Studies on Microsomal Cytochromes 467 



cytoclirome-likc material in cell tVactions indicate that it is specifically 

 concentrated and firmly bound in the microsome fraction, suggesting that in 

 the intact cell the cytochrome is an intrinsic component of the endoplasmic 

 reticulum or related membranous structures which give rise to the microsome 

 fraction by the usual fractionation procedures. The possible presence of 

 microsomal cytochromc-like substances in other cell compartments is not 

 excluded, but direct and compelling evidence is lacking. There is indirect 

 evidence (Spiro and Ball, 1958; Bailie and Morton, 1955, 1958; Spiro, 1959) 

 that the cytochrome may appear in certain cytoplasmic 'granules', which in 

 at least some cases appear to be derived from the endoplasmic reticulum, 

 being formed by accumulation of newly synthesized material within the 

 vacuoles of the endoplasmic reticulum (cf. Schneider and Hogeboom, 1956; 

 Siekevitz and Palade, 1958). In addition, Raw and Mahler (1959) reported 

 that pig liver mitochondrial preparations contained cytochrome b^-\ike 

 material in appreciable concentrations, although the unambiguous designa- 

 tion of this material as an intrinsic mitochondrial cytochrome b^ was not 

 possible. Previous studies with rat liver had indicated that the mitochondrial 

 fraction contains either no discernible cytochrome b^ or only a small amount 

 attributable to microsomal contamination (Strittmatter, 1952; Strittmatter 

 and Ball, 1952, 1954; Chance and Williams, 1955). 



Bailie and Morton (1955) have reported that bovine mammary gland 

 microsomes in the presence of dithionite show a strong 557 m/* band of 

 cytochrome b^ but no other detectable haemochrome bands, while the 

 mitochondria of this tissue showed a 'muscle-type' cytochrome spectrum 

 with no evidence of the microsomal cytochrome. 



The predominant haemochrome-like band of the adrenal medulla was 

 variously reported in early studies to be at 559 m/n by Huszak (1942) or 

 561 m/ii by Tsou (1951) and was considered by them to represent cytochrome 

 b. Spiro and Ball (1958, 1961) have recently reported that the medullary 

 haemochrome material responsible for this absorption band is apparently 

 concentrated in a 'microsomal fraction' and in the 'epinephrine-rich granules', 

 which probably represent distinct entities and are partially separable from a 

 'mitochondrial fraction' in which cytochromes b, c, a and a-^ appear to be 

 concentrated. The apparent localization, absorption spectrum and ready 

 reducibility of the predominant medullary haemochrome material by DPNH, 

 cysteine and ascorbate suggest a possible relation to the liver microsomal 

 cytochrome, but the problem awaits a more rigorous characterization and 

 establishment of the unitary nature of the medullary pigment. The presence 

 of a microsomal cytochrome-like substance in the adrenal is indicated anew by 

 the recent isolation of a 'cytochrome b-^' from a 'microsomal fraction' of 

 whole pig adrenals by Krisch and Staudinger (1958). The properties of this 

 preparation in general closely resemble those of the isolated liver microsomal 

 cytochrome, although the question of possible inhomogeneity posed by 



