1 Determining the Molar Extinction Coefficient of Antimycin A

Antimycin A is an antibiotic that inhibits the mitochondrial bc1 complex. It has become an important tool for studying the function of the bc1 complex and for modulating respiratory activity in cells. Antimycin A binds extremely tightly to its binding site, with a dissociation constant less than 1 nM, and induces spectral changes in the enzyme. This makes it useful for quantitating the amount of bc1 complex in a sample, and for determining accurate extinction coefficients for the cytochromes of the bc1 complex. Antimycin A is a mixture of compounds with slightly different side chains on the same pharmacophore. Its concentration is usually determined not by weight but by absorbance of the pharmacophore. To date the only determination of the extinction coefficient 2 is from the chemists who originally characterized it, and is presumably based on weight. We report an more accurate extinction coefficient based on acidometric titration of the phenolic OH in the pharmacaphore, which induces a distinctive spectral change in the compound. The extinction coefficient of antimycin at 320nm is at the range 5.19~5.36. Introduction and Backgroun