Mechanism of the synergistic antiviral and cytostatic activity of ( RS)-3-(adenin-9-YL)-2-hydroxypropanoic acid isobutyl ester and d,l-homocysteine

Abstract

In a previous report (De Clercq E, Cools M and Balzarini J, Biochem Pharmacol 38: 1771–1778, 1989) we showed that homocysteine (Hcy) enhanced the antiviral and cytostatic activity of S-adenosylhomocysteine (AdoHcy) hydrolase inhibitors. The mechanism of synergistic action between Hcy and the isobutyl ester of ( RS)-3-(adenin-9-yl)-2-hydroxypropanoic acid [( RS)-AHPA] has been the subject of the present study. The selectivity index of ( RS)-AHPA against vaccinia virus in murine L929 cells was significantly increased if the drug was combined with 1 or 3 mM Hcy. Even if Hcy was added as late as 12 hr after ( RS)-AHPA, a synergistic antiviral activity was noted. Treatment of the L929 cells with ( RS)-AHPA caused a significant increase in AdoHcy levels, and these levels were further increased if, in addition to ( RS)-AHPA, Hcy (1mM) was added to the cell cultures. Double-pulse label experiments showed that the additional AdoHcy built up after the combined treatment of ( RS)-AHPA with Hcy did not originate from S-adenosylmethionine (via transmethylation reactions), but resulted from residual AdoHcy hydrolase activity (in the synthetic direction). To maintain sufficient levels of AdoHcy, AdoHcy hydrolase activity must be inhibited in the hydrolytic direction.