462 C. F. Strittmatter 



Although the 'liver-type' cytochrome spectrum was subsequently observed, 

 with some variations, in homogenates or particulate preparations from 

 several mammalian tissues, its significance remained uncertain. The centre 

 of the broad band, whose full development required the presence of a fairly 

 strong reducing agent, such as dithionite, fell in the range of 555-560 m/f, 

 thus coinciding approximately with the absorption maximum of the microbial 

 component known as cytochrome b^ (Keilin, 1934) or b' (Fujita and Kodama, 

 1934). Keilin and Hartree (1940) consequently referred to the broad band 

 of the 'liver-type' spectrum as the bi band, and Yoshikawa (1951) ascribed 

 the band to a haemochrome-like cytochrome b', but the component or 

 components responsible were not more definitely established. The early 

 microbial studies (Fujita and Kodama, 1934) had indicated that at least some 

 b^-\[ke bands resulted from a fusion of the a-bands of cytochromes b and c, 

 and the studies by Keilin and Hartree (1949) of low temperature spectra 

 suggested that b^ bands in some cases reflected presence of cytochromes b 

 and c and a third haematin component absorbing between b and c, possibly 

 cytochrome e, now known as cytochrome fj. Slater (1949) also pointed out 

 that in some instances absorption between the bands of cytochromes b and 

 c may reflect presence of contaminants such as denatured protein-proto- 

 haemochrome, with an a-band at about 558 m/i. Extracts of liver prepara- 

 tions with organic solvents also showed a haemochrome spectrum with 

 maximum at 560 m/t which was considered to be possibly referable to 

 cytochrome b (Kun, 1951). 



Localization of a Distinct Microsomal Cytochrome 



The nature of the intrinsic cytochrome components responsible for the 

 Miver-type' haemochrome spectrum was established in the course of studies 

 in our laboratory on the intracellular distribution of cytochrome components 

 and oxidative enzymes in rat liver (Strittmatter and Ball, 1952, 1954). Spec- 

 troscopic and spectrophotometric examination of washed cellular particles 

 isolated from homogenates of perfused rat liver by differential centrifugation 

 indicated that a complete 'muscle-type' cytochrome system, with absorption 

 maxima at 605, 562, 551 and 520-530 m^i in the reduced state, was localized 

 in the mitochondria, although the cytochrome components were present in 

 lower concentration than in muscle. The localization of these cytochrome 

 components in the mitochondria was correlated with the concentration in 

 this particulate fraction of cytochrome oxidase and other oxidative enzyme 

 activities involving terminal electron transport to oxygen. The microsome 

 fraction, which showed only low capacities for terminal respiration, possessed 

 no spectroscopically discernible amounts of the typical cytochrome com- 

 ponents of the 'muscle-type' cytochrome system, but contained instead as its 

 major intrinsic pigment a cytochrome component with a-band at 557 m// 

 which was not observable in the mitochondria. The concentration of the 



