The mechanism of inhibition of two forms of human neutrophil collagenase (HNC) by six Au(I) compounds, some of which are used as chrysotherapeutic agents, has been investigated. The two forms of enzyme studied are active and latent HNC, the latter of which is activated by p-chloromercuribenzoate (PCMB). The effects of PCMB and Zn(II), which are normally included in the assays, on the activity of both forms of HNC and on their inhibition by these Au(I) compounds have also been studied. Zn(II) stimulates the activity of both the active and PCMB-activated latent forms of HNC up to a concentration of 50-100 microM, after which it inhibits markedly. PCMB activates latent HNC up to a concentration of 100 microM followed by inhibition at higher concentrations. Active HNC is not stimulated at PCMB concentrations below 100 microM, but is inhibited at higher concentrations. The stimulatory effects of Zn(II) and PCMB on HNC and its inhibition by PCMB are all attributable to binding at distinct sites. The inhibition of both active and PCMB-activated latent HNC by the Au(I) compounds is noncompetitive and is reversed by Zn(II). The inhibition of both forms of HNC by SKF 80544 and SKF 36914, which do not contain thiol ligands, is weak to moderate and is not influenced by the PCMB concentration. In contrast, PCMB markedly enhances the inhibition by Myocrisin, Sanocrisin, and Solganol by complexing to their thiol ligands to facilitate release of the Au(I) atom for binding to HNC. Cd(II) and Cu(II) also inhibit HNC noncompetitively, and inhibition is also reversed by Zn(II). Collectively, these data indicate that latent HNC contains a heavy metal binding site distinct from the active site at which Au(I), Cd(II), and Cu(II) bind to cause noncompetitive inhibition. Occupancy of this site by Zn(II) is characterized by retention of activity.