Studies on Microsomal Cytochromes 465 



the enzyme, as a result of differential complex formation of the oxidized and 

 reduced form of the nucleotide with the enzyme protein (Theorell and 

 Bonnichsen, 1951; Hayes and Velick, 1954). Alternatively the cytochrome 

 may have been altered during its isolation, perhaps in the incompletely defined 

 enzymic liberation from its particulate complex, although the retention of 

 specific enzymic activities and other properties provides no evidence of such 

 alteration. 



The isolated microsomal cytochrome was readily reduced by such reducing 

 agents as dithionite and high concentrations of cysteine. Unhke the particu- 

 late form, the purified cytochrome was not directly reduced by DPNH or 

 TPNH, and reduction by DPNH or TPNH required the presence of cyto- 

 chrome reductases, which have been obtained in soluble form from liver 

 microsomes (Strittmatter and Velick, 1956a, b). A flavin adenine dinucleotide 

 (FAD)-linked reductase specific for both DPNH and microsomal cytochrome, 

 but essentially inactive with TPNH as electron donor or cytochrome c as 

 acceptor, was highly purified from calf liver microsomes (Strittmatter and 

 Velick, 1957), and a reductase of similar specificities has been obtained from 

 pig liver particles (Mahler, Raw, Molinari and doAmaral, 1958). On the 

 other hand, the microsomal cytochrome was not reduced by typical DPNH- 

 cytochrome c reductase preparations from heart muscle (Strittmatter and 

 Vehck, 1956b). The reduced form of isolated microsomal cytochrome was 

 very rapidly oxidized by cytochrome c, with a direct cytochrome-to-cyto- 

 chrome electron transfer, by ferricyanide, and by various dyes of appropriate 

 potential; it was also oxidized, less rapidly, by oxygen (Strittmatter and 

 Vehck, 1956a). The highly active DPNH-microsomal cytochrome reductase 

 and the microsomal cytochrome therefore form an efficient microsomal 

 system for electron transport from DPNH to cytochrome c and this pathway, 

 which is antimycin A-insensitive, can account for at least one-half of the 

 total DPNH-cytochrome c reductase capacity observed with isolated micro- 

 somes and added cytochrome c (Strittmatter and Vehck, 1956b). 



In addition to the isolation from rabbit liver microsomes, the microsomal 

 cytochrome has been purified from liver microsomes of pig (Garfinkel, 1957; 

 Krisch and Staudinger, 1958), rat (Strittmatter, unpublished work) and calf 

 (Strittmatter and Velick, 1957), by somewhat similar procedures, and, 

 possibly, from pig liver particles (Raw, Mohnari, doAmaral and Mahler, 

 1958). In so far as the preparations have been compared, the purified cyto- 

 chromes from the various sources appear to be essentially the same protein, 

 with properties referable to a particular type of group b cytochrome. In 

 current usage, the term 'cytochrome b-^ is widely used in referring to the 

 microsomal cytochrome of mammalian liver, and to a number of apparently 

 similar pigments in other biological materials. 



