126 ALEXANDER L. DOUNCE 



for DNA, and did not represent direct determinations. It has been dis- 

 covered subsequently that such a procedure appears to give PNA values 

 which are considerably higher than those obtained by direct colorimetric 

 determination. An erroneously high value for the per cent of PNA in liver 

 nuclei isolated at pH 4 in dilute citric acid had also been reported by Bounce 



Until recently all direct PNA analyses done in the writer's laboratory 

 were not based on purified PNA standards, but instead were based on a 

 calibration curve for color production made with ATP. The technique of 

 Schneider*^ was used with correction for color caused by DNA, and the 

 result obtained was multiplied by two in order roughly to compensate for 

 the fact that pyrimidine nucleosides do not give a color with the orcinol 

 reagent, since they are not hydrolyzed. Recently it has become possible to 

 construct calibration curves using PNA isolated from the tissues being 

 studied by an improved procedure. ^^ Thus far only nuclei isolated from rat 

 liver at pH 4 have been analyzed but here the average value for PNA was 

 2.5% on a dry weight basis, a figure representing approximately one- 

 quarter of the DNA present. This figure is now in approximate agreement 

 with the values given by Hogeboom et al.,'^- Mclndoe and Davidson,*^ and 

 Mirsky et al.'^^ for rat liver nuclei obtained by methods involving the use 

 of aqueous and nonaqueous solvents, respectively. 



Since the best sample of the Behrens-type nuclei which was used in our 

 original work had been saved in the dry state in the refrigerator, it was 

 also possible to reanalyze this sample directly for PNA, using liver PNA 

 as a standard. The value obtained was approximately 3.0%. 



It appears therefore that Behrens-type nuclei do possess a somewhat 

 higher PNA content than nuclei isolated in aqueous media, and that the 

 value for PNA is somewhat higher than reported by Mirsky et al}^ for 

 their rat liver nuclei isolated by the Behrens technique and stated by them 

 to be impure. A comparison of different samples of nuclei on the bases of 

 DNA content would however be much more reliable. 



A few final words of caution should be added concerning estimations of 

 the PNA content of cell nuclei. In the first place, methods for determining 

 PNA may not be entirely precise even at the present time, and possibly 

 more than one procedure should be used in making the analysis. But more 

 important than this is the likelihood that the PNA content of cell nuclei 

 may vary with a number of factors, as does the protein content. A substance 

 with a very high rate of turnover, such as is shown by nuclear PNA,®*"' 

 S6-89 can hardly be expected to remain in fixed amount in the nucleus, since 



s^W. C. Schneider, J. Biol. Chem. 161, 293 (1945). 



s5 E. R. M. Kay and A. L. Bounce, J. Am. Chem. Soc. 75, 4041 (1953). 



86 A. Marshak and F. Calvet, J. Cellular Comp. Physiol. 34, 451 (1949). 



