86 S. S, COHEN 



currency, ATP, and other high energy phosphates, a number of different 

 biochemical mechanisms are available for the conservation of the energy 

 derived from energy-yielding reactions and for the formation of ester and 

 peptide hnkages, as well as carbon to carbon bonds. Reactions associated 

 with mitochondria, leading to the esterification of sulfate, fixation of COg, 

 and the formation of glutathione will be discussed in later sections. 



E. Enzymatic Systems in the Microsomal Fraction 



It was not mitil 1951 that an enzyme was found to be concentrated in this 

 fraction. Hers and collaborators (1951) reported on the distribution of 

 glucose-6-phosphatase and, as presented in Table X, it appeared that essen- 

 tially all of this enzyme in liver was present in the microsomal fraction. A 

 similar situation exists for kidney lactonase (Meister, 1952). 



Glucose-6-phosphatase is not inactivated by RNAase and may be solubil- 

 ized in an active nonsedimentable form by deoxycholate. Its inactivation by 

 trypsin and a lecithinase suggests a close relationship with hpoprotein, 

 presumably on the membranes of the endoplasmic reticulum (Beaufay and 

 de Duve, 1954). 



In the 48-hour fastmg rat, under conditions in which hver glycogen is 

 reduced to 5 % of the normal value, the glucose-6-phosphatase activity per 

 unit weight of hver is increased 60 % (Weber and Cantero, 1954). It is poss- 

 ible that there has been a synthesis of enzyme under conditions in which 

 glycogen is being mobihzed, and it is of interest that this apparently adaptive 

 phenomenon appears to be associated with the membrane component of the 

 reticulum. 



Some oxidation-reductio2i systems have been found concentrated in the 

 microsomal fraction.^ These include DPN- and TPN-cytochrome c reductases. 

 However, although the DPNH and TPNH-cytochrome c reductase of mito- 

 chondria are inhibited by antimycin, this antibiotic has no effect on the 

 reductases of the microsomal fraction. Cytochrome c is not present in the 

 microsomal fraction nor is it certain that cytochrome c reaUy exists outside 

 of mitochondria. It therefore seems likely that these reductases transfer 

 electrons from DPNH to other acceptors. The hver microsomal fraction has 

 a reddish hue, and a slowly autoxidizable protoporphyrin hemochromogen, 

 now termed cytochrome m, distinct from cytochromes a, b, and c, was 

 demonstrated in this fraction by Strittmatter and BaU (1952). More recent 

 work of Strittmatter and Vehck (1956) has shown that cytochrome m is the 

 true microsomal electron acceptor for the DPNH reductase of this fraction. 



^ Thiers and Vallee (1957) have recently analyzed subcellular fractions for 8 cations. 

 Microsomes are low in Cu and ]VIn and relatively rich in Fe, suggesting the presence of 

 Fe hemochromogens (see Fig. 5). 



