Structure-activity relationship, selectivity and mode of inhibition of terminal deoxyribonucleotidyltransferaseby streptolydigin analogs

Three of thirty-one streptolydigin analogs resembled the parent compound in selectively inhibiting terminal deoxyribonucleotidyltransferase (terminal transferase) when compared with cellular DNA polymerases α, β and γ, simian sarcoma virus DNA polymerase, herpes simplex virus-induced DNA polymerase and cellular RNA polvmerase II. The other twenty-eight compounds either did not inhibit any of these enzymes or inhibited all of these enzymes without selectivity for terminal transferase. Two analogs that selectively inhibited terminal transferase (Ki = 0.12 mM) were 2- to 3-fold more potent than streptolydigin (Ki = 0.32mM). All the selective inhibitors of termininal transferase are 3-acylte-tramic acids with various substituent groups at the 1-, 3- and 5-positions. One of these, a less potent inhibitor of terminal transferase than streptolydigin, lacks the 3- and 5-substituents of streptolydigin but has virtually the same 1-substituent. The substituent groups of the other two selective inhibitors are structurally different from those of streptolydigin but essentally identical to each other. The mode of inhibition of terminal transferase by selective inhibitors was the same as for streptolydigin, but different from an analog which non-selectively inhibited terminal transferase. Evidence suggested that the selective inhibitors specifically interacted with terminal transferase and not with initiator (oligo- or polydeoxyribonucleotide), substrate (deoxyribonucleoside 5'-triphosphates) or the divalent cation (Mn2+) required for enzyme activity. The data also implied that these compounds bind to the enzyme at a site(s) other than the initiator or substrate binding sites. In contrast, an analog which non-selectively inhibited terminal transferase apparently interacted with many proteins and polydeoxyribonucleotides non-specifically.