SULFUR K-EDGE X-RAY-ABSORPTION SPECTROSCOPY - A SPECTROSCOPIC TOOL TOEXAMINE THE REDOX STATE OF S-CONTAINING METABOLITES IN-VIVO

The sulfur K-edge x-ray absorption spectra for the amino acids cystein e and methionine and their corresponding oxidized forms cystine and me thionine sulfoxide are presented. Distinct differences in the shape of the edge and the inflection point energy for cysteine and cystine are observed. For methionine sulfoxide the inflection point energy is 2.8 eV higher compared with methionine. Glutathione, the most abundant th iol in animal cells, also has been investigated. The x-ray absorption near-edge structure spectrum of reduced glutathione resembles that of cysteine, whereas the spectrum of oxidized glutathione resembles that of cystine. The characteristic differences between the thiol and disul fide spectra enable one to determine the redox status (thiol to disulf ide ratio) in intact biological systems, such as unbroken cells, where glutathione and cyst(e)ine are the two major sulfur-containing compon ents. The sulfur K-edge spectra for whole human blood, plasma, and ery throcytes are shown. The erythrocyte sulfur K-edge spectrum is similar to that of fully reduced glutathione. Simulation of the plasma spectr um indicated 32% thiol and 68% disulfide sulfur. The whole blood spect rum can be simulated by a combination of 46% disulfide and 54% thiol s ulfur.