title: Efficient pyrophosphorolysis by a hepatitis B virus polymerase may be a primer-unblocking mechanism.
abstract: Effective antiviral agents are thought to inhibit hepatitis B virus (HBV) DNA synthesis irreversibly by chain termination because reverse transcriptases (RT) lack an exonucleolytic activity that can remove incorporated nucleotides. However, since the parameters governing this inhibition are poorly defined, fully delineating the catalytic mechanism of the HBV-RT promises to facilitate the development of antiviral drugs for treating chronic HBV infection. To this end, pyrophosphorolysis and pyrophosphate exchange, two nonhydrolytic RT activities that result in the removal of newly incorporated nucleotides, were characterized by using endogenous avian HBV replication complexes assembled in vivo. Although these activities are presumed to be physiologically irrelevant for every polymerase examined, the efficiency with which they are catalyzed by the avian HBV-RT strongly suggests that it is the first known polymerase to catalyze these reactions under replicative conditions. The ability to remove newly incorporated nucleotides during replication has important biological and clinical implications: these activities may serve a primer-unblocking function in vivo. Analysis of pyrophosphorolysis on chain-terminated DNA revealed that the potent anti-HBV drug beta-l-(-)-2',3'-dideoxy-3'-thiacytidine (3TC) was difficult to remove by pyrophosphorolysis, in contrast to ineffective chain terminators such as ddC. This disparity may account for the strong antiviral efficacy of 3TC versus that of ddC. The HBV-RT pyrophosphorolytic activity may therefore be a novel determinant of antiviral drug efficacy, and could serve as a target for future antiviral drug therapy. The strong inhibitory effect of cytoplasmic pyrophosphate concentrations on viral DNA synthesis may also partly account for the apparent slow rate of HBV genome replication.
authors.affiliation: Department of Medical Microbiology and Immunology and Glaxo-Wellcome Heritage Research Institute, University of Alberta, Edmonton, AB, Canada, T6G 2H7. sin@mrc-lmb.cam.ac.uk
authors.forename: S
authors.initials: S
authors.lastname: Urban
authors.forename: S
authors.initials: S
authors.lastname: Urban
authors.forename: K P
authors.initials: KP
authors.lastname: Fischer
authors.forename: D L
authors.initials: DL
authors.lastname: Tyrrell
mesh_terms: D000818:Animals; D000998:Antiviral Agents / Q000494:pharmacology; D002384:Catalysis / Q000187:drug effects; D004261:DNA Replication / Q000187:drug effects; D004279:DNA, Viral / Q000096:biosynthesis; D011756:Diphosphates / Q000378:metabolism* / Q000494:pharmacology; D004372:Ducks / Q000821:virology*; D016679:Genome, Viral; D015611:Hepatitis B Virus, Duck / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D006868:Hydrolysis / Q000187:drug effects; D007700:Kinetics; D019259:Lamivudine / Q000494:pharmacology; D008099:Liver / Q000821:virology; D012194:RNA-Directed DNA Polymerase / Q000378:metabolism*; D018894:Reverse Transcriptase Inhibitors / Q000494:pharmacology; D014771:Virion / Q000187:drug effects / Q000201:enzymology / Q000235:genetics; D016047:Zalcitabine / Q000494:pharmacology
chemical_list: D000998:Antiviral Agents; D004279:DNA, Viral; D011756:Diphosphates; D018894:Reverse Transcriptase Inhibitors; D019259:Lamivudine; D016047:Zalcitabine; D012194:RNA-Directed DNA Polymerase