THE MITOTIC CYCLE 



Phosphatase distribution 



By means of the well-known cytochemical methods of Gomori^^^ ^^^ 

 the distribution of phosphatases within the nucleus has been examined, 

 though the customary debates concerning the validity of methods for 

 cytochemical localization extend also to these procedures (Danielli/^* 

 Jacoby and Martin^^^). Wolf et alii^^^ have investigated the acid phos- 

 phatase of a number of types of cell, while several studies have been 

 devoted to the alkaline enzyme, such as those on chick cells in tissue 

 culture (WiLLMER,!*^ Chevremont and Firket^*^), on salivary gland 

 chromosomes (Danielli and Catcheside,^*^ Krugelis^** i*^), and on 

 mouse spermatocytes (Krugelis^*^). Both the nucleoli and the hetero- 

 chromatin of the interphase nucleus and the chromosomes during 

 mitosis give a strong alkaline phosphatase reaction. 



The evidence for the chemical nature of the alkaline phosphatases 

 has been reviewed by Moog.^*' They are proteins of the globulin type, 

 activated by metal ions. Alkaline kidney phosphatase is strongly bound 

 to zinc (Cloetens^^^) ; it is possible that the zinc reported by Heath^*® 

 in isolated deoxyribonucleoprotein may be related to this enzyme. 



The presence of an enzyme within the nucleus does not necessarily 

 indicate that it has some function connected with nuclear metabolism 

 per se, for the nucleus may be the site of origin of an enzyme, which 

 may then diffuse outwards from the nuclear membrane to operate 

 within the cytoplasm. This possibility is suggested by the work of 

 DouNCE and Beyer^^" on nuclear arginase, a characteristic enzyme of 

 the mammalian liver, which breaks down arginine to urea and ornithine. 

 Liver nuclei contain slightly more arginase per unit of dry weight than 

 does the cytoplasm, while kidney nuclei contain little or none. In a further 

 paper, Dounce et alii^^^ have confirmed that this enzyme is present in 

 rat liver nuclei. 



LIPOIDS OF the nucleus 



Both chemical analysis of isolated nuclei and microscopic techniques 

 have been used in the few studies which have been devoted to this 

 subject. In the original paper describing the citric acid method of 

 isolating nuclei Stoneberg^^^ used this technique for studying the 

 lipoids. It was found that in a number of organs there was a somewhat 

 greater lipoid content in the nuclei than in the whole tissue; phospho- 

 lipine and cholesterol were the main substances of this class to be found 

 within the nucleus. It has been found that in the nuclei of the mouse 

 liver, the percentage of phospholipines is equal to that of ribonucleic 

 acid, namely 3-4 (Bavaum et alii^^^). Williams et alii^^^ studied 

 the effect on the nuclear lipoids of the liver of feeding the carcinogen 

 butter yellow and found that the proportion of cholesterol present was 

 thereby increased. In the intact nucleus, the lipoids are bound to protein, 



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