Studies on Microsomal Cytochromes 469 



cytochrome h^ molecule is in question, since, as crystallized by Appleby and 

 Morton (1954), the molecule contained one unit each of riboflavin phosphate 

 and haem and possessed lactate dehydrogenase activity, while a crystalline 

 preparation of Yamashita et al. (1957) was devoid of intrinsic flavin or lactate 

 dehydrogenase activity. 



POSSIBLE BIOLOGICAL SIGNIFICANCE 

 The firm association of the microsomal cytochrome in significant and 

 rather constant amounts with specific metabolically-active intracellular 

 structures in the liver and possibly in other tissues bespeaks for it specific 

 biological roles, probably catalytic in nature, but it is not yet possible to 

 define with assurance the particular biological significance of this cytochrome. 

 However, it might be profitable to review again in the light of present in- 

 formation the several possible roles which we considered in earlier studies to 

 merit special consideration (Strittmatter, 1952; Strittmatter and Ball, 1952, 

 1954). 



Storage Form or Precursor 



The microsomal cytochrome might represent a storage form of haemin or 

 a precursor in the formation of other cytochromes or haemin components, 

 but there is no direct evidence that it serves in such relatively inert capacities. 

 Such a role was early suggested for cytochrome b^ in certain micro-organisms 

 (Keilin, 1933), and was apparently supported by observations that the 

 typical 'muscle-type' cytochrome spectrum of aerobically grown resting yeast 

 cultures is altered during anaerobic growth or in a phase of exponential 

 aerobic growth, with replacement of the cytochrome b and c bands by a b^ 

 band at about 555 m// (Ephrussi and Slonimski, 1950; Chaix and Heyman- 

 Blachet, 1957). However, low temperature studies suggest that the Z>i band 

 represents a fusion of bands (Chaix and Heyman-Blachet, 1957). It is likely 

 that essentially all intracellular haem is maintained in bound forms, since 

 addition of low concentrations of free haematin to cell-free preparations 

 markedly inhibits oxidative activities (Keilin and Hartree, 1947). Protohaem 

 might therefore be stored in bound forms with nitrogenous components, 

 most probably proteins, to form haemochromes that possess absorption 

 spectra very similar to that of the microsomal cytochrome, and in the 

 absence of more specific identification such bound forms might in some 

 preparations be mistaken for the cytochrome. 



Active Role in Electron Transfer 



The specific and highly active oxidation-reduction properties of the micro- 

 somal cytochrome suggest for it a more active role in biological oxidation- 

 reduction reactions. Liver microsomes possess a high capacity for oxidation 

 of DPNH and TPNH; indeed, the DPNH-cytochrome c reductase activity 



