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  • Title: Spin-label and fluorescence labeling studies of the thioester bonds in human alpha 2-macroglobulin.
    Author: Zhao BL, Musci G, Sugawara Y, Berliner LJ.
    Journal: Biochemistry; 1988 Jul 12; 27(14):5304-8. PubMed ID: 2458760.
    Abstract:
    Upon cleavage of the reactive thioester bonds (Cys-949-Glx-952) of tetrameric human alpha 2-macroglobulin (alpha 2M) by methylamine, one sulfhydryl group per alpha 2M subunit is exposed. These identical sulfhydryl group sites were labeled with the thiol-specific nitroxide spin-labels (1-oxy-2,2,5,5-tetramethyl-3-pyrrolin-3-yl)methyl methanethiosulfonate and (1-oxy-2,2,6,6-tetramethyl-4-piperidinyl)methyl methanethiosulfonate, a homologous series of maleimide spin-labels, and the thiol-specific fluorescent probe 2-[(4-maleimidophenyl)amino]naphthalene-6-sulfonic acid sodium salt (MANS). The ESR and fluorescence results showed that these sulfhydryl group sites were at the base of a narrow crevice that is greater than or equal to 8 A deep. Although the bound MANS fluorophore was slightly blue shifted with an enhanced quantum yield vs the free label in water, the environment of the sulfhydryl site appeared to be of a polar nature when compared with the emission maxima in several solvents of varying polarity. The Glx residue participating in the thioester linkage in the intact protein was labeled with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl. The distance between the Glx and Cys moieties was estimated at greater than or equal to 10-25 A from double spin-labeling experiments.
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