We describe a simple method for the analysis of activation systems in which a metal ion modifier may combine with either the enzyme or the substrate (or both) and the metal ion-substrate complex is the true substrate of the enzyme reaction. The suggested approach is essentially a ‘graphical’ method that both provides unbiased criteria for the choice of the activation mechanism and yields good rough estimates of the kinetic parameters. The procedure, tested on a variety of simulated models, produces appropriate and reliable results. Applying this treatment to isocitrate lyase, we confirmed the data previously reported for Mg2+ [Giachetti, Pinzauti, Bonaccorsi & Vanni (1988) Eur. J. Biochem. 172, 85-92], and we found that Mn2+ functions with the same mechanism as does Mg2+, but with quite different kinetic constants. In particular, its ratio of the Vmax, values of the activated and the non-activated enzyme is less than 1, and thus Mn2+ is to be considered an inhibitor rather than an activator.
