Involvement of a lysine residue in the inactivation of Leuconostoc mesenteroides NRRL B-512F dextransucrase by o-phthalaldehyde.

Leuconostoc mesenteroides NRRL B-512F dextransucrase was rapidly and irreversibly inactivated by o-phthalaldehyde. The dextransucrase-o-phthalaldehyde adduct showed a characteristic fluorescence maxima at 417 nm when excited at 337 nm. These results were consistent with the isoindole derivative formation in which the sulfhydryl group of cysteine and epsilon-amino group of lysine participate in the reaction. The stoichiometric determinations gave one isoindole derivative per enzyme molecule upon complete inactivation by o-phthalaldehyde. The enzyme showed no inhibition on treatment with thiol specific reagents. This indicated that cysteine is present in close proximity of the lysine and is involved in the isoindole derivative formation but is not participating in the catalysis. These results established for the first time that one lysine residue present at the active site is required for the activity of dextransucrase.