ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 119 



Behrens-type nuclei.*'*' Such nuclei should also be satisfactory for isola- 

 tion of histone. The only possible disadvantage in the Behrens-type nuclei, 

 apart from an uncertainty as to the purity, is that certain proteins of the 

 nucleus may be denatured by the organic solvents used, but there is as yet 

 little information available on this point. 



Nuclei isolated in dilute citric acid at pH 4.0 seem to be satisfactory for 

 the subsequent isolation of histone, but these nuclei are in general not satis- 

 factory for work with proteins other than histone. They may be suitable, 

 however, for a study of the type of protein present in the residual chromo- 

 somes described by Mirsky. 



Nuclei isolated at pH 6.0 in very dilute citric acid can be used to some 

 extent for protein studies, but it would no doubt be preferable to use nu- 

 clei obtained by one of the procedures involving calcium chloride-sucrose 

 solutions. 



It would be well however to make a comparison of the latter nuclei with 

 the Behrens-type nuclei, since too little information is available to make 

 possible any final judgment on the best procedure to use for isolating nuclei 

 for the study of a given protein or protein fraction. Possible losses and gains 

 of proteins isolated in aqueous media already have been discussed in the 

 introduction. 



e. Choice of Method for Study of Enzymes 



The same remarks apply that were made concerning proteins. For the 

 time being at least, it will be necessary in the opinion of the writer to con- 

 tinue work with nuclei prepared by different procedures. The Behrens type 

 of procedure and the procedure using calcium chloride with the small 

 homogenizer described above, are to be particularly recommended. Nuclei 

 obtained at pH 4.0 with dilute citric acid are probably useful only occasion- 

 ally in enzyme studies, and nuclei obtained at pH 6.0 suffer from the defect 

 caused by the adsorption of fine mitochondrial fragments. 



/. Choice of Method for Study of Vitamins 



For studies of water-soluble vitamins and coenzymes, the Behrens-type 

 procedure is undoubtedly the only one which would be generally reliable. 

 For fat-soluble vitamins, nuclei obtained in dilute citric acid at pH 4.0 

 might be used, or nuclei isolated on a large scale using calcium chloride and 

 the colloid mill could be tried. 



g. Choice of Method for Study of Minerals 



For work with minerals, the Behrens-type procedure is the only one that 

 can be recommended at the present time. Work that has been done on the 

 mineral content of nuclei prepared in aqueous media is probably only of 

 slight significance. 



