26 INTERMEDIARY METABOLISM AND GROWTH I 



character. The dehydrogenases, if actually present in the nucleus, presumably 

 function as mutases. 



Mention has been made in the previous section of the postulated electron trans- 

 port system of plants mediated by glutathione reductase, dehydroascorbic 

 dehydrogenase, and ascorbic oxidase. These enzymes are apparently also localized 

 in the soluble cytoplasm of the cell (Mapson and Moustafa, 1956; Rail and Lehnin- 

 ger, 1952). The flavin enzx'mes, xanthine oxidase and aldehyde oxidase, are found 

 in the soluble cytoplasm of liver. 



Microsomes represent an active site of protein and cholesterol synthesis. The 

 oxidation of certain drugs has been observed in liver microsomes (Brodie et al., 

 1 955) and the enzymatic steps for the 2 1 -hydroxylation of i 7-a-hydroxyprogesterone 

 have been shown in beef adrenal microsomes (Ryan and Engel, 1956). Interest- 

 ingly enough liver microsomes actively reduce exogenous cytochrome c via TPNH 

 or DPNH linked reactions, although cytochrome c and cytochrome oxidase are 

 absent from microsomal particles (Hogeboom et al., 1953; Strittmatter and Ball, 

 1954). However, a cytochrome pigment, designated as cytochrome m, with reduced 

 absorption spectra showing peaks at 5570, 5270, and 4240 A is to be found in liver 

 microsomal suspensions (Strittmatter and Ball, 1954). The E'^ of cytochrome m 

 was estimated to be — 0.12V. This low oxidation-reduction potential might favor 

 the participation of cytochrome m in reductive synthetic reactions. 



Liver cytochrome m has recently been solubilized with pancreatic lipase and 

 purified (Strittmatter and Velick, 1956a). The molecular weight was estimated to 

 be 16,900 and the concentration of flavin not significant. The standard potential 

 of the purified cytochrome m, calculated on the basis of equilibrium measurements, 

 was + 0.02V, in contrast to the value estimated for the cytochrome m of the mi- 

 crosomal suspension (Velick and Strittmatter, 1956). In the presence of either 

 TPNH or DPNH, microsomal suspensions are capable of reducing cytochrome 

 m (Strittmatter and Velick, 1956b). A reductase specific for both DPNH and for 

 inicrosomal cytochrome m which was completely inactive with TPNH as electron 

 donor or with cytochrome c as electron acceptor has been isolated. Exogenous 

 cytochrome c is however reduced by the reduced microsomal cytochrome m, 

 thereby completing a DPNH-cytochrome c transfer. The microsomal cytochrome 

 m reductase contains flavin but the role of this nucleotide in electron transfer has 

 not as yet been established. The cytochrome m reductase is capable of reducing 

 2,6-dichlorophenolindophenol, thereby exhibiting the so called diaphorase activity. 



Several of the energy yielding cycles are localized in the mitochondria. Thus, 

 the enzymes concerned with tricarboxylic acid cycle oxidations, the enzymes of 

 the fatty acid oxidative spiral, the succinic oxidase system, the electron transport 

 particle (Green and Beinert, 1955) all are found in the mitochondria fraction of cell 

 homogenates (Hogeboom et al., 1953). Isocitric dehydrogenase and aconitase, are, 

 however, notable examples of tricarboxylic acid cycle enzymes which have been 

 found in high concentration in the soluble cytoplasm of the cell. Mitochondria 

 particles manifest dehydrogenase activity towards various other substrates in- 

 cluding choline, betaine, D-amino acids, and amines. Catalase activity is also 

 found associated with the mitochondria. The nitrate reductase system of microor- 

 ganisms is localized in the particulate matter of the cell. 



