Measurement of uracil-DNA glycosylase activity by matrix assisted laser desorption/ionization time-of-flight mass spectrometry technique.

Uracil-DNA glycosylase (UDG) is a highly conserved DNA repair enzyme that acts as a key component in the base excision repair pathway to correct hydrolytic deamination of cytosine making it critical to genome integrity in living organisms. We report here a non-labeled, non-radio-isotopic and very specific method to measure UDG activity. Oligodeoxyribonucleotide duplex containing a site-specific G:U mismatch that is hydrolyzed by UDG then subjected to Matrix Assisted Laser Desorption/Ionization time-of-flight mass spectrometry analysis. A protocol was developed to maintain the AP product in DNA without strand break then the cleavage of uracil was identified by the mass change from uracil substrate to AP product. From UDG kinetic analysis, for G:U substrate the Km is 50 nM, Vmax is 0.98 nM/s and Kcat = 9.31 s-1. The method was applied to uracil glycosylase inhibitor measurement with an IC50 value of 7.6 pM. Single-stranded and double-stranded DNAs with uracil at various positions of the substrates were also tested for UDG activity albeit with different efficiencies. The simple, rapid, quantifiable, scalable and versatile method has potential to be the reference method for monofunctional glycosylase measurement, and can also be used as a tool for glycosylase inhibitors screening.