Effect of vanillin on methylene blue plus light-induced single-strand breaks in plasmid pBR322 DNA

The ability of vanillin (4-hydroxy-3-methoxybenzaldehyde), a naturally occu rring food flavouring agent, in inhibiting photosensitization-induced singl e-strand breaks (ssbs) in plasmid pBR322 DNA has been examined in an in vit ro system, independent of DNA repair/replication processes. Photosensitizat ion of DNA with methylene blue, visible light and oxygen, induced ssbs resu lting in the production of open circular form (OC form) in a concentration- dependent manner. The yield of OC form induced by photosensitization was in creased several-fold by deuteration of the buffer and was found to be inhib ited by sodium azide, a scavenger of singlet oxygen (O-1(2)). Vanillin, per se, did not induce but inhibited photosensitization-induced ssbs in plasmi d DNA, at millimolar concentrations. The inhibitory effect of vanillin was both concentration- and time-dependent. On a molar basis, vanillin was, how ever, less effective than trolox, a water-soluble analogue of alpha-tocophe rol. Photosensitization by methylene blue system generates singlet oxygen, as one of the major components of ROS. Therefore, interaction of singlet ox ygen with vanillin was investigated. The rate constant of vanillin with O-1 (2) was estimated to be 5.93 x 10(7) M-1 s(-1) and that of sodium azide as 2.7 x 10(8) M-1 s(-1). The present investigations show that vanillin can pr otect against photosensitization-induced ssbs in the plasmid pBR322 DNA, an d this effect may partly be due to its ability to scavenge O-1(2). (C) 2000 Elsevier Science B.V. All rights reserved.