The pH dependence of the hydrolysis of chromogenic substrates of the type, Lys-Pro-Xaa-Yaa-Phe-(NO2)Phe-Arg-Leu, by selected aspartic proteinases: evidence for specific interactions in subsites S3 and S2.

Variation in the kinetic parameters, kcat and Km, with pH has been used to obtain evidence for significant acid-dissociation processes in the hydrolysis of octapeptide substrates by three aspartic proteinases. These substrates are all cleaved at the peptide bond between a Phe (P1) and a p-nitroPhe (P1') residue resulting in a shift in absorbance at 300 nm that facilitates kinetic measurements. The substrates differ in the amino-acid residues present in the P3 and the P2 positions. Porcine pepsin, calf chymosin, and the aspartic proteinase from Endothia parasitica all show pH dependencies that imply that favorable or unfavorable interactions can occur with the S3 or S2 areas of the enzyme-active site. Examination of the crystallographically determined structure of the E. parasitica proteinase and consideration of the amino-acid sequence differences between the three enzymes suggests that the origin of the pH effects arises from favorable interactions between Glu-13 (COO-) of pig pepsin and Thr (OH) or His (ImH+) in P3 of a substrate. Similarly, Lys-220 (NH3+) of chymosin and a Glu (COO-) in P2 of a substrate may produce a favorable interaction and Asp-77 (COO-) of E. parasitica proteinase and a Glu (COO-) in P2 of a substrate may produce an unfavorable interaction. These results lead to possible explanations for subtle specificity differences within a family of homologous enzymes, and suggest loci for study by site-directed mutagenesis.

Immunolocalization of cathepsin D in normal and neoplastic human tissues.

The aspartic proteinase cathepsin D was purified from human spleen and localised in various formalin fixed paraffin embedded human tissues using the peroxidase-antiperoxidase (PAP) technique. Cathepsin D was shown not only in macrophages but also in other connective tissue cells, and in epithelium. It was present in spleen (littoral cells and cells within Malpighian bodies), liver (hepatocytes and Kupffer cells), lung (alveolar macrophages and bronchial epithelium), brain (neurones), lymph nodes (histiocytes in germinal centres, sinusoid lining cells) and stomach (parietal and mucous neck cells). Cathepsin D was also found in carcinomas of bronchus, stomach, colon, kidney, breast, ovary, bladder and pancreas, both in neoplastic epithelium and in stromal cells, but was seldom present in connective tissue neoplasms. A group of malignant lymphomas also contained the enzyme within scattered cells. The distribution of cathepsin D seems to be much wider than that of the structurally related aspartic proteinases pepsin, gastricsin, and renin.

Identification of the acid proteinase in human seminal fluid as a gastricsin originating in the prostate.

On the basis of a) kinetic data obtained with a synthetic substrate and two peptide inhibitors and b) immunological cross-reactivity, it is shown that the aspartic proteinase of human seminal fluid is a gastricsin. The source of the precursor (progastricsin) in the male genital tract is identified to be the prostate.

The selectivity of action of the aspartic-proteinase inhibitor IA3 from yeast (Saccharomyces cerevisiae).

The ability of the aspartic-proteinase inhibitor IA3 from yeast (Saccharomyces cerevisiae) to affect the activities of a range of mammalian and microbial aspartic proteinases was examined. The inhibitor appeared to be completely selective in that only the aspartic proteinase A from yeast was inhibited to any significant extent. IA3 thus represents the first example of a totally specific, naturally occurring, aspartic-proteinase inhibitor.

Activation of spin-labeled chicken pepsinogen.

Chicken pepsinogen has been spin-labeled by the attachment of four nitroxides to epsilon-amino groups near the protein's amino terminus. Acidification results in a bond cleavage, generating a nonlabeled, enzymatically active protein. Electron spin resonance spectra of the spin-labeled zymogen, acidified in the presence or absence of pepstatin, are identical and indicate that the nitroxides are quite mobile, compared to the nonacidified zymogen. This mobilization is interpreted as the freeing of the peptide to which the spin-labels are attached, from the protein, subsequent to the acidification that causes a peptide bond cleavage. The rate at which the peptide leaves the protein is 1 order of magnitude slower than the cleavage of the peptide bond, measured by the rate of appearance of milk-clotting activity (first-order rate constants of 0.3 min-1 vs. 6 min-1 at pH 2, 22 degrees C). The inclusion of pepstatin, at molar ratios above 2 during activation, decreases the rate of peptide leaving. These observations, and those previously reported for activation of spin-labeled pig pepsinogen, are incorporated into a model of pepsinogen activation.

A systematic series of synthetic chromophoric substrates for aspartic proteinases.

The hydrolysis of the chromogenic peptide Pro-Thr-Glu-Phe-Phe(4-NO2)-Arg-Leu at the Phe-Phe(4-NO2) bond by nine aspartic proteinases of animal origin and seven enzymes from micro-organisms is described [Phe(4-NO2) is p-nitro-L-phenylalanine]. A further series of six peptides was synthesized in which the residue in the P3 position was systematically varied from hydrophobic to hydrophilic. The Phe-Phe(4-NO2) bond was established as the only peptide bond cleaved, and kinetic constants were obtained for the hydrolysis of these peptide substrates by a representative selection of aspartic proteinases of animal and microbial origin. The value of these water-soluble substrates for structure-function investigations is discussed.

Slow moving proteinase. Isolation, characterization, and immunohistochemical localization in gastric mucosa.

Human gastric mucosa contains three immunochemically distinguishable aspartic proteinases, pepsinogen I (pepsinogen A), pepsinogen II (pepsinogen C, progastricsin), and a nonpepsinogen proteinase also termed slow moving proteinase (SMP). The properties of SMP, and in particular its relationship to another aspartic proteinase, cathepsin D, were examined in this study. Slow moving proteinase and cathepsin D were isolated, respectively, from gastric mucosa and human spleen. Antiserum specific to each proteinase was prepared in rabbits. Rabbit anti-SMP did not recognize cathepsin D, and conversely, anticathepsin D did not react with SMP. Immunohistochemical studies localized SMP to surface epithelial cells in both the fundic and pyloric gland areas of the stomach. In contrast, cathepsin D was found mainly in mononuclear cells in the lamina propria and in parietal cells. Slow moving proteinase exhibited considerably lower Km values for its interaction with two chromogenic substrates than did cathepsin D. An even greater distinction between the two enzymes was found with the protein inhibitor from Ascaris lumbricoides; the activity of SMP was inhibited very strongly, whereas that of cathepsin D was not affected. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis under denaturing conditions, SMP consisted of two subunits with apparent molecular weights of 42,500 and 41,000. The last two properties characterize a less-well-known aspartic proteinase, cathepsin E. We conclude that SMP is not cathepsin D, but that it may be cathepsin E.

High resolution X-ray analyses of renin inhibitor-aspartic proteinase complexes.

Inhibitors of the conversion of angiotensinogen to the vasoconstrictor angiotensin II have considerable value as antihypertensive agents. For example, captopril and enalapril are clinically useful as inhibitors of angiotensin-converting enzyme. This has encouraged intense activity in the development of inhibitors of kidney renin, which is a very specific aspartic proteinase catalysing the first and rate limiting step in the conversion of angiotensinogen to angiotensin II. The most effective inhibitors such as H-142 and L-363,564 have used non-hydrolysable analogues of the proposed transition state, and partial sequences of angiotensinogen (Table 1). H-142 is effective in lowering blood pressure in humans but has no significant effect on other aspartic proteinases such as pepsin in the human body (Table 1). At present there are no crystal structures available for human or mouse renins although three-dimensional models demonstrate close structural similarity to other spartic proteinases. We have therefore determined by X-ray analysis the three-dimensional structures of H-142 and L-363,564 complexed with the aspartic proteinase endothiapepsin, which binds these inhibitors with affinities not greatly different from those measured against human renin (Table 1). The structures of these complexes and of that between endothiapepsin and the general aspartic proteinase inhibitor, H-256 (Table 1) define the common hydrogen bonding schemes that allow subtle differences in side-chain orientations and in the positions of the transition state analogues with respect to the active-site aspartates.