is most active during the premeiotic period and declines in the early 

 stages of prophase. These events could well account for the increased 

 oxygen consumption associated with the premitotic period. 



Recently, Allfrey and coworkers (1957, and later) have demon- 

 strated that isolated thymus nuclei can synthesize ATP through opera- 

 tion of a coupled oxidation-phosphorylation mechanism. The reaction 

 mechanism mediating this synthesis remains to be investigated. How- 

 ever, their studies indicate that synthesis of ATP by the nucleus requires 

 oxygen and is inhibited by azide, cyanide, and 2,4-dinitrophenol. Carbon 

 monoxide, which is considered to be a specific inhibitor of cytochrome 

 oxidase, has little or no effect on nuclear ATP synthesis. While the 

 studies of the Allfrey group will have to be extended to nuclei of other 

 cell types before their general significance can be determined, they do 

 raise the point that the metabolism of the cell nucleus is probably much 

 more complex than originally supposed. If it should turn out that the 

 capacity of the nucleus for aerobic ATP synthesis is of general occur- 

 rence, the high ATP-ase content of nuclei isolated from other tissues 

 (Siebert and Smellie, 1957) and the nuclear synthesis of DPN from 

 nicotinamide mononucleotide and ATP (Hogeboom and Schneider, 

 1952) would have functional significance in terms of oxidative metabo- 

 lism in the nucleus. Likewise, the synthesis of nucleotide coenzymes 

 which Brachet (1957) has suggested may be an important activity of 

 the nucleus, would have a basis for interpretation. 



It is well established that certain glycolytic enzymes (e.g., lactic de- 

 hydrogenase, triosephosphate dehydrogenase, aldolase, enolase) are 

 present in reasonably high concentration in the nuclei of plant and ani- 

 mal cells (Bounce, 1954, 1955; Lang and Siebert, 1951, 1952). On the 

 other hand, the enzyme systems characteristically associated with respi- 

 ration and oxidative phosphorylation in the cytoplasm are presumably 

 absent (Siebert and Smellie. 1957). A list of the enzymes which cannot 

 be demonstrated in nuclei is shown in Table 6-1. The only known excep- 

 tion to this finding is that the nucleated erythrocyte of the bird has been 

 shown to contain cytochrome oxidase and. succinic dehydrogenase in the 

 nucleus. The explanation, as pointed out by Brachet (1957), seems to be 

 that this type of cell does not possess mitochondria. The functional sig- 

 nificance of the presence of glycolytic enzymes in the nucleus is not 

 known; however, the possibility exists that a nuclear glycolytic pathway 

 may function in the initial stages of carbohydrate breakdown to pro- 

 duce energy, and operate to reduce DPN (Stern and Mirsky, 1952). 

 Also glycolysis within the nucleus may be necessary to supply certain 

 intermediate metabolites needed for special intranuclear metabolism 



MECHANICS AND PHYSIOLOGY OF CELL DIVISION / 157 



