Magnesium stimulation of catalytic activity of horse liver aldehyde dehydrogenase. Changes in molecular weight and catalytic sites
Takahashi, K.; Weiner, H.
Journal of Biological Chemistry 255(17): 8206-8209
1980
ISSN/ISBN: 0021-9258 PMID: 7410360 Accession: 005843400
The addition of .apprx. 0.5 mM Mg2+ ions enhances the specific activity of the pI [isoelectric point] isozymeof horse liver aldehyde dehydrogenase by a factor of 2. The mechanism of the Mg enhancement of the activity was investigated by means of presteady state and steady state kinetic assays and MW determination. The magnitude of the presteady state burst formation of NADH increased from 2 mol/mol of tetrameric enzyme to 4 mol/mol of tetrameric enzyme in the presence of 0.7 mM Mg2+. Both the steady state velocity and the burst magnitude increased to the same degree if less than 0.7 mM Mg2+ was employed. The curves of the burst magnitude and steady state velocity, as determined by the linear portion of the stopped flow run, were sigmoidal with respect to increasing Mg2+ ion concentrations. The steady state velocity as determined by nonstopped flow (static) spectrophotometric assays was hyperbolic. The MW of aldehyde dehydrogenase, as determined by sedimentation equilibrium, decreased from 260,000 in the absence of Mg2+ to 130,000 in the presence of 0.4 mM Mg2+. The dependency of apparent MW on Mg2+ ion concentrations was hyperbolic and essentially parallel to that found for the steady state assay. The presteady state burst doubled in the presence of Mg2+ suggests that the enzyme exhibits half-of-sites reactivity in the absence of metal and all-of-sites reactivity in its presence. The metal also caused a dissociation of tetrameric enzyme into dimers. Assuming that the tetrameric and dimeric forms of the enzyme can function with half-of-sites and all-of-sites reactivities, respectively. An excellent correlation exists between the apparent MW and the specific activity of the enzyme at any Mg2+ concentration.