THE CYTOPLASM 227 



chondria are choline, betaine aldehyde, and tyramine oxidases. According 

 to Claude, '^^ D-amino acid oxidase is also concentrated in mitochondria, 

 but complete data supporting this statement have not been published. 

 Dianzani'^^ has reported the recovery in mitochondria of 60 % of the a-gly- 

 cerophosphate dehydrogenase of liver homogenates (cf. footnote 137), 

 but the enzyme assay was probably inadequate in that it was based on 

 oxygen uptake determinations without added cofactors such as DPN and 

 cytochrome c. Furthermore, this finding could not be confirmed in un- 

 published experiments in the writers' laboratory, since the reverse reaction, 

 i.e., the oxidation of reduced DPN in the presence of dihydroxyacetone 

 phosphate, was largely carried out by the soluble fraction of liver homog- 

 enates. 



Of the other enzymes listed in Table III, several deserve special com- 

 ment. Recent experiments have suggested that uricase, which is con- 

 centrated in both the mitochondrial and microsomal fractions, is associated 

 with particles devoid of respiratory enzyme activity and containing much 

 more PNA than the mitochondrial fraction. ^^-^^^ This finding emphasizes 

 the importance of studying the biochemical homogeneity of the cell frac- 

 tions, a problem that will be discussed in a later section. 



It is rather surprising that ribonuclease and deoxyribonuclease should be 

 concentrated in the mitochondrial fraction, since the native substrates 

 for both of these enzymes are present in other cell fractions. It should be 

 noted, however, that Lang et al.,^^° using a viscosimetric method of enzyme 

 assay, have reported that the deoxyribonculease of kidney is mainly re- 

 covered in the nuclear fraction. The determination of the deoxyribonu- 

 clease activity concentrated in liver mitochondria was based on the rate 

 of release from highly polymerized DNA of ultraviolet-absorbing fragments 

 soluble in dilute perchloric acid. The possibility must therefore be borne 

 in mind that successive steps in the depolymerization of DNA may be 

 carried out by more than one type of cell structure. 



The synthesis of p-aminohippuric acid, a reaction somewhat analagous 

 to the synthesis of a peptide bond and studied in detail by Cohen and Mc- 

 Gilvery,!^^ was found by Kielley and Schneider^" to be almost entirely 

 localized in mitochondria (Table III). Leuthardt and Nielsen^^- have re- 

 ported the synthesis of hippuric acid and Muller and Leuthardt^"*-'^' the 

 synthesis of citrulline by isolated mitochondria. 



'" A. Claude, Am. Assoc. Advancement Sci. Research Conf. on Cancer p. 223 (1944). 



138 M. U. Dianzani, Arch, fisiol. 50, 187 (1951). 



"9 E. L. Kuff and W. C. Schneider, J. Biol. Chem. 206, 677 (1954). 



i^o K. Lang, G. Siebert, I. Baldus, and A. Corbet, Experientia 6, 59 (1950). 



1" P. P. Cohen and R. W. McGilvery, J. Biol. Chem. 171, 121 (1947). 



i« F. Leuthardt and H. Nielsen, Helv. Chim. Acta 34, 1618 (1951). 



i« A. F. Muller and F. Leuthardt, Helv. Chim. Acta 32, 2289 (1949). 



