Pentapeptide (pepstatin) inhibition of brain acid proteinase

In vitro selection of macrocyclic peptide inhibitors containing cyclic γ2,4-amino acids targeting the SARS-CoV-2 main protease

γ-Amino acids can play important roles in the biological activities of natural products; however, the ribosomal incorporation of γ-amino acids into peptides is challenging. Here we report how a selection campaign employing a non-canonical peptide library containing cyclic γ^2,4-amino acids resulted in the discovery of very potent inhibitors of the SARS-CoV-2 main protease (M^pro). Two kinds of cyclic γ^2,4-amino acids, cis-3-aminocyclobutane carboxylic acid (γ¹) and (1R,3S)-3-aminocyclopentane carboxylic acid (γ²), were ribosomally introduced into a library of thioether-macrocyclic peptides. One resultant potent M^pro inhibitor (half-maximal inhibitory concentration = 50 nM), GM4, comprising 13 residues with γ¹ at the fourth position, manifests a 5.2 nM dissociation constant. An M^pro:GM4 complex crystal structure reveals the intact inhibitor spans the substrate binding cleft. The γ¹ interacts with the S1' catalytic subsite and contributes to a 12-fold increase in proteolytic stability compared to its alanine-substituted variant. Knowledge of interactions between GM4 and M^pro enabled production of a variant with a 5-fold increase in potency.

Bovine brain cathepsin D: inhibition by pepstatin and binding to concanavalin A

1. Cathepsin D from bovine brain has been purified 1100-fold in 46% recovery. Three isozymes are present with pI (+/- 0.05) = 6.10, 6.30 and 6.40. 2. The isozymes are single polypeptide chains with apparent Mr = 42,000 and are similar with respect to substrate binding and cleavage; the pH-optimum is 3.5 with virtually no activity at neutral pH. 3. Pepstatin inhibits the enzyme and kinetic data are consistent with a "tight binding" mechanism. 4. The dissociation constant for the concanavalin A-enzyme complex is Kd = 19 nM at pH 5.0. 5. Under conditions where 90% of the enzyme is bound to soluble concanavalin A, full enzymatic activity is observed.

Acidic lipids enhance cathepsin D cleavage of the myelin basic protein

Some acidic lipids including sulfatide and phosphatidylinositol were found to increase greatly the rate of cathepsin D cleavage of the myelin basic protein. Since a similar effect was seen when the substrate was changed to cytochrome C, but not when the enzyme was changed to pepsin, these acidic lipids seem to be acting on cathepsin D rather than on myelin basic protein itself. Even so, chemical modification studies suggest that this phenomenon is only seen when the myelin basic protein is in its native conformation. Succinylation of MBP increases its rate of cleavage by cathepsin D by at least tenfold and, in addition, with this modified and presumably denatured MBP as substrate, activation of cathepsin D is no longer seen with acidic lipids. These findings suggest that the native conformation of MBP is both an important determinant of its rate of cleavage by cathepsin D and is also essential for observing activation of this reaction by acidic lipids. The acidic lipids seem to alter the "extended active site" of cathepsin D in such a way as to enable this enzyme better to utilize the native myelin basic protein as a substrate. Cathepsin D has previously been implicated as the protease responsible for the release into cerebrospinal fluid in multiple sclerosis patients of an encephalitogenic fragment derived from myelin basic protein. It is possible that the elevated levels of cathepsin D and sulfatide that have previously been found associated with multiple sclerosis plaques in vivo act in concert to bring about the rapid cleavage and subsequent loss of the myelin basic protein from these localized regions in the myelin sheath.

Proteases of human brain

Growing appreciation of the multiple functions of proteolytic enzymes in intracellular protein degradation and post-translational modification, in the release of biologically active macromolecules and peptides from precursors and in cellular protein regulation and quality control has stimulated interest in proteases in neurobiology and neuropathology. In this article, the proteinases and peptidases thus far studied in the human central nervous system are reviewed with respect to their enzymology, anatomical and cytological distributions and contributions to neurological and psychiatric disease states. Though information concerning brain proteases in man is fragmentary, it suffices to establish the importance of these complex systems for advancing knowledge of human cerebral function in health and disease.

The identity of hexokinase activities from mitochondrial and cytoplasmic fractions of rat brain homogenates

Cytoplasmic hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) was purified from the soluble fraction of a rat brain homogenate by a procedure that included a unique affinity elution of the enzyme from Blue Dextran-Sepharose. The purified enzyme was examined with respect to properties in which the impure cytoplasmic enzyme has been reported to differ from the solubilized mitochondrial enzyme. These included the ability to bind to mitochondria, inhibition by quercetin, effect of pH on activity, and kinetics. In all regards the purified mitochondrial and cytoplasmic enzymes appeared identical. In addition, comparative peptide maps after partial proteolysis showed no detectable differences. These results do not support the view that there exist distinct mitochondrial and cytoplasmic forms of hexokinase, the latter being permanently relegated to a cytoplasmic location and unable to participate in a dynamic equilibrium with the mitochondrially-bound enzyme. Alternatives are proposed to explain previous results that had been interpreted as indirect evidence for the existence of a distinct cytoplasmic hexokinase.

Mitigation of experimental allergic encephalomyelitis by cathepsin D inhibition

Intraperitoneal treatment with the enzyme inhibitor, pepstatin, of BSVS mice, guinea pigs and Lewis rats which were sensitized with guinea pig spinal cord and pertussis vaccine resulted in complete or partial suppression of paralysis dependent on the species studied and alterations of histological signs of experimental allergic encephalomyelitis (EAE). The effect was dose-dependent but had no relationship to the age of the experimental animal at the time of the experiment.

Immunoglobulin elution from multiple sclerosis brain

Immunoglobulin (Ig) was eluted from multiple sclerosis (MS) brain tissue. Razor thin slices of white matter from 10 g of MS and control brains were washed with 5 liters of phosphate-buffered saline (PBS), then treated for 90 sec with acetic acid, pH 2.5, containing Pepstatin and epsilon-aminocaproic acid. The protein concentrations of the PBS washes and neutralized acid eluates were determined, and the eluates were assayed for Ig by competitive microradioimmunoassay using rabbit anti-human F(ab1)2. Successive PBS washes reduced extracellular protein to a very low level. Equivalent quantities of protein were recovered from 7 MS and 6 non-MS brain samples after PBS washing and acetic acid elution. However, the amount of protein needed for 50% inhibition of [125I]IgG binding to anti-human F(ab1)2 was significantly less in MS brain than in non-MS brain (P less 0.05). The Ig in brain eluates was present in the void volume of a DEAE cellulose column. The techniques described above facilitate the isolation and characterization of cell-surface Ig in MS brain.

Hypothalamic cathepsin D: assay and isoenzyme composition

A sensitive and convenient method of endopeptidase assay using as substrate globin modified with pyridoxal-5-phosphate was used for determination of acid proteinases in bovine hypothalamus separated by isoelectric focusing. The soluble protein fraction of hypothalamus upon elution from Sephadex gave five peaks of proteinase activity at pH 3.2. The properties indicate that these peaks of endopeptidase activity are isoenzyme forms of cathepsin D.

Electrophoresis of immunoglobulin G. Facilitated migration of minute amounts in agarose

Migration of very small amounts of immunoglobulin (20 ng) is restricted in agarose electrophoresis. Incorporation of a stable protein matrix (rabbit gamma globulin 1 mg/ml) in the agarose permits unrestricted migration so that immunoelectrophoresis of this quantity of radiolabelled antibody is possible. Very small amounts of radiolabelled and non-radiolabelled antibody were subjected to successful crossed immunoelectrophoresis through barriers of antigen under conditions which provide favorable ratios of antibody to antigen. These methods should be useful for studies of antibody eluted from tissue in acquired and autoimmune diseases associated with tissue bound immunoglobulin.

A technique for the elution of cell-surface antibody from human brain tissue

An artificial system is described in which anti-dinitrophenyl (DNP) antibody attaches to hapten-conjugated human brain tissue. Treatment of this material with acetic acid at pH 2.5 to 3.0 for 90 seconds followed by immediate neutralization results in dissociation of antibody from the hapten-conjugated brain. Less than 1 ng of specific antibody can be detected, as determined by the ability of eluted material to combine with DNP in an established radioimmunoassay system. Similar methods may be applied to neurological disorders in which immunoglobulins are thought to be produced in the central nervous system.

Treatment of experimental allergic encephalomyelitis with an inhibitor of cathepsin D (pepstatin)

Intraperitoneal administration of pepstatin (2 mg/day for 5 weeks) to Lewis rats subjected to experimental allergic encephalomyelitis (EAE) (induced by guinea pig spinal cord and pertussis vaccine) suppressed the appearance of clinical signs of disease, and reduced the severity and incidence of CNS lesions normally associated with this disease. Administration of pepstatin for shorter periods to Lewis rats, or BSVS mice, or guinea pigs challenged with myelin basic protein delayed, but did not prevent clinical signs of EAE, but was accompanied in all cases by a less severe histopathology.

Purification and partial characterization of rat brain acid proteinase (isorenin)

1. Isorenin was purified 2000-fold from rat brain by a simple 3-step procedure involving affinity chromatography on pepstatinyl-Sepharose, The preparation appears as a homogenous protein in analytical polyacrylamide gel electrophoresis. Sodium dodecyl sulfate gel electrophoresis indicated an apparent molecular weight of 45 000. Isoelectric focusing separated isoenzymes with isoelectric points at pH 5.45, 5.87, 6.16 and 7.05. 2. The enzyme generates antiotensin I from tetradecapeptide (pH optimum 4.7) and from sheep angiotensinogen (pH optima 3.9 and 5.5). The rate of angiotensin I formation from tetradecapeptide was 30 000 times higher than that from sheep angiotensinogen. The enzyme has acid protease activity at pH 3.2 with hemoglobin as the substrate and pepstatin is a potent inhibitor of the enzyme with a Ki of less than 10(-9) M. 3. The properties of the enzyme strongly suggest that it is identical with cathepsin D.

Acid proteinase of hypothalamus. Purification, some properties, and action on somatostatin and substance P

In a continuing study of the physiological role of protein breakdown in the hypothalamus, acid proteinase from bovine hypothalamus was purified about 1000-fold. The molecular weight of the enzyme was approximately 50,000. Masimal activity against hemoglobin was obtained at pH 3.2-3.5; serum albumin was split much more slowly. Hypothalamus acid proteinase was partially inhibited by beta-phenyl pyruvate, or benzethonium Cl, and was completely inhibited by low concentrations of pepstatin. This proteinase splits somatostatin, substance P, and analogs of substance P. The probable sites of enzyme action on these peptides were determined by the end group dansyl technique. The enzyme, most likely cathepsin D, may play an important role in the formation and breakdown of peptide hormones in the hypothalamus.

Comparison of the primary structures of acidic proteinases and of their zymogens

From the scattered information about primary structures of aspartate proteinases the following general features appear: 1) Sequence determinations show that two catalytically active aspartic acid residues are located in highly conservative surroundings. 2) Zymogens have so far only been found for extracellular aspartate proteinases of the vertebrates. The zymogens from the gastric mucosa are converted into active enzymes by a limited proteolysis releasing 42 to 44 residues from the NH2-terminus. A common pattern of basic and apolar residues is observed in this NH2-terminal segment. 3) In the presently known structures gastric proteinases and their zymogens have about 40% of all residues in common. The sequence of penicillopepsin shows 18% of identity with the gastric proteinases.

Proteolytic activity associated with human erythrocyte membranes. Self-digestion of isolated human erythrocyte membranes

At least two kinds of enzymes are active in the proteolytic self-digestion of erythrocyte membranes. The specific activities of these enzymes do not decrease with repeated washings of purified stroma. The effects of a variety of inhibitors on the membrane preparation's capacity to digest 125-I-labelled casein, covalently linked to latex beads, have been examined. Pepstatin-inhibitable enzyme, active at low pH, digests the membrane extensively to small polypeptide fragments. Spectrin, located at the internal part of the membrane, is readily degraded. Diisopropylfluorophosphate-inhibitable enzyme, active at pH 8-9, has only limited digestive capacity. Some of the membrane components, such as the small molecular weight glycoproteins, are resistant to digestion. The restricted capacity of digestion is due to the membrane molecular arrangement; increased disaggregation removes the restriction and increases the activity. Spectrin is not digested unless the membrane topography is disrupted by NP-40 neutral detergent. These observations suggest that the enzymes active at basic pH are located external to the cell. Intact cells do possess a limited capacity to degrade 125-I-labelled casein when their surfaces are brought into contact with substrate-coated beads.