Human platelet-type 12-lipoxygenase (12-LOX) and its metabolites play a crucial role in tumor angiogenesis. A "10-23" deoxyribozyme (DNAzyme) and its phosphorothioate-modified version were designed and synthesized against the 12-LOX mRNA. Both DNAzymes were able to cleave their substrate efficiently in a time- and concentration-dependent manner in vitro. Under a multiple turnover condition, both performed well at 37 degreesC, showing the k(cat) of 1 and 0.26 min(-1), respectively. The phosphorothioate modification of the DNAzyme significantly increased its stability in cells without a substantial loss of kinetic efficiency in vitro. In a cell culture system, transfection of the DNAzymes into HEL cells resulted in a significant down-regulation of the 12-LOX mRNA. Furthermore, the cell extracts from the DNAzyme-transfected cells exhibited a marked reduction in the 12-LOX enzyme activity. The present results indicated the potential use of DNAzyme technology for gene function study and cancer therapy.