Specificity of Spleen Acid DNAase

The kinetics of liberation of 3′‐phosphate terminal and of 5′‐hydroxy terminal and penultimate nucleotides (termini) by spleen acid DNAase was investigated using as substrates calf thymus DNA, three bacterial DNAs of different base composition, and poly‐[d(A‐T] · d(A‐T)]. The relative amounts of the nucleotides deoxyguanosine, deoxyadenosine, thymidine and deoxycytidine in the termini released by the enzyme from all DNAs were found to be constant in the terminal phase of digestion. In contrast, some variations were seen during the middle phase, particularly in the amount of 3′‐phosphate terminal deoxyguanosine, which decreased with digestion time, specially in the calf thymus DNA digest. This phenomenon appears to be due to an initial accumulation of breaks in some nucleotide sequences which are preferentially split at the beginning of digestion, possibly by a diplotomic mechanism, with predominant release of 3′‐phosphate deoxyguanosine. In contrast, no change in the composition of 3′‐phosphate terminals released from poly[d(A‐T) · d(A‐T)] took place in the 40–15 average size range; deoxyadenosine and thymidine formed 80% and 20%, respectively, of these terminals. The composition of the 3′‐phosphate terminal nucleotides was very little or not at all affected by changes in the ionic environment of the incubation mixture, whereas it was significantly changed in degradations of native DNA digested at neutrality and low ionic strength, or of heat‐denatured DNA. Linear relationships were found to hold, at all degradation stages investigated here, between the relative amounts of termini liberated from different bacterial DNAs and their (dG + dC) contents. In contrast, the termini obtained from calf thymus DNA were released in relative amounts which did not fit the relationships established for bacterial DNAs.