Mechanistic evaluation of new plant-derived compounds that inhibit HIV-1 reverse transcriptase.

Swertifrancheside [1], a new flavonone-xanthone glucoside isolated from Swertia franchetiana, 1 beta-hydroxyaleuritolic acid 3-p-hydroxybenzoate [2], a triterpene isolated from the roots of Maprounea africana, and protolichesterinic acid [3], an aliphatic alpha-methylene-gamma-lactone isolated from the lichen Cetraria islandica, were found to be potent inhibitors of the DNA polymerase activity of human immunodeficiency virus-1 reverse transcriptase (HIV-1 RT), with 50% inhibitory doses (IC50 values) of 43, 3.7, and 24 microM, respectively. They were not cytotoxic with cultured mammalian cells. The kinetic mechanisms by which compounds 1-3 inhibited HIV-1 RT were studied as was their potential to inhibit other nucleic acid polymerases. Swertifrancheside [1] bound to DNA and was shown to be a competitive inhibitor with respect to template-primer, but a mixed-type competitive inhibitor with respect to TTP. On the other hand, 1 beta-hydroxyaleuritolic acid 3-p-hydroxybenzoate [2] and protolichesterinic acid [3] were mixed-type competitive inhibitors with respect to template-primer and noncompetitive inhibitors with respect to TTP. Therefore, the mechanism of action of 1 beta-hydroxyaleuritolic acid 3-p-hydroxybenzoate [2] and protolichesterinic acid [3] as HIV-1 RT inhibitors involves nonspecific binding to the enzyme at nonsubstrate binding sites, whereas swertifrancheside [1] inhibits enzyme activity by binding to the template-primer.