A continuous assay for α-methylacyl-coenzyme A racemase using circular dichroism

alpha-Methylacyl-coenzyme A racemase (AMACR) catalyzes the epimerization of (2R)- and (2S)-methyl branched fatty acyl-coenzyme A (CoA) thioesters. AMACR is a biomarker for prostate cancer and a putative target for the development of therapeutic agents directed against the disease. To facilitate development of AMACR inhibitors, a continuous circular dichroism (CD)-based assay has been developed. The open reading frame encoding AMACR from Mycobacterium tuberculosis (MCR) was subcloned into a pET15b vector, and the enzyme was overexpressed and purified using metal ion affinity chromatography. The rates of MCR-catalyzed epimerization of either (2R)- or (2S)-ibuprofenoyl-CoA were determined by following the change in ellipticity at 279 nm in the presence of octyl-beta-D-glucopyranoside (0.2%). MCR exhibited slightly higher affinity for (2R)-ibuprofenoyl-CoA (K-m = 48 +/- 5 mu M, k(cat) = 291 +/- 30 s(-1)), but turned over (2S)-ibuprofenoyl-CoA (K-m = 86 +/- 6 mu M, k(cat) = 450 +/- 14 s(-1)) slightly faster. MCR expressed as a fusion protein bearing an N-terminal His(6)-tag had a catalytic efficiency (k(cat)/K-m) that was reduced 22% and 47% in the 2S -> 2R and 2R -> 2S directions, respectively, relative to untagged enzyme. The continuous CD-based assay offers an economical and efficient alternative method to the labor-intensive, fixed-time assays currently used to measure AMACR activity. (C) 2009 Elsevier Inc. All rights reserved