Role of calcium in the release of EC 3.4.24.15 and EC 3.4.24.16 from endothelial cells

Summary The two closely related soluble zinc metalloendopeptidases EC 3.4.24.15 (EP24.15) and EC 3.4.24.16 (EP24.16) readily hydrolyze the vasocative peptide bradykinin in vitro, and therefore may play a role in cardiovascular regulation. Although primarily soluble cytosolic enzymes, both secreted and membrane-associated forms of both peptidases have been reported. However, these enzymes have neither a transmembrane domain nor a signal sequence; thus, the mechanisms of membrane anchoring and secretion are unknown. In the present study, secreted/released EP24.15 and EP24.16 activity from aortic endothelial cells in culture was assessed by the cleavage of a specific quenched fluorescent substrate. An increase in enzyme activity released from endothelial cells, which express both peptidases, was seen following incubation with calcium-free media. In the AtT-20 endocrine cell (mouse pituitary corticotrope), which predominantly expresses EP24.15, the release of activity into media was unaffected by calcium removal. The release of enzyme activity from endothelial cells was inversely proportional to calcium concentrations ranging between 0.01 mM (activity equivalent to calcium-free media) and 0.5 mM (activity equivalent to normal media). Cleavage of the EP24.16-specific substrate AcNT_8–13 indicated that the increase in enzyme activity released upon incubation with calcium-free medium was due at least in part to the release of EP24.16. These results suggest that EP24.15 and EP24.16 are secreted from endothelial cells, and that removal of calcium selectively enhances the release of EP24.16 by an as yet unknown mechanism.     

Secretion of metalloendopeptidase 24.15 (EC 3.4.24.15).

The metalloendopeptidase EP24.15 (EC3.4.24.15) is a neuropeptide-metabolizing enzyme present in neural and endocrine tissues, presumably functioning extracellularly. Because the majority of the EP24.15 activity is identified in the soluble fraction of cellular homogenates, suggesting that the enzyme is primarily an intracellular protein, we addressed the issue of how EP24.15 arrives in the extracellular environment. We utilized a model system of neuroendocrine secretion, the AtT20 cell. According to both enzymatic activity and immunologic assays, EP24.15 was synthesized in and released from AtT20 cells. Under basal conditions and after stimulation by corticotropin-releasing hormone or the calcium ionophore A23187, EP24.15 activity accumulated in the culture medium. This secretion was not attributable to cell damage, as judged by the absence of release of cytosolic enzyme markers and the ability to exclude trypan blue dye. Pulse-chase analysis and subcellular fractionation of AtT20 cell extracts suggested that the mechanism of EP24.15 secretion is not solely via classical secretory pathways. Additionally, drugs which disrupt the classical secretory pathway, such as Brefeldin A and nocodazole, blocked A23187-stimulated EP24.15 release yet had no effect on basal EP24.15 release, suggesting differences in the basal and stimulated pathways of secretion for EP24.15. In summary, EP24.15 appears to be secreted from AtT20 pituitary cells into the extracellular milieu, where the enzyme can participate in the physiologic metabolism of neuropeptides.     

Endopeptidases 3.4.24.15 and 24.16 in endothelial cells: potential role in vasoactive peptide metabolism

The closely related metalloendopeptidases EC (EP24.15; thimet oligopeptidase) and 24.16 (EP24.16; neurolysin) cleave a number of vasoactive peptides such as bradykinin and neurotensin in vitro. We have previously shown that hypotensive responses to bradykinin are potentiated by an inhibitor of EP24.15 and EP24.16 (26), suggesting a role for one or both enzymes in bradykinin metabolism in vivo. In this study, we have used selective inhibitors that can distinguish between EP24.15 and EP24.16 to determine their activity in cultured endothelial cells (the transformed human umbilical vein endothelial hybrid cell line EA.hy926 or ovine aortic endothelial cells). Endopeptidase activity was assessed using a specific quenched fluorescent substrate [7-methoxycoumarin-4-acetyl-Pro-Leu-Gly-d-Lys(2,4-dinitrophenyl)], as well as the peptide substrates bradykinin and neurotensin (assessed by high-performance liquid chromatography with mass spectroscopic detection). Our results indicate that both peptidases are present in endothelial cells; however, EP24.16 contributes significantly more to substrate cleavage by both cytosolic and membrane preparations, as well as intact cells, than EP24.15. These findings, when coupled with previous observations in vivo, suggest that EP24.16 activity in vascular endothelial cells may play an important role in the degradation of bradykinin and/or other peptides in the circulation.     

Bradykinin analogues with beta-amino acid substitutions reveal subtle differences in substrate specificity between the endopeptidases EC 3.4.24.15 and EC 3.4.24.16.

The closely related zinc metalloendopeptidases EC 3.4.24.15 (EP24.15) and EC 3.4.24.16 (EP24.16) cleave many common substrates, including bradykinin (BK). As such, there are few substrate-based inhibitors which are sufficiently selective to distinguish their activities. We have used BK analogues with either alanine or beta-amino acid (containing an additional carbon within the peptide backbone) substitutions to elucidate subtle differences in substrate specificity between the enzymes. The cleavage of the analogues by recombinant EP24.15 and EP24.16 was assessed, as well as their ability to inhibit the two enzymes. Alanine-substituted analogues were generally better substrates than BK itself, although differences between the peptidases were observed. Similarly, substitution of the four N-terminal residues with beta-glycine enhanced cleavage in some cases, but not others. beta-Glycine substitution at or near the scissile bond (Phe5-Ser6) completely prevented cleavage by either enzyme: interestingly, these analogues still acted as inhibitors, although with very different affinities for the two enzymes. Also of interest, beta-Gly8-BK was neither a substrate nor an inhibitor of EP24.15, yet could still interact with EP24.16. Finally, while both enzymes could be similarly inhibited by the D-stereoisomer of beta-C3-Phe5-BK (IC50 approximately 20 microM, compared to 8 microM for BK), EP24.16 was relatively insensitive to the L-isomer (IC50 12 approximately microM for EP24.15, >40 microM for EP24.16). These studies indicate subtle differences in substrate specificity between EP24.15 and EP24.16, and suggest that beta-amino acid analogues may be useful as templates for the design of selective inhibitors.     

Metalloendopeptidase EC3.4.24.15 is constitutively released from the exofacial leaflet of lipid rafts in GT1-7 cells

Metalloendopeptidase EC3.4.24.15 (EP24.15) is a physiologically important neuropeptide-degrading enzyme involved in the metabolism of multiple neuropeptides. The mechanism of release of EP24.15 from neuronal cells is multimodal, being both constitutive and stimulatable. Previous studies have characterized stimulated EP24.15 secretion, yet little is understood concerning constitutive release of the peptidase. Utilizing the mouse hypothalamic neuronal GT1-7 cell line, we demonstrate that EP24.15 exists within lipid rafts in the plasma membrane, and that the enzyme is localized to the exofacial leaflet of lipid rafts. Further, we have found that biotinylated EP24.15 on the extracellular surface is released into the cell media in a fashion similar to constitutive release. In addition, classical and non-classical secretion pathway inhibitors were employed to understand the release of EP24.15 into surrounding cell media. The non-classical secretion inhibitor glyburide, a blocker of ATP-sensitive K+ channels, decreased the amount of constitutively released EP24.15 in cell media of GT1-7 cells. With these data, we conclude that EP24.15 association with lipid rafts on the extracellular surface precedes constitutive release of the peptidase into the extracellular milieu for its action on neuropeptides.     

The intracellular distribution and secretion of endopeptidases 24.15 (EC 3.4.24.15) and 24.16 (EC 3.4.24.16)

Endopeptidase 24.15 (EC 3.4.24.15; EP24.15) and endopeptidase 24.16 (EC 3.4.24.16; EP24.16) are enzymes involved in general peptide metabolism in mammalian cells and tissues. This review will focus on morphological and biochemical aspects related to the subcellular distribution and secretion of these homologous enzymes in the central nervous system. These are important issues for a better understanding of the functions of EP24.15 and EP24.16 within neuroendocrine systems.     

Confocal microscopy reveals thimet oligopeptidase (EC 3.4.24.15) and neurolysin (EC 3.4.24.16) in the classical secretory pathway

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) and neurolysin (EC 3.4.24.16; EP24.16) are closely related enzymes involved in the metabolic inactivation of bioactive peptides. Both of these enzymes were previously shown to be secreted from a variety of cell types, although their primary sequence lacks a signal peptide. To investigate the mechanisms responsible for this secretion, we examined by confocal microscopy the subcellular localization of these two enzymes in the neuroendocrine cell line AtT20. Both EP24.15 and EP24.16 were found by immunohistochemistry to be abundantly expressed in AtT20 cells. Western blotting experiments confirmed that the immunoreactivity detected in the soma of these cells corresponded to previously cloned isoforms of the enzymes. At the subcellular level, both enzymes colocalized extensively with the integral trans-Golgi network protein, syntaxin-6, in the juxtanuclear region. In addition, both EP24.15 and EP24.16 were found within small vesicular organelles distributed throughout the cell body. Some, but not all, of these organelles also stained positively for ACTH. These results demonstrate that both EP24.15 and EP24.16 are present within the classical secretory pathway. Their colocalization with ACTH further suggests that they may be targeted to the regulated secretory pathway, even in the absence of a signal peptide.     

Metalloendopeptidases EC 3.4.24.15 and EC 3.4.24.16: potential roles in vascular physiology

The zinc metalloendopeptidases EC 3.4.24.15 (EP24.15) and EC 3.4.24.16 (EP 24.16) are closely relatedubiquitous enzymes, which have well-defined in vitroactivities in generation and degradation of a range ofspecific peptide targets. Despite this, little is knownregarding their roles in whole animal physiology. One of thepeptides degraded by these enzymes in vitro isbradykinin, a mediator with potent effects on the vasculatureat both systemic and local levels. This review summarises thework that has examined the role of EP 24.15/24.16 inregulation of the vascular effects of bradykinin invivo. This work was made possible by the development of aspecific stable inhibitor of these enzymes, JA-2. Use of thisinhibitor has shown that EP 24.15/24.16 are capable ofregulating responses induced by exogenous bradykinin. Thiseffect was observed at a systemic level with an increase inthe hypotensive effect of intravenous bradykinin. Further workis required to determine whether these enzymes also regulatebradykinin produced endogenously.     

Inhibitors of metalloendopeptidase EC 3.4.24.15 and EC 3.4.24.16 stabilized against proteolysis by the incorporation of beta-amino acids

The enzyme EC 3.4.24.15 (EP 24.15) is a zinc metalloendopeptidase whose precise function in vivo remains unknown but is thought to participate in the regulated metabolism of a number of specific neuropeptides. The lack of stable and selective inhibitors has hindered the determination of the exact function of EP 24.15. Of the limited number of EP 24.15 inhibitors that have been developed, N-[1-(R,S)-carboxy-3-phenylpropyl]-Ala-Ala-Tyr-p-aminobenzoate (CFP) is the most widely studied. CFP is a potent and specific inhibitor, but it is unstable in vivo due to cleavage between the alanine and tyrosine residues by the enzyme neprilysin (EP 24.11). This cleavage by EP 24.11 generates a potent inhibitor of angiotensin converting enzyme, thereby limiting the use of CFP for in vivo studies. To develop specific inhibitors of EP 24.15 that are resistant to in vitro and potentially in vivo proteolysis by EP 24.11, this study incorporated beta-amino acids replacing the Ala-Tyr scissile alpha-amino acids of CFP. Both C2 and C3 substituted beta-amino acids were synthesized and substituted at the EP 24.11 scissile Ala-Tyr bond. Significant EP 24.15 inhibitory activity was observed with some of the beta-amino acid containing analogues. Moreover, binding to EP 24.11 was eliminated, thus rendering all analogues containing beta-amino acids resistant to degradation by EP 24.11. Selective inhibition of either EP 24.15 or EP 24.16 was also observed with some analogues. The results demonstrated the use of beta-amino acids in the design of inhibitors of EP 24.15 and EP 24.16 with K(i)'s in the low micromolar range. At the same time, these analogues were resistant to cleavage by the related metalloendopeptidase EP 24.11, in contrast to the alpha-amino acid based parent peptide. This study has therefore clearly shown the potential of beta-amino acids in the design of stable enzyme inhibitors and their use in generating molecules with selectivity between closely related enzymes.     

A structure-based site-directed mutagenesis study on the neurolysin (EC 3.4.24.16) and thimet oligopeptidase (EC 3.4.24.15) catalysis

Neurolysin (EP24.16) and thimet oligopeptidase (EP24.15) are closely related metalloendopeptidases. Site-directed mutagenesis of Tyr(613) (EP24.16) or Tyr(612) (EP24.15) to either Phe or Ala promoted a strong reduction of k(cat)/K(M) for both enzymes. These data suggest the importance of both hydroxyl group and aromatic ring at this specific position during substrate hydrolysis by these peptidases. Furthermore, the EP24.15 A607G mutant showed a k(cat)/K(M) of 2x10(5) M(-1) s(-1) for the Abz-GFSIFRQ-EDDnp substrate, similar to that of EP24.16 (k(cat)/K(M)=3x10(5) M(-1) s(-1)) which contains Gly at the corresponding position; the wild type EP24.15 has a k(cat)/K(M) of 2.5x10(4) M(-1) s(-1) for this substrate.     

Metalloendopeptidases EC 3.4.24.15 and EC 3.4.24.16: potential roles in vascular physiology

Summary The zinc metalloendopeptidases EC 3.4.24.15 (EP 24.15) and EC 3.4.24.16 (EP 24.16) are closely related ubiquitous enzymes, which have well-defined in vitro activities in generation and degradation of a range of specific peptide targets. Despite this, little is known regarding their roles in whole animal physiology. One of the peptides degraded by these enzymes in vitro is bradykinin, a mediator with potent effects on the vasculature at both systemic and local levels. This review summarises the work that has examined the role of EP 24.15/24.16 in regulation of the vascular effects of bradykinin in vivo. This work was made possible by the development of a specific stable inhibitor of these enzymes, JA-2. Use of this inhibitor has shown that EP 24.15/24.16 are capable of regulating responses induced by exogenous bradykinin. This effect was observed at a systemic level with an increase in the hypotensive effect of intravenous bradykinin. Further work is required to determine whether these enzymes also regulate bradykinin produced endogenously.     

Metalloendopeptidases EC 3.4.24.15/16 regulate bradykinin activity in the cerebral microvasculature

Bradykinin is a vasoactive peptide that has been shown to increase the permeability of the cerebral microvasculature to blood-borne macromolecules. The two zinc metalloendopeptidases EC (EP 24.15) and EC (EP 24.16) degrade bradykinin in vitro and are highly expressed in the brain. However, the role that these enzymes play in bradykinin metabolism in vivo remains unclear. In the present study, we investigated the role of EP 24.15 and EP 24.16 in the regulation of bradykinin-induced alterations in microvascular permeability. Permeability of the cerebral microvasculature was assessed in anesthetized Sprague-Dawley rats by measuring the clearance of 70-kDa FITC dextran from the brain. Inhibition of EP 24.15 and EP 24.16 by the specific inhibitor N-[1-(R,S)-carboxy-3-phenylpropyl]-Ala-Aib-Tyr-p-aminobenzoate (JA-2) resulted in the potentiation of bradykinin-induced increases in cerebral microvessel permeability. The level of potentiation was comparable to that achieved by the inhibition of angiotensin-converting enzyme. These findings provide the first evidence of an in vivo role for EP 24.15/EP 24.16 in brain function, specifically in regulating alterations in microvessel permeability induced by exogenous bradykinin.     

Calcium modulates endopeptidase 24.15 (EC 3.4.24.15) membrane association, secondary structure and substrate specificity

The metalloendopeptidase 24.15 (EP24.15) is ubiquitously present in the extracellular environment as a secreted protein. Outside the cell, this enzyme degrades several neuropeptides containing from 5 to 17 amino acids (e.g. gonadotropin releasing hormone, bradykinin, opioids and neurotensin). The constitutive secretion of EP24.15 from glioma C6 cells was demonstrated to be stimulated linearly by reduced concentrations of extracellular calcium. In the present report we demonstrate that extracellular calcium concentration has no effect on the total amount of the extracellular (cell associated + medium) enzyme. Indeed, immuno-cytochemical analyses by confocal and electron microscopy suggested that the absence of calcium favors the enzyme shedding from the plasma membrane into the medium. Two putative calcium-binding sites on EP24.15 (D93 and D159) were altered by site-directed mutagenesis to investigate their possible contribution to binding of the enzyme at the cell surface. These mutated recombinant proteins behave similarly to the wild-type enzyme regarding enzymatic activity, secondary structure, calcium sensitivity and immunoreactivity. However, immunocytochemical analyses by confocal microscopy consistently show a reduced ability of the D93A mutant to associate with the plasma membrane of glioma C6 cells when compared with the wild-type enzyme. These data and the model of the enzyme's structure as determined by X-ray diffraction suggest that D93 is located at the enzyme surface and is consistent with membrane association of EP24.15. Moreover, calcium was also observed to induce a major change in the EP24.15 cleavage site on distinctive fluorogenic substrates. These data suggest that calcium may be an important modulator of ep24.15 cell function.     

The neuropeptide processing enzyme EC 3.4.24.15 is modulated by protein kinase A phosphorylation

The metalloendopeptidase EC (EP24.15) is a neuropeptide-metabolizing enzyme expressed predominantly in brain, pituitary, and testis, and is implicated in several physiological processes and diseases. Multiple putative phosphorylation sites in the primary sequence led us to investigate whether phosphorylation effects the specificity and/or the kinetics of substrate cleavage. Only protein kinase A (PKA) treatment resulted in serine phosphorylation with a stoichiometry of 1.11 +/- 0.12 mol of phosphate/mol of recombinant rat EP24.15. Mutation analysis of each putative PKA site, in vitro phosphorylation, and phosphopeptide mapping indicated serine 644 as the phosphorylation site. Phosphorylation effects on catalytic activity were assessed using physiological (GnRH, GnRH(1-9), bradykinin, and neurotensin) and fluorimetric (MCA-PLGPDL-Dnp and orthoaminobenzoyl-GGFLRRV-Dnp-edn) substrates. The most dramatic change upon PKA phosphorylation was a substrate-specific, 7-fold increase in both K(m) and k(cat) for GnRH. In both rat PC12 and mouse AtT-20 cells, EP24.15 was serine-phosphorylated, and EP24.15 phosphate incorporation was enhanced by forskolin treatment, and attenuated by H89, consistent with PKA-mediated phosphorylation. Cloning of the full-length mouse EP24.15 cDNA revealed 96.7% amino acid identity to the rat sequence, and conservation at serine 644, consistent with its putative functional role. Therefore, PKA phosphorylation is suggested to play a regulatory role in EP24.15 enzyme activity.     

Novel natural peptide substrates for endopeptidase 24.15, neurolysin,and angiotensin-converting enzyme

Endopeptidase 24.15 (EC; ep24.15), neurolysin (EC; ep24.16), and angiotensin-converting enzyme (EC; ACE) are metallopeptidases involved in neuropeptide metabolism in vertebrates. Using catalytically inactive forms of ep24.15 and ep24.16, we have identified new peptide substrates for these enzymes. The enzymatic activity of ep24.15 and ep24.16 was inactivated by site-directed mutagenesis of amino acid residues within their conserved HEXXH motifs, without disturbing their secondary structure or peptide binding ability, as shown by circular dichroism and binding assays. Fifteen of the peptides isolated were sequenced by electrospray ionization tandem mass spectrometry and shared homology with fragments of intracellular proteins such as hemoglobin. Three of these peptides (PVNFKFLSH, VVYPWTQRY, and LVVYPWTQRY) were synthesized and shown to interact with ep24.15, ep24.16, and ACE, with K(i) values ranging from 1.86 to 27.76 microm. The hemoglobin alpha-chain fragment PVNFKFLSH, which we have named hemopressin, produced dose-dependent hypotension in anesthetized rats, starting at 0.001 microg/kg. The hypotensive effect of the peptide was potentiated by enalapril only at the lowest peptide dose. These results suggest a role for hemopressin as a vasoactive substance in vivo. The identification of these putative intracellular substrates for ep24.15 and ep24.16 is an important step toward the elucidation of the role of these enzymes within cells.     

Oligomerization of the cysteinyl-rich oligopeptidase EP24.15 is triggered by S-glutathionylation

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) is a thiol-rich metallopeptidase ubiquitously distributed in mammalian tissues and involved in oligopeptide metabolism both within and outside cells. Fifteen Cys residues are present in the rat EP24.15 protein, seven are solvent accessible, and two are found inside the catalytic site cleft; no intraprotein disulfide is described. In the present investigation, we show that mammalian immunoprecipitated EP24.15 is S-glutathionylated. In vitro EP24.15 S-glutathionylation was demonstrated by the incubation of bacterial recombinant EP24.15 with oxidized glutathione concentration as low as 10 microM. The in vitro S-glutathionylation of EP24.15 was responsible for its oxidative oligomerization to dimer and trimer complexes. EP24.15 immunoprecipitated from cells submitted to oxidative challenge showed increased trimeric forms and decreased S-glutathionylation compared to immunoprecipitated protein from control cells. Our present data also show that EP24.15 maximal enzymatic activity is maintained by partial S-glutathionylation, a mechanism that apparently regulates the protein oligomerization. Present results raise the possibility of an unconventional property of protein S-glutathionylation, inducing oligomerization by interprotein thiol-disulfide exchange.     

Specific inhibition of endopeptidase 24.16 by dipeptides.

The inhibitory effect of various dipeptides on the neurotensin-degrading metallopeptidase, endopeptidase 24.16, was examined. These dipeptides mimick the Pro10-Tyr11 bond of neurotensin that is hydrolyzed by endopeptidase 24.16. Among a series of Pro-Xaa dipeptides, the most potent inhibitory effect was elicited by Pro-Ile (Ki approximately 90 microM) with Pro-Ile greater than Pro-Met greater than Pro-Phe. All the Xaa-Tyr dipeptides were unable to inhibit endopeptidase 24.16. The effect of Pro-Ile on several purified peptidases was assessed by means of fluorigenic assays and HPLC analysis. A 5 mM concentration of Pro-Ile does not inhibit endopeptidase 24.11, endopeptidase 24.15, angiotensin-converting enzyme, proline endopeptidase, trypsin, leucine aminopeptidase, pyroglutamyl aminopeptidase I and carboxypeptidase B. The only enzyme that was affected by Pro-Ile was carboxypeptidase A, although it was with a 50-fold lower potency (Ki approximately 5 mM) than for endopeptidase 24.16. By means of fluorimetric substrates with a series of hydrolysing activities, we demonstrate that Pro-Ile can be used as a specific inhibitor of endopeptidase 24.16, even in a complex mixture of peptidase activities such as found in whole rat brain homogenate.     

Stability and Specificity of the Cell Wall-Associated Proteinase from Lactococcus lactis subsp. cremoris H2 Released by Treatment with Lysozyme in the Presence of Calcium Ions

The cell wall-associated proteinase from Lactococcus lactis subsp. cremoris H2 (isolate number 4409) was released from the cells by treatment with lysozyme, even in the presence of 50 mM calcium chloride. Cell lysis during lysozyme treatment was minimal. The proteinase activity released by lysozyme treatment fractionated on ion-exchange chromatography as three main forms, the molecular masses of which were determined by gel exclusion chromatography and polyacrylamide gel electrophoresis. Two of the enzyme forms released, 137 and 145 kDa, were the same as those released by incubation of cells in calcium-free phosphate buffer. In the presence of calcium, lysozyme treatment also resulted in the release of a 180-kDa enzyme molecule. The total proteinase activity released by lysozyme treatment (in the presence or absence of calcium) was not only greater than that released by phosphate buffer but was also greater than that initially detectable on the surface of whole cells, suggesting an unmasking of enzyme on the cell surface. The presence of calcium during release treatment resulted in increased stability of the crude enzyme preparations. For the proteinase preparation released by using lysozyme with 50 mM CaCl₂, the half-life of proteinase activity at 37°C was 39 h, compared with 0.22 h for the calcium-free phosphate buffer-released preparation. In all cases, maximum stability was observed at pH 5.5. Comparison of β-casein hydrolysis by the three forms of the enzyme showed that the products of short-term (5- to 30-min) digestions were very similar, although subtle differences were detected with the 180-kDa form.     

Metalloendopeptidase EC 3.4.24.15 is necessary for Alzheimer's amyloid-beta peptide degradation.

We have investigated the functional relationship between metalloendopeptidase EC 3.4.24.15 (MP24.15) and the amyloid precursor protein involved in Alzheimer's disease (AD) and discovered that the enzyme promotes Abeta degradation. We show here that conditioned medium (CM) of MP24.15 antisense-transfected SKNMC neuroblastoma has significantly higher levels of Abeta. Furthermore, synthetic-Abeta degradation was increased or decreased following incubation with CM of sense or antisense-transfected cells, respectively. Soluble Abeta1-42 was degraded more slowly than soluble Abeta1-40, while aggregated Abeta1-42 showed almost no degradation. Pretreatment of CM with serine proteinase inhibitors 4-(2-aminoethyl)benzenesulfonyl fluoride and diisopropyl fluorophosphate completely inhibited Abeta degradation. Additionally, alpha1-antichymotrypsin (ACT), a serpin family inhibitor tightly associated with plaques and elevated in AD brain, blocked up to 60% of Abeta degradation. Interestingly, incubation of CM of MP24. 15-overexpressing cells with ACT formed an SDS-resistant ACT complex, suggesting an ACT-serine proteinase interaction. Recombinant MP24. 15 alone did not degrade Abeta. 14C-Diisopropyl fluorophosphate-radiolabeled CM from MP24.15-overexpressing cells contained increased levels of several active serine proteinases, suggesting that MP24.15 activates one or more Abeta-degrading serine proteases. Thus, ACT may cause Abeta accumulation by inhibiting an Abeta-degrading enzyme or by direct binding to Abeta, rendering it degradation-resistant. Identification of the Abeta-degrading enzyme and MP24.15's role in its activation is underway. Pharmacological modulation of either enzyme may provide a means of regulating Abeta in the brain.