ACTIVE-SITE LABELING OF ERYTHROCYTE TRANSGLUTAMINASE BY O-PHTHALALDEHYDE

Tissue-type transglutaminase is inactivated in a time-dependent way du ring incubation with submillimolar concentrations of o-phthalaldehyde, with affinity labeling kinetics. The rate of inactivation by the reag ent is greatly enhanced in the presence of the essential enzyme cofact or calcium and is decreased by GTP, an allosteric inhibitor. A fluores cent isoindole derivative is formed during the modification apparently through crosslinkage of active site Cys 277 to a lysine residue. Thes e data and the quenching of fluorescence by addition of calcium ions s uggest that the enzyme active site is directly involved in the inactiv ation process.