10 



SUBCELLULAR PARTICLES 



OXIDATIVE PHOSPHORYLATION 



ATP 



AMINO ACID ACTIVATION 



ATP 



+ 



AMINO AGIO 



i 



P P 



+ 



AMINO ACYL 



ADENYLATE 



1. 



Fig. 8. Schematic representation of complexity and interrelatedness of subcellular units in 

 the rat liver cell. [From (35).] Abbreviations: DPN, diphosphopyridine nucleotide; NMN, 

 nicotinamide mononucleotide; ATP, adenosine trijihosphate; PP, inorganic pyrophosphate; AH:;, 

 oxidizable substrate; A, oxidized substrate: ADP, adenosine diphosphate; P, orthophosphate; 

 RNA, ribonucleic acid. 



The work of Preiss and Handler (44) suggests that perhaps the reaction for 

 DPN synthesis which the diagram (fig. 8) shows, first described by Romberg 

 (19), may not be the important pathway of diphosphopyridine nucleotide (DPN) 

 synthesis in the cell. However, it is of interest that at least one of the steps of 

 DPN synthesis from free nicotinic acid (44) is also recovered in the nuclear 

 fraction (43). 



From the work Dr. Stephenson describes (49) there is little doubt that amino 

 acid incorporation, probably an integral part of protein synthesis, occurs in the 

 microsomal ribonucleoprotein granules, although as Loftfield (22) says in his 

 masterful review, "the final step in forming the protein may be disturbed" in 

 these cell-free systems. 



Irrespective of changes which will be required in this diagram, the data from 

 isolated subcellular fractions make the conclusion inescapable that subcellular 

 particles are interdependent metabolically. The diagram does not attempt to 

 show interrelationships among nucleotides, RNA and DNA (deoxyribonucleic 

 acid). Nor does it show lysosomes, dense bodies and other complexities of the 

 liver cell. These are shown diagrammatically in figure 9; this, too, omits some 

 known structures and greatly oversimplifies others. It is based not only upon 



