Inhibitors of the isoprenylated protein endoprotease

The isoprenylation pathway requires an endoprotease that cleaves the modified protein at the isoprenylated cysteine residue. This endoprotease was readily assayed with simple tetrapeptide substrates of the type N-acetyl-S-farnesyl-L-Cys-(AFC)-Val-Ile-Met, where AFC and the tripeptide are the products of the hydrolysis. The endoprotease proved to be unaffected by (1) serine protease inhibitors, including (4-amidinophenyl)methanesulfonyl fluoride, aprotinin, and leupeptin, by (2) cysteine protease inhibitors, including E-64 and leupeptin [the enzyme is, however, inhibited by p-(hydroxymercuri)benzoate], by (3) metalloprotease inhibitors, including phosphoramidon, EDTA, and 1,10-phenanthroline, or by (4) the aspartyl protease inhibitor pepstatin. The conclusion from these data is that the enzyme is probably not a metalloenzyme. N-Boc-S-all-trans-farnesyl-L-cysteine (BFC) derivatives containing a statine moiety are also not inhibitory, strongly suggesting that the enzyme is not an aspartyl protease. However, the enzyme is potently inhibited by the aldehyde derivative of BFC (K1 = 1.9 microM), which is consistent with the idea that the enzyme is a serine or cysteine protease. Potent tetrapeptide-based competitive inhibitors were prepared. Analogs with the scissile bond modified so that hydrolysis could not occur were excellent inhibitors. An analog containing BFC-statine-Val-Ile-Met inhibited the endoprotease with a K1 = 64 nM. The equivalent pseudopeptide psi (CH2-NH) analog was almost as potent, indicating that the statine moiety simply represents a nonhydrolyzable linker.