La Dégradation Enzymatique de L'Acide Désoxyribonucléique. III — Le Mode d'Action des Désoxyribonucléases I et II

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Abstract
The hydrolysis of thymus DNA by pancreatic DNase I and spleen DNase II has been studied by viscometry, U. V. spectrometry, determination of acidosoluble products, titration of secondary phosphate groups and ultracentrifugation. By the action of DNase II, an important lag time exists in the increase of optical density of DNA whereas the viscosity immediately drops; with DNase I, the lag time is much more important in the viscosity variation. Ultracentrifugation of hydrolysates at several stages do not show separate boundaries. It is concluded that both DNases simultaneously attack all DNA molecules. Titration curves of hydrolyzed DNA (pH 3 to 10) are compared to those of denatured DNA and of mixture of nucleotides. Shifts of the pK's, occuring during hydrolysis, are evidenced; 15% and 35% of phosphate groups respectively appear after prolonged action of DNase II and I. A method based on titration between pH 5.5 and 8.5 is devised for determining the number of phosphate groups esterified by the enzymes. The liberation of secondary phosphate by DNase II stops at the same time as the variation in optical density at 260 mμ. On the contrary DNase I continues to liberate phosphate after the maximum optical density has been reached.It is concluded that both DNases have some kind of specificity in their sites of action. Moreover it is supposed that DNase II is only able to split simultaneously double chains at the same level; DNase I would also attack single chains after their separation.