ABSTRACT
Interest in the beneficial effects of green tea has led to investigations on activities by the main catechin (-)-epigallocatechin-3-gallate (EGCG). This antioxidative compound could contribute to cancer chemoprevention by acting antigenotoxic. To further explore this hypothesis we investigated antigenotoxic potentials of low EGCG concentrations in human peripheral leucocytes. Leucocytes isolated from whole blood were (1) stimulated with phytohaemagglutinin, (2) damaged with genotoxic bleomycin, and (3) post-incubated to allow DNA repair. After each phase DNA integrity was measured with the comet assay. EGCG (2, 20, 100 microM) was added either during phases 1, 2 or 3 or during the whole process (1-3), to delineate mechanisms of antigenotoxicity reflecting induction of detoxification (phase 1), scavenging of radicals (phase 2), stimulation of repair (phase 3), respectively. Bleomycin induced breaks and endonuclease III specific damage, but EGCG did not affect damage or repair of these lesions when added during phases 1, 2 or 3. However, the application of EGCG during phases 1 and 2 significantly reduced both bleomycin-induced breaks and endonuclease III sensitive sites. EGCG added during all phases impaired persistence of damage. Our studies show that the continuous presence of EGCG can reduce radical-induced DNA damage in primary leucocytes, possibly due to a combination of different mechanisms. Together the findings support the hypotheses that EGCG acts protective in human cells.
Subject(s)
Antibiotics, Antineoplastic/antagonists & inhibitors , Bleomycin/antagonists & inhibitors , Catechin/analogs & derivatives , DNA Damage/drug effects , Leukocytes/drug effects , Antibiotics, Antineoplastic/toxicity , Bleomycin/toxicity , Catechin/pharmacology , Comet Assay , Deoxyribonucleases, Type III Site-Specific/antagonists & inhibitors , Deoxyribonucleases, Type III Site-Specific/metabolism , Humans , In Vitro Techniques , Phytohemagglutinins/chemistryABSTRACT
While it has been demonstrated that AdoMet is required for DNA cleavage by Type III restriction enzymes, here we show that in the presence of exogenous AdoMet, the head-to-head oriented recognition sites are cleaved only on a supercoiled DNA. On a linear DNA, exogenous AdoMet strongly drives methylation while inhibiting cleavage reaction. Strikingly, AdoMet analogue sinefungin results in cleavage at all recognition sites irrespective of the topology of DNA. The cleavage reaction in the presence of sinefungin is ATP dependent. The site of cleavage is comparable with that in the presence of AdoMet. The use of EcoP15I restriction in presence of sinefungin as an improved tool for serial analysis of gene expression is discussed.
Subject(s)
Adenosine/analogs & derivatives , DNA, Superhelical/metabolism , DNA/metabolism , Deoxyribonucleases, Type III Site-Specific/metabolism , S-Adenosylmethionine/pharmacology , Site-Specific DNA-Methyltransferase (Adenine-Specific)/metabolism , Adenosine/pharmacology , DNA, Circular/metabolism , Deoxyribonucleases, Type III Site-Specific/antagonists & inhibitors , Gene Expression Profiling/methods , Repressor Proteins/pharmacology , Site-Specific DNA-Methyltransferase (Adenine-Specific)/antagonists & inhibitorsABSTRACT
The EcoP15 modification methylase gene from the p15B plasmid of Escherichia coli 15T-has been cloned and expressed at high levels in a plasmid vector system. We have purified the enzyme to near homogeneity in large amounts and have studied some of its enzymatic properties. Initial rates of methyl transfer are first order in methylase concentration and, with pUC19 DNA as substrate, the reaction proceeds by a random mechanism in which either DNA or S-adenosylmethionine can bind to the free enzyme. After methyltransfer to DNA, the methylated DNA and S-adenosylhomocysteine appear to dissociate in random order. As expected in such a mechanism, S-adenosylhomocysteine is a non-competitive inhibitor by S-adenosylmethionine at concentrations not much above its KM suggests that release of methylated DNA may be the rate-limiting step. This suggestion is strengthened by the fact that a mutant of the closely related EcoP1 does not show such substrate inhibition.
