236 GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



It may be noted that the properties of the mitochondrial membrane are 

 of great importance in a consideration of the vaUdity of enzyme assays. 

 Thus in studies of the distribution of enzymes among cell fractions it would 

 now appear to be imperative to determine routinely by disruption of the 

 mitochondria whether the membrane acts as a barrier between substrate 

 and enzyme. 



3. The Microsomal Fraction 



An event of major importance in both cytology and cytochemistry was 

 Claude's^^ isolation of a particulate fraction from tissue extracts by means 

 of prolonged centrifugation at gravitational forces of the order of 20,000. 

 Although these particles were at first thought to be mitochondria, it soon 

 became apparent that they were too small to be resolved by the light micro- 

 scope, and they were therefore referred to as submicroscopic particles or 

 microsomes. This cell fraction accounts for a considerable proportion of the 

 total dry weight or nitrogen of most tissues,^ — ^as much as 20 to 25% in 

 the case of rat or mouse liver. It was recognized fairly early in semiquantita- 

 tive analyses of the microsomes that the particles contained comparatively 

 high concentrations of pentose nucleic acid and phospholipid,^^ -^^ but ad- 

 ditional information concerning their biochemical properties has not been 

 available until recently and is still rather meager. 



Table VII lists those properties of the microsomal fraction that have 

 been found in a concentration exceeding that in the homogenate. Omachi 

 and coworkers^^^ were the first investigators to offer clear-cut evidence that 

 an enzyme, namely, an esterase (methylbutyrase) , was associated with the 

 microsomes. Shortly thereafter, the important electron-transporting sys- 

 tem, DPN-cytochrome c reductase, was also found to be concentrated in 

 the fraction. ^^ Until Hers et al."^ described their work on the specific glucose- 

 6-phosphatase of hver, however, none of the properties studied appeared 

 to be confined exclusively to microsomes. Thus the cytochrome c reductase 

 and uricase were also concentrated in the mitochondrial fraction, PNA and 

 esterase were diffusely distributed among all other cell fractions, and DPN 

 nucleosidase was found in the nuclear and soluble fractions. The recovery 

 of glucose-6-phosphatase in the microsomes, on the other hand, was nearly 

 complete, the small amount of activity remaining in the other fractions 

 being readily explained on the basis of contamination by microsomes. A 

 similar situation appears to hold for triacetic acid lactonase of rat kidney, 

 as reported by Meister.^^" Schotz, Rice, and Alfin-Slater, in a personal com- 



"8 A. Omachi, C. P. Barnum, and D. Glick, Proc. Soc. Exptl. Biol. Med. 67, 133 (1948). 

 1" H. G. Hers, J. Berthet, L. Berthet, and C. de Duve, Bull. soc. chim. biol. 33, 21 



(1951). 

 18" A. Meister, Science 115, 521 (1952). 



