Cloning, expression, and enzymatic activity of Acinetobacter baumannii and Klebsiella pneumoniae acetyl-coenzyme A carboxylases.

Pathogenic Gram-negative bacteria are a major public health concern because they are causative agents of life-threatening hospital-acquired infections. Due to the increasing rates of resistance to available antibiotics, there is an urgent need to develop new drugs. Acetyl-coenzyme A carboxylase (ACCase) is a promising target for the development of novel antibiotics. We describe here the expression, purification, and enzymatic activity of recombinant ACCases from two clinically relevant Gram-negative pathogens, Acinetobacter baumannii and Klebsiella pneumoniae. Recombinant ACCase subunits (AccAD, AccB, and AccC) were expressed and purified, and the holoenzymes were reconstituted. ACCase enzyme activity was monitored by direct detection of malonyl-coenzyme A (malonyl-CoA) formation by liquid chromatography tandem mass spectrometry (LC-MS/MS). Steady-state kinetics experiments showed similar k(cat) and K(M) values for both enzymes. In addition, similar IC(50) values were observed for inhibition of both enzymes by a previously reported ACCase inhibitor. To provide a higher throughput assay suitable for inhibitor screening, we developed and validated a luminescence-based ACCase assay that monitors ATP depletion. Finally, we established an enzyme activity assay for the isolated AccAD (carboxyltransferase) subunit, which is useful for determining whether novel ACCase inhibitors inhibit the biotin carboxylase or carboxyltransferase site of ACCase. The methods described here could be applied toward the identification and characterization of novel inhibitors.

Acoustic Droplet Ejection and Open Port Interface for Rapid Analysis of Metabolic Stability Assays.

In vitro absorption, distribution, metabolism and elimination (ADME) assays are widely used for profiling compounds in pharmaceutical drug discovery programs. Many compounds are screened in metabolic stability assays, using liver microsomes as a model of intrinsic hepatic clearance. Analysis of metabolic stability assays has relied on high throughput LC-MS/MS techniques to keep up with automated assays and compound profiling needs. An experimental alternative to sample analysis via fast chromatography employs an open port interface (OPI) which dilutes and directs acoustically-ejected droplets from microtiter plates to a conventional electrospray ion source for ionization and introduction into a mass spectrometer. Metabolic stability assays of 37 commercial drug compounds using in human, dog, rat and mouse liver microsomes (LMs), were analyzed by LC-MS/MS and an experimental breadboard version of an ADE-OPI-MS/MS system. Results from the experiments comparing intrinsic clearance (CLint) generated with ADE-OPI-MS/MS vs fast LC-MS/MS for all compounds showed ≥86% of CLint values were within a factor of two with R2 ≥ 0.86 using 25 nL and 5 nL sample ejection volumes on the ADE-OPI-MS/MS instrument. Throughput with the experimental ADE-OPI-MS/MS system used in this study was more than ten-fold faster than analysis by the fast LC-MS/MS at 1.3 s/sample versus 17.2 s/sample, respectively.

Cherry-picking in an orchard: unattended LC/MS analysis from an autosampler with >32,000 samples online.

The 1,536-well microplate format has widely supplanted the 384-well microplate format for high-throughput screening and for IC(50) assays. Previously, liquid chromatography/mass spectrometry (LC/MS) analyses of such samples required manual transfers of the wells of interest from a 1,536-well plate into a 384-well plate. Because this manual transfer introduced a source of potential error, it became clear that a more appropriate solution would be to sample directly from the 1,536-well plates. Currently, commercially available 1,536-well plate auto samplers are not compatible with Waters LC/MS systems. The authors have modified their CTC PAL autosampler to support injection from up to twenty-four 1,536-well plates. This allows them to cherry-pick any sample from up to 36,864 wells on the autosampler. Because of its success at this Institute, sampling from 1,536-well plates has not only become the preferred method for LC/MS analysis from IC(50) plates but also become the standard format used for the handling of and the sampling from large combinatorial libraries.