Studies on Miciosonial Cylocfironies 463 



microsomal cytochrome was of the order of 6 x 10^° molcs/mg protein in 

 rat Hver microsomes, accounting for nearly 50% of the total haem found in 

 these particles, and no other haemochrome component was discernible 

 provided that the liver had been perfused, or the isolated microsomes 

 adequately washed to reduce to a minimum such contaminants as adsorbed 

 haemoglobin or denatured protein haemochromes. The microsomal cyto- 

 chrome was a firmly bound, intrinsic component and could be removed 

 from its particulate binding only by enzymic digestion or by treatment with 

 lipid solvents or bile salts, whereas a single washing sufficed to remove 

 readily adsorbed contaminants such as haemoglobin (Strittmatter and Ball, 

 1952; Strittmatter, 1952 and unpublished work). As it was specifically 

 concentrated in the microsome fraction, the microsomal cytochrome was 

 tentatively labelled 'cytochrome /;/', pending its more definitive character- 

 ization (Strittmatter, 1952; Strittmatter and Ball, 1954). The haemochrome 

 absorption spectrum of liver was therefore quantitatively interpretable as 

 due to masking of the rather weak 'muscle-type' cytochrome spectrum of the 

 mitochondria by the relatively intense absorption by a greater concentration 

 of the microsomal cytochrome. 



Properties of Particulate Microsomal Cytochrome 



The properties and possible biological function of the microsomal cyto- 

 chrome were first examined in isolated rat liver microsomes, with the cyto- 

 chrome still in particulate form and presumably still in essentially physiologic 

 state (Strittmatter and Ball, 1952, 1954; Strittmatter, 1952). In the oxidized 

 state, the visible range of the spectrum showed only a slight broad hump 

 with maxima at about 530 and 565 m/n and an intense Soret band at 414 m/(. 

 When reduced by addition of dithionite to a suspension of microsomes, the 

 typical haemochrome spectrum of the cytochrome appeared, with a sharp 

 a-band at 557 m/n, a ^-band at 527 m^ and the Soret band shifted to 

 423 m/Li. Neither carbon monoxide nor cyanide (10~- m) at neutral pH altered 

 the oxidized or reduced spectra appreciably. The prosthetic group of the 

 microsomal cytoclii-ome was identified as iron protoporphyrin from spectro- 

 scopic studies on the haem extracted from the microsomes and from the 

 spectrum of the pyridine haemochrome formed with intact microsomes. 

 The apparent standard potential of the microsomal cytochrome in its parti- 

 culate complex was roughly estimated to be about —0-12 V at pH 7 from 

 reduction titrations in the presence of oxidation-reduction indicators. 



As a microsomal component, the cytochrome was reduced rapidly and 

 essentially completely by reduced di- and tri-phosphopyridine nucleotides 

 (DPNH and TPNH), suggesting that it may serve as acceptor of electrons 

 from reduced pyridine nucleotide-cytochrome reductases concentrated in 

 the rhicrosomes. The cytochrome was also readily reduced by cysteine and 

 partially reduced by ascorbate, but was not appreciably reduced by succinate 



