ABSTRACT: Tervalent cations of the lanthanide (rare-earth) elements reversibly inhibit bacterial collagenase (clostridiopeptidase A; EC 3.4.24.3). Sm(3+), whose ionic radius is closest to that of Ca(2+), is the most effective inhibitor, completely suppressing clostridiopeptidase activity at a concentration of 100mum in the presence of 5mm-Ca(2+). Er(3+) and Lu(3+), which both have ionic radii smaller than either Ca(2+) or Sm(3+), inhibit less efficiently, and La(3+), which is slightly larger than Ca(2+) or Sm(3+), inhibits only weakly. These findings indicate a closely fitting, stereospecific, Ca(2+)-binding pocket in clostridiopeptidase, which excludes ions that are only slightly larger than Ca(2+) [ionic radius 0.099nm (0.99 A)]. By contrast, trypsin, an enzyme whose activity does not depend on Ca(2+), requires lanthanide concentrations 50-100-fold greater for inhibition. Furthermore, the relative efficiency of inhibition of trypsin by lanthanides increases as the lanthanide ions become smaller and the charge/volume ratio increases. At a concentration of 50mum, Sm(3+) lowers the apparent K(m) for the hydrolysis of Pz-peptide by clostridiopeptidase from 5.4mm to 0.37mm and the apparent V(max.) from 0.29 Wünsch-Heidrich unit to 0.018 unit. Thus Sm(3+) enhances the affinity of this enzyme for its substrate; inhibition of hydrolysis of Pz-peptide may result from the excessive stability of the enzyme-Sm(3+)-substrate complex. Inhibition by Sm(3+) is competitive with regard to Ca(2+). The apparent dissociation constant, K(d), of Ca(2+) is 0.27mm, where the K(i) for Sm(3+) is 12mum. Clostridiopeptidase is more thermolabile in the absence of Ca(2+). With Sm(3+), thermoinactivation of the enzyme at 53 degrees C or 60 degrees C is initially accelerated, but then becomes retarded as heating continues. Lanthanide ions bind to gelatin and collagen. In so doing, they appear to protect these substrates from lysis by clostridiopeptidase through mechanisms additional to supplanting Ca(2+) at its binding site on the enzyme. Collagen and gelatin sequester sufficient lanthanide ions to gain partial protection from clostridiopeptidase in the absence of an extraneous source of these inhibitors.
