MEDICAL HYPOTHESES AND RESEARCH
VOL. 1, No. 1, January 2005


W. J. Lee and B. T. Zhu [2005] Med Hypotheses Res 2: 325-337.


Modulation of the Rate of Enzymatic DNA
Methylation by Catechol-O-Methyltransferase


Won Jun Lee and Bao Ting Zhu*

Department of Basic Pharmaceutical Sciences, College of Pharmacy, University of South
Carolina, Columbia, SC 29208, USA



Abstract. We examined the modulating effects of catechol-O-methyltransferase
(COMT) and S-adenosyl-L-homocysteine (AdoHcy) on DNA methylation catalyzed by
prokaryotic SssI DNMT and human DNMT1. The presence of COMT (at physiologically-
relevant concentrations) enhanced the rate of DNA methylation catalyzed by SssI DNMT
and human DNMT1, by up to 1-fold over the control rate. Transfections of the human
COMT siRNAs into the cultured MCF-7 human breast cancer cells for 10 days caused
~50% reduction of the COMT expression, and this reduction in COMT levels was
accompanied by a slight decrease in the methylation status of the RARβ gene. Kinetic
analyses showed that AdoHcy strongly and noncompetitively inhibited the methylation of
DNA by competing S-adenosyl-L-methionine off the DNMT, thus shifting more enzyme
molecules to a form that was bound with AdoHcy. Consequently, the Vmax values were
reduced when AdoHcy was present, but the Km values were not markedly altered. As
expected, the presence of COMT increased the Vmax for the enzymatic DNA methylation,
but the Km values were essentially not changed. In conclusion, the COMT-mediated
enhancement of DNA methylation likely is due to the sequestration of AdoHcy by binding
to COMT, which reduces the intracellular bioavailability of the free AdoHcy for DNMT
inhibition.



*Address all correspondence to: Dr. Bao Ting Zhu, Department of Basic Pharmaceutical
Sciences, College of Pharmacy, University of South Carolina, Room 617 of Coker Life
Sciences Building, 700 Sumter Street, Columbia, SC 29208 (USA).
PHONE: 803-777-4802; FAX: 803-777-8356; E-MAIL:
BTZhu@cop.sc.edu


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