Adaptive enhancement and kinetics of nucleotide excision repair in humans

An adaptive response, low doses of a mutagen rendering cells more able to s ubsequently cope with higher doses of that or a related challenging mutagen , enhances nucleotide excision repair in human fibroblasts. After fibroblas ts were flashed with 20 J/m(2) of UVC, the cyclopyrimidine dimer frequency at any single dinucleotide position remained unchanged for several hours be fore abruptly displaying first order kinetics of repair. These kinetics wer e determined by ligation-mediated PCR along exon 9 of the human p53 gene. W hen a chronic dose of quinacrine mustard (QM) preceded the UVC challenge, t he duration of the cyclobutane pyrimidine dimer (CPD) repair lags were redu ced by a factor of three and the kinetic half-lives for CPD repair were red uced by a factor of three. The observed repair kinetics are consistent with the following model. The UVC dose required (K-m) to generate a substrate c oncentration which half-saturates the cell's repair capacity is 3 J/m(2) fo r the high affinity (6-4) photoproducts and greater than 100 J/m(2) for the low affinity cyclobutane dimers. After 20 J/m2 of UVC, the repair enzyme i s saturated with (6-4) photoproducts: these competitively inhibit CPD repai r by binding all available repair enzyme. After the (6-4)s are repaired, th e CPD concentration is less than K-m(CPD) and so CPD repair kinetics initia te with first order kinetics. QM-induced enhancement, by increasing the con centration, V-max, of repair enzyme, shortens the duration of (6-4) saturat ion and increases the rate constant for cyclobutane dimer repair. The data exactly fit the expectations from Michaelis kinetics. Transcription coupled repair is less amenable to Michaelis interpretations and enhanced global r epair was almost as rapid as the slightly enhanced transcription coupled re pair. We infer that repair enhancement is unable to proportionally increase the number of matrix attachment sites necessary for transcription coupled repair. Understanding competitive inhibition between adduct classes and ada ptive enhancement of V-max is important to understanding the effects of hig h doses of mutagen mixtures. (C) 1999 Elsevier Science B.V. All rights rese rved.