STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 87 



Strittmatter and Ball (1952) have suggested that this system plays an 

 important role in the reductive syntheses of lipids. The microsomal fraction 

 of various plants has also been found to contain a hemeprotein, called 

 cytochrome bg, which is localized exclusively in this fraction (Martin and 

 Morton, 1957). This cytochrome is reduced enzymatically by the addition of 

 reduced DPN. 



Many of the key steps of cholesterol synthesis, as well as the hemochromo- 

 gen, do appear to be centered in the vesicular structures of the microsomal 

 fraction (Bucher and McGarrahan, 1956). In short term experiments with 

 injected acetate-C^^, over 90 % of the formed cholesterol is formed in the 

 microsomal fraction. However, in in vitro systems, the synthesis of cholesterol 

 requires the participation of glycolytic systems and oxygen as well as other 

 systems derived from the soluble fraction. Squalene accumulates in the 

 absence of air and this hydrocarbon is converted to cholesterol only in the 

 presence of oxygen. 



Mueller and Miller (1949) and Brodie and his collaborators (1955; Brodie, 

 1956; GiUette et al., 1957) have studied the detoxication of drugs by liver 

 microsomes. Only nonpolar compounds are degraded by these metabolic 

 systems and it may be imagined that this has provided a mechanism for the 

 survival of "normal" metabohtes, which are predominantly polar in char- 

 acter. However, these systems have not been found in microsomes of other 

 cells. These reactions involve TPNH and oxygen and the TPNH oxidase 

 system involved is not cyanide-sensitive. The end product of the TPNH 

 oxidase acti\aty is hydrogen peroxide. The system is also inhibited by 

 detergents. 



Of outstanding interest to this discussion is the fact that the microsomal 

 fraction is a major site of protein and RNA synthesis.^ Various aspects of 

 these polymer syntheses will be discussed in detail below. 



F. Enzymatic Activities of Cell Sap 



This fraction is rich in protein and it may be suspected of containing many 

 enzymatic activities. Many enzymes have indeed been found. Phosphorylase 

 is mainly contained in this fraction, as is the largest part of cell glycogen 

 (Sacks et al., 1957). Glycolytic enzymes are found largely in this fraction, 

 although certain enzymes, such as hexokinase, may be bound in considerable 

 degree to sedimentable material in particular organs. Enzymes of the 

 oxidative hexose monophosphate shunt are also present in this fraction, as 

 are some enzymes of the tricarboxyHc acid cycle, e.g., isocitric dehydrogenase, 



^ It has been discovered that the heavier components of a homogenate of hen oviduct 

 appear most active in protein synthesis (Hendler, 1956). On closer investigation, it 

 appears that the endoplasmic reticulum of this tissue is better preserved after homogen- 

 ization, in contrast to rat liver, and is therefore sedimented rather readily (Hendler et al., 

 1957). 



