Continuous colorimetric screening assay for detection of d-amino acid aminotransferase mutants displaying altered substrate specificity.

D-Amino acid aminotransferase (DAAT) catalyzes the synthesis of numerous d-amino acids, making it an attractive biocatalyst for the production of enantiopure d-amino acids. To bolster its biocatalytic applicability, improved variants displaying increased activity toward non-native substrates are desired. Here, we report the development of a high-throughput, colorimetric, continuous coupled enzyme assay for the screening of DAAT mutant libraries that is based on the use of d-amino acid oxidase (DAAO). In this assay, the d-amino acid product of DAAT is oxidized by DAAO with concomitant release of hydrogen peroxide, which is detected colorimetrically by the addition of horseradish peroxidase and o-dianisidine. Using this assay, we measured apparent KM and kcat values for DAAT and identified mutants displaying altered substrate specificity via the screening of cell lysates in 96-well plates. The DAAO coupled assay is sensitive in that it allowed the detection of a DAAT mutant displaying an approximately 2000-fold decrease in kcat/KM relative to wild type. In addition, the DAAO assay enabled the identification of two DAAT mutants (V33Y and V33G) that are more efficient than wild type at transaminating the non-native acceptor phenylpyruvate. We expect that this assay will be useful for the engineering of additional mutants displaying increased activity toward non-native substrates.

Enhancement of cisplatin cytotoxicity by disulfiram involves activating transcription factor 3.

BACKGROUND: Activating transcription factor 3 (ATF3), a stress-inducible gene, is a regulator of cisplatin-induced cytotoxicity, and enhancement of the ATF3 expression potentiates this cytotoxicity. MATERIALS AND METHODS: ATF3 expression and its binding to the transcription target CHOP were evaluated by western blot and chromatin immunoprecipitation (ChIP), respectively, in a panel of five cell lines (WI38, MCF7, PC3, A549). MTT assays were employed to assess the effects of many drugs, including disulfiram, on cell viability. RESULTS: ATF3 protein expression was up-regulated after cytotoxic doses of cisplatin treatment and it directly bound to the CHOP gene promoter, increasing this pro-apoptotic protein's expression. In a library screen of 1200 compounds, disulfiram, a dithiocarbamate drug, was identified as an enhancer of the cytotoxic effects of cisplatin. This increased cytotoxic action was synergistic and likely due to their ability to induce ATF3 independently. CONCLUSION: Understanding the role of ATF3 in cisplatin-induced cytotoxicity will lead to novel therapeutic approaches that could improve this drug's efficacy.