1. L-trans-Epoxysuccinyl-leucylamido(4-guanidino)butane (E-64) at a concentration of 0.5 mM had no effect on the serine proteinases plasma kallikrein and leucocyte elastase or the metalloproteinases thermolysin and clostridial collagenase. In contrast, 10 muM-E-64 rapidly inactivated the cysteine proteinases cathepsins B, H and L and papain (t0.5 = 0.1-17.3s). The streptococcal cysteine proteinase reacted much more slowly, and there was no irreversible inactivation of clostripain. The cysteine-dependent exopeptidase dipeptidyl peptidase I was very slowly inactivated by E-64. 2. the active-site-directed nature of the interaction of cathepsin B and papain with E-64 was established by protection of the enzyme in the presence of the reversible competitive inhibitor leupeptin and by the stereospecificity for inhibition by the L as opposed to the D compound. 3. It was shown that the rapid stoichiometric reaction of the cysteine proteinases related to papain can be used to determine the operational molarity of solutions of the enzymes and thus to calibrate rate assays. 4. The apparent second-order rate constants for the inactivation of human cathepsins B and H and rat cathepsin L by a series of structural analogues of E-64 are reported, and compared with those for some other active-site-directed inhibitors of cysteine proteinases. 5. L-trans-Epoxysuccinyl-leucylamido(3-methyl)butane (Ep-475) was found to inhibit cathepsins B and L more rapidly than E-64. 6. Fumaryl-leucylamido(3-methyl)butane (Dc-11) was 100-fold less reactive than the corresponding epoxide, but was nevertheless about as effective as iodoacetate.

A new thiol protease inhibitor, named E-64, was isolated from the extract of a solid culture of Aspergillus japonicus TPR-64 freshly isolated from soil.E-64 was obtained as white needles and the empirical formula was estimated to be C15N5-H27O5 (M. W. 357). This was almost neutral in its electrophoretic behavior and proved to be a specific and strong inhibitor toward thiol protease such as papain and cathepsin B. They combine equimolecularly and irreversibly.

New derivatives of E-64 (compound CA-030 and CA-074) were tested in vitro and in vivo for selective inhibition of cathepsin B. They exhibited 10,000-30,000 times greater inhibitory effects on purified rat cathepsin B than on cathepsin H and L: their initial Ki values for cathepsin B were about 2-5 nM, like that of E-64-c, whereas their initial Ki values for cathepsins H and L were about 40 200 microM. In in vivo conditions, such as intraperitoneal injection of compound CA-030 or CA-074 into rats, compound CA-074 is an especially potent selective inhibitor of cathepsin B, whereas compound CA-030 does not show selectivity for cathepsin B, although both compounds CA-030 and CA-074 show complete selectivity for cathepsin B in vitro.

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Ethyl ( + )-(2S,3S)-3-[ (S)-3-methyl-1-(3-methylbutylcarbamoyl)butylcarbamoyloxiranecarboxylate (EST; la) is expected to be useful as an oral therapeutic agent for muscular dystrophy on the basis of its potent inhibitory activities against the cysteine proteinases involved in the myofibrillar protein degradation that occurs in the disease. Through extensive investigations aimed at developing a new synthetic method for la that would be suitable for industrial application, it has been found that L-arginine can be used as a new, efficient resolving agent to obtain optically pure Ltrans-epoxysuccinic acid (3a), and the active ester method using p-nitrophenol is very effective in the coupling reaction of ethyl L-trans-epoxysuccinate (7a) and L-leucine isoamylamide (8a) because of the extremely low formation of by-products.To examine the contribution of the stereochemistry of the trans-epoxysuccinic acid and leucine moieties to the inhibitory activity against cysteine proteinases, the diastereomers (lb-d) of la were synthesized by a similar method and the rate constants of inactivation of papain by la-d were measured. Compound la, having L-trans-epoxysuccinic acid and L-leucine moieties, showed the most potent activity among them.

In order to investigate the active site of inhibition of E-64 against papain, the constituents of E-64 and their derivatives were synthesized and their activities on papain were assayed. It was consequently found trans-epoxysuccinic acid was essential for the activity.

Inactivators of cysteine endopeptidases were tested as inhibitors of the cytokine-stimulated release of proteoglycan from cartilage. The test system consisted of bovine nasal septum cartilage maintained in organ culture, and the stimulus was provided by recombinant human interleukin 1 alpha. L-3-Carboxy-2,3-trans-epoxypropionyl-leucylamido-(4-guanidin o)butane (E64) and L-3-carboxy-2,3-trans-epoxypropionyl-leucylamido-(3-methyl)b utane (Ep475) showed no inhibition at concentrations up to 100 microM. In contrast, trans-epoxysuccinyl-leucylamido-(3-methyl)butane ethyl ester (Ep453), a 'prodrug' of Ep475, was an effective inhibitor. The LL-, LD- and DL-isomers gave significant inhibition at 10 microM, and the DD-isomer was inhibitory at 100 microM. None of the isomers had any detectable effect on protein synthesis or glycolysis, and their inhibitory effects were reversible. Iodoacetate inhibited proteoglycan release by a general toxic effect. Our results suggest that cysteine endopeptidase(s) play a part in cytokine-stimulated cartilage breakdown, but that effective inhibitors must pass through membranes.

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