RESUMO
4-Nitroquinoline-1-oxide (4NQO) produced unstable and stable purine adducts to the DNA in human cells. Alkali-labile single-strand breaks (AL-SSBs) arose from 40% of the total adducts immediately after a 30 min alkali-denaturation of DNA. Near-ultraviolet (NUV) induced photochemical SSBs at each site of the remaining 60% adducts. Normal human, xeroderma pigmentosum group A (XPA) and C (XPC) cells removed rapidly all of the AL-SSBs and half of the 60% photobreakable adducts in a similar fashion. Thus, 60-70% labile 4NQO adducts in cells were suggested to be depurinated. Only 30-40% photo-breakable stable adducts of the total were excised almost completely in 24 h by nucleotide excision repair in normal cells, but remained unexcised in XPA cells. XPC cells excised approximately 50% of such excisable adducts in 6 h without further greater loss, as revealed by the kinetics of cumulative unscheduled DNA synthesis, excision-break accumulation and reduction in photochemical SSBs. Bimodal alkali-sucrose sedimentation profiles of photolysed DNA of growing and quiescent XPC cells following a 6 h repair of 4NQO damage presented the normal preferential excision repair in 50% of genomic DNA domains and no or greatly retarded repair in the remaining domains. In XPC cells, such a larger fraction of domain-limited repair of excisable 4NQO damage than a 10-20% fraction for UV damage was the molecular basis for more resistance to 4NQO than to UV. An XP47TO strain heterogeneous within XPC had the normal 4NQO resistance and nearly normal random repair of stable 4NQO adducts throughout genomic DNA.
