MASS-SPECTROMETRIC METHODS FOR DISTINGUISHING STRUCTURAL ISOMERS OF GLUTATHIONE CONJUGATES OF ESTRONE AND ESTRADIOL

Collisionally activated decompositions (CAD) of [M+H](+) ions from two sets (estrone and estradiol) of three isomeric glutathione (GSH) conj ugates were studied by using five tandem mass spectrometric methods: ( 1) low energy (LE) CAD in an ion trap, (2) LE CAD in a triple quadrupo le, (3) electrospray ionization (ESI)-source CAD in a tandem four sect or, (4) high energy (HE) CAD of both ESI-produced and fast-atom bombar dment (FAB)-produced ions in a tandem four-sector mass spectrometer, a nd (5) metastable-ion decompositions of FAB-produced ions. Four types of fragment ions are produced. The first type, formed from cleavage of the peptide backbone, gives rise to modified b(2), modified y(2), y(2 ), and b(1) ions. These fragments are observed with all the methods an d show that the catechol estrogen attachment is at the cysteine moiety of the GSH. Internal fragment ions are the second type, and they also support that the modification is at cysteine. The third type involves fragmentation of the C-S bond to give an ion containing the steroid b onded to the sulfur. The fourth type of fragment ion is similar to the third but involves oxidation of the steroid ring and reduction of the GSH moiety; it is the most isomer specific of the four. The isomer-sp ecific ions are of relatively low abundance in the product-ion spectra taken on the triple quadrupole and ion tray, but their abundances can be improved by increasing the collision energy. ESI source-CAD and th e HE-CAD spectra of the isomers are the most distinctive because abund ant product ions of all four types are seen in a single spectrum. (C) 1998 American Society for Mass Spectrometry.