The hydrolysis of [Leu]enkephalin and substance P by purified pig kidney endopeptidase (EC 3.4.24.11) and synaptic membranes prepared from pig caudate nuclei has been compared. The hydrolysis of an enkephalin analogue (Tyr-D-Ala-GlyPhe-Leu) at the Gly-Phe bond was completely inhibited by phosphoramidon. The IC50 concentration (8 nM) was similar to that reported for [Leu] Biochem. J. 203,[519][520][521][522]. Seven peptides were produced when substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-GlyLeu-Met-NH2) was hydrolyzed by the kidney endopeptidase. These were formed by cleavage at bonds Gln-Phe (positions 6 and 7), Phe-Phe (positions 7 and 8), and Gly-Leu (positions 9 and 10). Synaptic membranes generated peptides with the same HPLC retention times and hydrolysis of substance P by either preparation was inhibited completely by 10 FM phosphoramidon. The most susceptible bond appeared to be Gly-Leu (positions 9 and 10). A specific polyclonal antibody raised in rabbits to purified pig endopeptidase inhibited the hydrolysis of [Leu]enkephalin and substance P by detergent-solubilized kidney microvilli or synaptic membranes; the titration curves were essentially identical. We conclude that the endopeptidase, which we suggest should be designated "endopeptidase-24.11," is present in caudate synaptic membranes and could play an important role in the hydrolysis of neuropeptides.When the inactivation of biologically active peptides has been reported it has become commonplace to name the peptidase responsible in a way that implies the existence of peptide-specific enzymes. Although peptidases of high specificity do exist, such a close relationship between peptidase and substrate is unusual. The field of neuropeptide metabolism in brain has, in particular, been plagued by this restrictive nomenclature [e.g., enkephalinase and substance P-degrading enzyme (1-4)]-a limitation that has consequently hindered comparisons with wellcharacterized peptidases from other tissues. It is our contention (5) that a limited number of peptidases mediate a wide range of functions and that their cellular and subcellular localization, rather than peptide specificity, defines their roles at different sites.

Neutral endopeptidase (EC 3.4.24.11) from pig kidney hydrolyses [125I]iodo-insulin B-chain and leucine-enkephalin. Both activities were equally sensitive to inhibition by phosphoramidon [N-(alpha-L-rhamnopyranosyloxyhydroxyphosphinyl)-L-leucyl-L-tryptophan] and thiorphan [N-(DL-2-benzyl-3-mercaptopropionyl)glycine]. Thermolysin hydrolysis of insulin B-chain was also sensitive to both inhibitors. The hydrolysis of the Gly3-Phe4 bond of Leu-enkephalin by synaptic membranes prepared from pig brain was partially inhibited by phosphoramidon and thiorphan. Synaptic membranes appear to contain another endopeptidase activity that is insensitive to these reagents. These observations suggest that enzymes similar to the kidney endopeptidase may play a general role in neuropeptide metabolism.

The purification of detergent-solubilized kidney microvillar endopeptidase (EC 3.4.24.11) by immuno-adsorbent chromatography is described. The product (the d-form) was 270-fold purified compared with the homogenate of kidney cortex and was obtained in a yield of 5%. It was free of other peptidase activities and homogeneous by electrophoretic analyses. It contained about 15% carbohydrate and one Zn atom/subunit. Two trypsin-treated forms were also characterized. One (dt-form) was obtained by treatment of the d-form. The other (tt-form) was the result of solubilizing the membrane by treatment with toluene and trypsin. All three forms had apparent subunit Mr values of approx. 89 000, but the d-form appeared to be slightly larger than the other two. Estimates of Mr by gel filtration showed that of the tt-form to be 216 000 whereas those of the other forms were 320 000. An estimate of the detergent (Triton X-100) bound to the d- and dt-forms accounted for this difference. By several criteria, including charge-shift crossed immunoelectrophoresis and hydrophobic chromatography, the d- and dt-forms were shown to be amphipathic molecules. In contrast, the tt-form was hydrophilic in its properties. Differences in ionic properties were also noted, consistent with the loss, in the case of the dt-form, of a positively charged peptide. The results indicate that the native endopeptidase is a dimeric molecule, each subunit being anchored in the membrane by a relatively small region of the polypeptide close to one or other terminus. The d- and dt-forms had similar enzyme activity when assayed by the hydrolysis of 125I-insulin B-chain. Chelating agents and phosphoramidon inhibited the endopeptidase. The kinetic constants were determined by a new two-stage fluorimetric assay using glutarylglycylglycylphenylalanine 2-naphthylamide as substrate and aminopeptidase N (EC 3.4.11.2) to hydrolyse phenylalanine 2-naphthylamide. The Km was 68 microM and Vmax. 484nmol X min-1 X (mg of protein)-1.

Pig kidney microvillar proteins were extracted with octyl beta-glucoside and reconstituted in liposomes prepared from microvillar lipids of known composition. Four peptidases, namely endopeptidase (EC 3.4.24.11), aminopeptidases N (EC 3.4.11.2) and A (EC 3.4.11.7) and dipeptidyl peptidase IV (EC 3.4.14.5), were shown to be reconstituted. At lipid/protein ratios greater than 4:1, about half the detergent-solubilized protein and nearly all of the activity of the four peptidases were reconstituted. Dissolution of the liposomes with Triton X-100 did not increase the activity of any of these peptidases, a result consistent with an asymmetric, 'right-side-out', orientation of these enzymes. When purified, endopeptidase was subjected to the same procedure; the two amphipathic forms of the enzyme (the detergent form and the trypsin-treated detergent form) were fully reconstituted. The amphiphilic form, purified after toluene/trypsin treatment, failed to reconstitute. Electron microscopy of microvilli showed that the appearance of the surface particles was profoundly altered by treatment with papain. Before treatment, the microvilli were coated with particles of stalk lengths ranging from 2.5 to 9 nm. After papain treatment nearly all the particles had stalks of 2-3 nm. Reconstituted microvillar proteins in liposomes showed the same heterogeneity of stalk length. In contrast, liposomes containing reconstituted endopeptidase revealed a very homogeneous population of particles of stalk length 2 nm. Since the smallest dimension of a papain molecule is 3.7 nm, the ability of papain, and other proteinases of similar molecular size, to release microvillar enzymes is crucially affected by the length of the junctional peptide that constitutes the stalk of this type of membrane protein.

Endopeptidase-24.11 (EC 3.4.24.11) was purified from pig intestinal microvilli by immunoadsorbent chromatography, using antibodies raised to kidney endopeptidase-24.11. In many respects, the kidney and intestinal enzymes were indistinguishable, but some structural differences were demonstrated. In particular, the detergent form of the intestinal enzyme had an apparent subunit Mr of 95000, which, on treatment with trypsin, fell to a value of 89000, identical with that of the kidney form. The intestinal enzyme contained 3-4% more carbohydrate and many more fucose residues than that from kidney. Although these results show that post-translational processing was different in the two cell types, the possibility that the primary translation products also differed cannot be excluded.