STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 85 



COOH 

 COOH COOH COOH CH, 



^2 



Soccinyl-SCoAl ^^^h — CO2, CH, ^CH^ CH^ 



^'y^'"" ^ CH2 CH, C C 



C=0 C=0 C CH 



Porphyrin 



HCNHj CH2NH2 (/h,NH2 



COOH 



oe- amino /3-l<.eto <x- amino Porphobilinogen 



adipic acid levulinic acid 



(11) 



The syntliesis of the key pyrimidine, orotic acid, from NHg, CO 2, aspartate, 

 and ATP is also effected by mitochondrial enzymes, whereas the further 

 conversion of orotic acid to the nucleotide, uridylic acid (UMP), occurs in a 

 soluble fraction (Hurlbert and Eeichard, 1954; Reichard, 1954). 



Other reactions of amino acid biosynthesis also occur in mitochondria. 

 These include the amination of a-ketoglutarate to glutamate, transamination 

 from amino acid to another keto acid, and a number of synthetic reactions 

 of the citruUine > arginine • > ornithine cycle. 



It will be noted that this brief and incomplete hst of synthetic reactions 

 localized in mitochondria does no more than skirt the areas of polymer 

 synthesis with which we shall be mainly concerned. The major reason for this 

 is the fact that the biosynthesis of nucleic acid and protein has not yet been 

 found to be very active in these particles. At least this is so in terms of such 

 crude measurements as the incorporation of P^^ and C^^-amino acids into 

 total mitochondrial nucleic acid and protein. One might ask if this were true 

 with respect to the synthesis of a mitochondrial enzyme or RNA. Are mito- 

 chondrial nucleic acids and proteins made in mitochondria? WiU not the 

 syntheses of mitochondrial-specific entities occur at a maximal rate in these 

 organelles? These questions have not yet been posed experimentally. With 

 respect to the general problem of polymer syntheses, existing methods of 

 measurement and data have compelled us to shift our attention away from 

 these critical organelles. 



However, it can be seen in our examination of reactions occurring in 

 mitochondria, that, in addition to the generation of a utilizable chemical 



