238 GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



munication to the writers, have reported that the enzyme hydrolyzing 

 cholesterol acetate is localized exclusively in liver microsomes. It is of 

 interest that the latter group of investigators^*^ have recovered most of the 

 free cholesterol of liver in the microsomal fraction and all of the esterified 

 cholesterol in a lipid fraction that migrates centripetally in the centrifuge. 



Although other complete studies of the distribution of lipids in liver 

 homogenates apparently have not been reported, investigations of in- 

 dividual fractions have indicated that the microsomes contain much higher 

 concentrations of lipid than does any other cell fraction.^^-^**'** Approxi- 

 mately 40% of the dry weight of microsomes is comprised of lipid, which is 

 mainly in the form of phospholipid. The fact that liver microsomes are 

 pigmented, showing a distinctly reddish hue, has also interested a number 

 of investigators. Bensley'*® first suggested that the pigment represents the 

 products of oxidation of unsaturated lipids, particularly phospholipids, 

 but later^*'' stated that the microsomal color is due to adsorbed hemoglobin. 

 Strittmatter and Ball,'** however, have presented evidence indicating that 

 it is a protoporphyrin hemochromagen resembling but nevertheless distinct 

 from cytochromes a, b, and c. 



Obviously, it is not at present possible to present anything like a com- 

 plete picture of the role of microsomes in cellular metabolism. Except for 

 their apparent ability to transfer electrons between cytochrome c and the 

 pyridine nucleotides, the particles evidently do not play an important part 

 in oxidative or respiratory metabolism. The high PNA content of the frac- 

 tion and an apparent relationship between PNA and protein synthesis 

 (Chapter 28) have, however, led to some speculation concerning the possible 

 role of microsomes in the synthesis of proteins. In this respect, more recent 

 in vivo experiments have shown that after injection of labeled amino acids, 

 the proteins of microsomes show a higher specific activity than do the 

 proteins of any other fraction isolated from liver. '*^''^'' Siekevitz'"" has 

 extended these observations by showing that, when respiring liver ho- 

 mogenates are incubated in the presence of alanine-C'^, both the rates 

 of incorporation and the total incorporation of the amino acid are much 

 higher in microsomes subsequently isolated from the reaction mixture than 



'82 M. C. Schotz, L. I. Rice, and R. B. Alfin-Slater, J. Biol. Chem. 204, 19 (1953). 

 183 G. L. Ada, Biochem. J. 45, 422 (1949). 



'8'* C. P. Barnum and R. A. Huseby, Arch. Biochem. and Biophys. 19, 17 (1948). 

 '85 R. A. Huseby and C. P. Barnum, Arch. Biochem. and Biophys. 26, 187 (1950). 

 '86 R. R. Bensley, Anat. Record 98, 609 (1947). 

 '8'' R. R. Bensley, J. Histochem. and Cytochem. 1, 179 (1953). 

 '88 C. F. Strittmatter and E. G. Ball, Proc. Natl. Acad. Sci. U. S. 38, 19 (1952). 

 '83 E. B. Keller, Federation Proc. 10, 206 (1951). 



'9" N. D. Lee, J. A. Anderson, R. Miller, and R. H. Williams, J. Biol. Chem. 192, 

 733 (1951). 



