IV NUCLEIC ACIDS AND NUCLEOPROTEINS 913 



to cellular division have appeared recently. Colchicine, in spite of its action in 

 blocking mitosis, does not inhibit the synthesis of DNA (Bloch, 1953). During 

 a 26 h. period following the addition of colchicine to cultured rat fibroblasts, 

 DNA synthesis proceeded at a normal rate. However, this agent caused a progres- 

 sive increase in the number of cells belonging to the higher DNA classes. Stevens 

 et al. (1953, 1953a) demonstrated that approximately twice as much DNA was 

 synthesized in a group of normal cells prior to cell division as had previously been 

 present. This was interpreted as evidence that the DNA of the cell is replaced 

 by newly synthesized DNA before mitosis occurs. The findings of Kelly (1956) 

 would indicate that the Ehrlich ascites cell, following irradiation, synthesizes 

 DNA until it reaches the premitotic content of DNA (octaploid). These cells are 

 then arrested since they are unable to go through mitosis. 



An intriguing study of the distribution of the newly synthesized DNA in mitotic 

 division has been carried out by Plant and Mazia (1956, 1956a). A partial answer 

 was obtained as to whether in the course of division, the components of the old 

 chromosome are partitioned equally between the two daughter cells or whether 

 the old chromosome remains and a new unit is formed. Cells of the plant, Crepis 

 papillaris were exposed to radiolabeled precursors of DNA for a period sufficiently 

 long for cells to proceed from interphase synthesis to anaphase or telephase and 

 then fixed. It was reasoned that the radioactivity in the whole of the anaphase or 

 telophase cell is a measure of DNA synthesis in the preceeding interphase. There- 

 fore the distribution of radioactivity between the two halves of this cell makes it 

 possible to ascertain the mitotic division of the newly synthesized DNA. Radio- 

 autographic techniques were employed, and it was demonstrated that this newly 

 synthesized DNA was not distributed equally between the products of the nuclear 

 division. The extension of these important observations may result in some new 

 concepts of the mechanism of chromosomal reduplication. 



v. OTHER METABOLIC OR COMPOSITIONAL FEATURES OF 

 NEOPLASTIC TISSUES 



(a) Enzymes 



The most extensive compilation of the activities of enzyme, or enzyme co-fac- 

 tors in neoplastic tissues has been made by Greenstein (1954, 1956). Only the 

 more recent findings will be included in this discussion. 



Burstone (1956) employing histochemical techniques has studied the proteolytic 

 activity in human tumors. The connective tissue stroma adjacent to the tumors 

 exhibited an intense staining reaction for aminopeptidase. Stromal elements at 

 greater distances from the tumor had less activity or none at all. Peripheral tumor 

 areas adjacent to normal muscle or skin were found to exhibit higher dipeptidase 

 and cathepsin activities than the central areas of the tumor (Sylven and Malmgren, 

 1955). Homogenates of regenerating liver or hepatoma had higher specific cathep- 

 tic activities than normal rat liver. The nuclear activity of cathepsin was higher in 

 some hepatomas than in normal liver nuclei (Maver et al., 1952). Fiore (1952) 

 reported that there was less lysozyme in benign tumors than in malignant mam- 

 mary neoplasms. Also the metastatic lesions had more lysozyme activitv than the 



Lilerature p. gig 



