondria. 

 It would be gratifying from a teleological standpoint if all the enzymes 

 of the Krebs cycle were locahzed in mitochondria. In spite of statements 

 to this effect (cf. footnote 13), the data indicate that some of these enzymes, 

 isocitric dehydrogenase^ and aconitase^^^ being notable examples, are 



1" J. R. Stern and S. Ochoa, /. Biol. Chem. 191, 161 (1951). 



1'* G. Kalnitsky, /. Biol. Chem. 179, 1015 (1949). 



"* A. L. Lehninger, personal communication. Recent work by S. R. Dickman and 

 J. F. Spe\^er [J . Biol. Chem. 206, 67 (1954)], published since this manuscript went 

 to press, indicate that under certain conditions of enzyme assay, aconitase ap- 

 pears to be concentrated to some e.xtent in the mitochondrial fraction. 



