Somatic mutations induced by oxidative damage of
DNA might play important roles in
atherogenesis. However, the underlying mechanisms remain poorly understood.
Myeloperoxidase, a
heme protein expressed by select populations of artery wall macrophages, initiates one potentially mutagenic pathway by generating
hypochlorous acid. This potent chlorinating agent reacts rapidly with primary
amines to yield long-lived, selectively reactive N-
chloramines. In the current studies, we demonstrate that
myeloperoxidase produced by human macrophages differentiated in the presence of
granulocyte macrophage colony-stimulating factor generates
5-chlorouracil, a mutagenic
thymine analog. The primary
amine taurine fails to block the reaction, suggesting that N-haloamines produced by macrophages might oxidize
uracil. Model system studies demonstrated that N-
chloramines convert
uracil to
5-chlorouracil. Interestingly, the tertiary
amine nicotine dramatically enhances
uracil chlorination, suggesting that cigarette
smoke might promote nucleobase oxidation by N-
chloramines. To look for evidence that
myeloperoxidase promotes
uracil oxidation in vivo, we measured
5-chlorouracil levels in human aortic tissue, using
isotope dilution gas chromatography-mass spectrometry. The level of
5-chlorouracil was 10-fold higher in atherosclerotic aortic tissue obtained during
vascular surgery than in normal aortic tissue, suggesting that halogenated nucleobases produced by macrophages might contribute to
atherogenesis. Because
5-chlorouracil can be incorporated into nuclear
DNA, our observations raise the possibility that halogenation reactions initiated by phagocytes provide one pathway for mutagenesis, phenotypic modulation, and cytotoxicity during
atherogenesis.