Originally published in Arkiv for Kemi 24 A, 27 (1947). 



78. APPLICATION OF LABELLED SUBSTRATES IN THE 

 STUDY OF ENZYMIC PROCESSES 



L. Ahlstrom, H. Eulbr and G. Hevesy 

 From the Institute for Research in Organic Chemistry, Stockholm 



The application of labelled substrates in the study of enzymic processes 

 has two advantages: 



(a) The sensitivity of the determination of the enzyme can be much 

 increased. 



(b) The determination of the reaction product, for example, radio- 

 phosphate in phosphatase studies, is not influenced by non-labelled 

 phosphate, even if present in a high concentration. Accordingly, the 

 determination is based on the appearance of radioactivity in an initially 

 inactive system, and not on the increase in concentration of the inor- 

 ganic phosphate. In our experiments we used labelled desoxyribonucleic 

 acid and studied the respective actions of blood plasma phosphatase, 

 sarcoma phosphatase, and intestinal mucosa phosphatase on the labelled 

 substrate. 



PREPARATION OF SUBSTRATE 



Labelled sodium phosphate of an aggregate activity of 10 millicuries 

 was administered to rats which were sacrificed after a few days. Then the 

 desoxyribonucleic acid was extracted from the Jensen-sarcoma, the liver, 

 the intestinal mucosa, or other organs, and was carefully purified so as 

 to reprove all other labelled phosphorus compounds, as described pre- 

 viously(i\ The best yields in respect of both quantity and activity are 

 obtained when the nucleic acid is extracted from the sarcoma or the 

 intestinal mucosa. No advantage is gained by letting the rat live longer 

 then 3 to 4 days, as we have to reckon not only with a decay of the 

 activity of radiophosphorus at a rate of about 4 per cent per day, but 

 also with an exodus of the labelled phosphate from the nucleic acids 

 and other compounds of the organs. This exodus is particularly 

 pronounced in such tissues as the intestinal mucosa in which the nucleic 

 acid content is renewed at a comparatively high rate. 



(i>H. EuLEB and G. Hevesy, X^Z.Dansifce Vid. Selsk. Biol. Medd.18, No 8 (1945). 



