The enzymatic processing of bovine collagen I by neutrophil collagenase (MMP-8) has been monitored at 37°C, envisaging the occurrence of multiple intermediate steps, following the initial cleavage, which leads to the formation of 1 ⁄4 and 3 ⁄4 fragments. Further, the first cleavage event has been investigated at 37°C as a function of pH, and catalytic parameters have been obtained through a global analysis of steady-state kinetic data, such as to get an overall consistent picture of k cat /K m , k cat , and K m . These data have been compared with those obtained from the catalysis by MMP-8 of two synthetic fluorogenic substrates under the same experimental conditions. The overall behavior can be accounted for by the existence of five protonating groups, which vary to a different extent their pK a values for the three substrates investigated. The main observation concerns the fact the two of these residues, which play a relevant role in the enzymatic activity of MMP-8, are relatively far from the primary recognition site, and they are coming into action only for large macromolecular substrates, such as bovine collagen I. This finding opens the question of appropriate testing for inhibitors of the enzymatic action of MMP-8, which must take into account, and also of these relevant interactions occurring only with natural substrates.
Matrix metalloproteinases (MMP)1 are a class of proteolytic enzymes characterized by the presence of a Zn 2ϩ atom in the active site, which are able to process enzymatically several components of the extracellular matrix, such as interstitial and basement membranes, collagen, fibronectin, and laminin. Because of their crucial role in the extracellular matrix degradation, they are implicated in the tissue remodeling processes associated to the growth and development and in several pathological conditions, such as rheumatoid arthritis and tumor invasion (1-3). Collagenases, such as fibroblast collagenase (MMP-1), neutrophil collagenase (MMP-8), and collagenase-3 (MMP-13), are a class of MMPs that specifically cleave several types of native triple-helical collagen (4 -6), and in particular collagen I, at a specific peptide bond between Gly 775 and the residue in position 776 (which can be either Leu or Ile) (4, 7), leading to the formation of 1 ⁄4 and 3 ⁄4 fragments. This event is a crucial one because of the fairly slow rate of collagen turnover within the cartilage (8), and it is closely related to the occurrence of important physiological and pathological events (9 -11).Collagen I is one of the major constituents of the extracellular matrix for several tissues, such as skin, tendon, blood vessels, cartilage, bones, and basal laminas (12), and its triplehelical structural arrangement has been the object of several recent studies both on the native molecule (Ref. 13 and references quoted therein) and on model peptides (14, 15). The investigation on the action of MMPs on collagen has been carried out for a long time, and preliminary measurements have been carried out on the activation ener...