In our previous study we demonstrated selective, dose-dependent binding of matrix metalloproteinase-9 (MMP-9), a neutrophil collagenase, to fibrin. Here we investigated the mechanism of this interaction. We found that MMP-9 to fibrin was dependent on formation of a calcium-phosphate intermediate. The intermediate was precipitable by centrifugation and contained a Ca/P ratio of 1.52-1.54, consistent with amorphous calcium-phosphate (ACP). ACP formation exhibited a temperature optimum at 37 degrees C. Gelatin zymography revealed that interaction of ACP with MMP-9 resulted in formation of a high molecular weight ACP:MMP-9 complex which was required for MMP-9 binding to fibrin. Complex formation was dependent on the generation of viable ACP that required both calcium (7.5-10 mM) and phosphate (225-250 microM) (Ca x P product range, 1.7-2.5 mM2). Carbonate (CO3) and sulfate (SO4) were ineffective as calcium counteranions. Preformed ACP rapidly complexed MMP-9. Thus ACP formation was rate-limiting for MMP-9 fibrin binding activity. No MMP-9 fibrin binding activity was noted at 25 degrees C, an observation consistent with lack of ACP production. The significance of these findings is discussed with respect to normal and pathologic wound healing.