Studies on polynucleotides. X. Enzymic degradation. Some properties and mode of action of spleen phosphodiesterase.

As a part of a program of studies aiming at the development of methods for the end group and sequential analysis of polynucleotides (l), we have been investigating the properties and mode of action of certain phosphodiesterases. Studies of this kind have been facilitated by the availability of a variety of substrates and, in particular, synthetic oligonucleotides of known size and structure (2). In previous publications, detailed studies on the venom phosphodiesterase were reported (3, 4) and it was established that this enzyme, if given a 3’hydroxyl group at the end of a polynucleotide chain, acted stepwise from that end and caused the successive release of nucleoside 5’-phosphate units. As these studies were continued, attention was turned to the spleen phosphodiesterase (5), an enzyme which has already proved very useful in structural studies of the nucleic acids (6-8). Heppel et al. (9-11) have shown that the enzyme attacks phosphodiester bonds to release nucleoside 3’-phosphate units. Our studies on this enzyme have now shown that the action on oligonucleotide chains is stepwise and that it begins from the end of the chain bearing a 5’-hydroxyl group, this general mode of action being complementary to that of the venom phosphodiesterase. A distinctive feature of the spleen phosphodiesterase, however, is the tendency of the enzyme to catalyze transfer reactions (12, 13) which can lead to the formation of higher homologues (or oligonucleotides) as intermediates during the degradation. The present paper contains a detailed report of the experiments on the kinetic studies of the action of the spleen phosphodiesterase on a number of synthetic oligonucleotides and describes some general properties of this enzyme. Brief reports of a part of this work have already appeared (1, 14-17).