Monovalent cations requirements for the stability of betaine aldehyde dehydrogenase from Pseudomonas aeruginosa, porcine kidney and amaranth leaves

Betaine aldehyde dehydrogenase from the human pathogen Pseudomonas aeruginosa requires K+ ions for maintenance of its active conformation. In order to explore if this property is shared by other BADHs of different origins and to further understand the mechanism underlying the effects of these ions, we carried out a comparative study on the stability and quaternary structure of P. aeruginosa, porcine kidney and amaranth leaves BADHs in the absence of K+ ions. At low enzyme concentrations, the bacterial and porcine enzymes were totally inactivated upon removal of K+ following biphasic and monophasic kinetics, respectively, whereas the amaranth enzyme retained its activity. Inactivation of P. aeruginosa BADH was much faster than that of the porcine enzyme. The oxidized coenzyme protected both enzymes against inactivation by the absence of K+, whereas betaine aldehyde afforded partial protection to the bacterial BADH and increased the inactivation rate of the porcine. Reactivation of the inactive enzymes, by adding back to the incubation medium K+ ions, was dependent on enzyme concentration, suggesting that enzyme dissociation takes place in the absence of K+. In the bacterial enzyme, NH4+ but not Na+ ions could mimic the effects of K+, whereas the three cations tested reactivated porcine BADH, indicating a requirement of this enzyme for high ionic strength rather than for a specific monovalent cation. Size exclusion chromatography of the inactivated enzymes confirmed that K+ ions or other monovalent cations are required for the maintenance of the quaternary structure of these two BADHs. At pH 7.0, in the absence of K+ in a buffer of low ionic strength, the active tetrameric form of P. aeruginosa BADH dissociated into inactive monomers and that of porcine kidney BADH into inactive dimers. Once reactivated, both enzymes reassociated into active tetramers. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.