       Document 0003
 DOCN  M9610003
 TI    The intracellular phosphorylation of (-)-2'-deoxy-3'-thiacytidine (3TC)
       and the incorporation of 3TC 5'-monophosphate into DNA by HIV-1 reverse
       transcriptase and human DNA polymerase gamma.
 DT    9601
 AU    Gray NM; Marr CL; Penn CR; Cameron JM; Bethell RC; Department of
       Virology, Glaxo Research and Development Ltd.,; Stevenage,
       Hertfordshire, U.K.
 SO    Biochem Pharmacol. 1995 Sep 28;50(7):1043-51. Unique Identifier :
       AIDSLINE MED/96028078
 AB    (-)-2'-deoxy-3'-thiacytidine (3TC) has been shown to be a potent,
       selective inhibitor of HIV replication in vitro, which requires
       phosphorylation to its 5'-triphosphate for antiviral activity. The
       intracellular concentration of 3TC 5'-triphosphate in
       phytohaemagglutinin (PHA)-stimulated peripheral blood lymphocytes (PBL)
       shows a linear dependence on the extracellular concentration of 3TC up
       to an extracellular 3TC concentration of 10 microM. At this
       extracellular concentration of 3TC, the resulting intracellular
       concentration of 3TC 5'-triphosphate is 5 microM. This value is similar
       to the inhibition constant (Ki) values for the competitive inhibition of
       human immunodeficiency virus type 1 (HIV-1) reverse transcriptase and
       human DNA polymerases (10-16 microM) by 3TC 5'-triphosphate. Since the
       concentration of 3TC producing 90% inhibition (IC90) of HIV replication
       in PBLs has been reported to be 76 nM, the antiviral activity of 3TC
       requires intracellular concentrations of 3TC 5'-triphosphate, which
       would result in very little inhibition of reverse transcriptase if its
       sole mode of action was competitive inhibition. This apparent
       discrepency may be explained by the ability of 3TC 5'-triphosphate to
       act as a substrate for reverse transcriptase. Primer extension assays
       have shown that 3TC 5'-triphosphate is a substrate for HIV-1 reverse
       transcriptase and DNA polymerase gamma, resulting in the incorporation
       of 3TC 5'-monophosphate into DNA. In the case of DNA polymerase gamma,
       the product of this reaction (i.e. double-stranded DNA with 3TC
       5'-monophosphate incorporated at the 3'-terminus of the primer strand)
       is also a substrate for the 3'-5' exonuclease activity of this enzyme.
       This may explain the low levels of mitochondrial toxicity observed with
       3TC.
 DE    Base Sequence  Deoxycytidine Monophosphate/*ANALOGS &
       DERIVATIVES/METABOLISM  DNA/*METABOLISM  DNA Polymerase III/*METABOLISM
       Hela Cells  Human  Kinetics  Lymphocytes/DRUG EFFECTS/*METABOLISM
       Molecular Sequence Data  Phosphorylation  Phytohemagglutinins  Reverse
       Transcriptase Inhibitors/*METABOLISM  RNA-Directed DNA
       Polymerase/*METABOLISM  Stereoisomers  Zalcitabine/*ANALOGS &
       DERIVATIVES/METABOLISM/PHARMACOLOGY  JOURNAL ARTICLE

       SOURCE: National Library of Medicine.  NOTICE: This material may be
       protected by Copyright Law (Title 17, U.S.Code).

