Tissue-type transglutaminase is inactivated in a time-dependent way during incubation with submillimolar concentrations of o-phthalaldehyde, with affinity labeling kinetics. The rate of inactivation by the reagent is greatly enhanced in the presence of the essential enzyme cofactor calcium and is decreased by GTP, an allosteric inhibitor. A fluorescent isoindole derivative is formed during the modification apparently through crosslinkage of active site Cys 277 to a lysine residue. These data and the quenching of fluorescence by addition of calcium ions suggest that the enzyme active site is directly involved in the inactivation process.
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