Furin, a human subtilisin-related proprotein convertase (SPC), is emerging as an important pharmaceutical target because it processes vital proteins of many aggressive pathogens. Furin inhibitors reported as yet are peptide derivatives and proteins, with the exception of andrographolides, which are natural compounds. Here we report that the small and highly stable compounds M(chelate)Cl(2) (M is copper or zinc) inhibit furin and Kex2, with Cu(TTP)Cl(2) and Zn(TTP)Cl(2) as the most efficient inhibitors. (TTP is 4'-[p-tolyl]-2,2 ':6',2"-terpyridine.) Inhibition is irreversible, competitive with substrate, and affected by substituents on the chelate. The free chelates are not inhibitors. Solvated Zn(2+) is less potent than its complexes. This is true also for copper and Kex2. However, solvated Cu(2+) (k(on) of 25,000 +/- 2,500 s(-1)) is more potent than Cu(TTP)Cl(2) (k(on) = 140 +/- 13 s(-1) and allows recovery of furin activity prior to a second inhibition phase. A mechanism that involves coordination to the catalytic histidine is proposed for all inhibitors. Target specificity is indicated by the fact that these metal chelate inhibitors are much less potent toward Kex2, the yeast homologue of furin. For example, k(on) with Zn(TTP)Cl(2) is 120 +/- 20 s(-1) for furin, but only 1.2 +/- 0.1 s(-1) for Kex2.