414 



N. M. SISAKYAN 



example, we have obtained data for the invertase of the plastids of the sugar 

 beet (Table 6). 



Table 6 



Changes in the firmness of the bond between invertase and the 

 protein complex of the plastids 



Thus, with reference to the form of the bonding of enz>Tnic systems with 

 structural elements of cells we may distinguish at least four main categories: 

 a group of enzymes which is active only when associated with structures; 

 enzymes which are present in the absence of structures; enzymes which lose 

 their activity when their association with structures is interfered with; and 

 enzymes which lose their catalytic activity in the bound state. 



CONCLUSIONS 



On the basis of the numerous experiments which have been done during 

 recent years with cellular fractions from rats' livers, there emerges a rough 

 picture of the localization of biochemical processes within the cell. 



In the nuclear fraction of the animal cell we find the enzymes responsible for 

 nucleotide metabolism — adenosine 5-phosphatase, adenosine deaminase, DPN- 

 nucleosidase, nucleoside Phosphorylase and others. Furthermore, the nucleus 

 is the site of synthesis of the basic nucleotide and uridine coenzymes (DPN and 

 UTP), which take part in glycolytic processes, which would appear to occur in 

 the soluble cellular fraction. It is quite likely that the enzymes of glycolytic 

 metabolism are also localized within the nuclei of the cells but are washed out 

 of them during the process of isolation. Enzymes of the glycolytic system have 

 been found in nuclei which have been isolated in a non-aqueous medium. 



The mitochondria form the special metaboUc centre of the cell. In them arc 

 localized the fundamental processes associated with oxidative energy metabolism, 

 namely the tricarboxylic acid cycle, the cytochrome system, the system of 

 oxidative phosphorylation, the oxidation of fatty acids. In addition the mito- 

 chondria take part in the metabolism of amino acids and lipids. Several syntheses 

 (e.g. that of urea) cither begin or culminate in them. The activity of many of 

 the enzymes of the mitochondria, especially those which are closely associated 

 with their structure, depends on the conditions of the surrounding medium. 

 Great changes are brought about in the activity by alterations in the osmotic 

 pressure, concentration of electrolytes and other factors which affect the struc- 

 ture of the mitochondria. 



