Phosphonate analogues of carboxypeptidase A substrates are potent transition-state analogue inhibitors

Analogues of tri- and tetrapeptide substrates of carboxypeptidase A in which the scissile peptide linkage is replaced with a phosphonate moiety (-PO2--O-) were synthesized and evaluated as inhibitors of the enzyme. The inhibitors terminated with either L-lactate or L-phenyllactate [designated (O) Ala and (O) Phe, respectively] in the P1' position. Transition-state analogy was shown for a series of 14 tri- and tetrapeptide derivatives containing the structure RCO-AlaP-(O)Ala [RCO-AP(O)A, AP indicates the phosphonic acid analogue of alanine] by the correlation of the Ki values for the inhibitors and the Km/kcat values for the corresponding amide substrates. This correlation supports a transition state for the enzymatic reaction that resembles the tetrahedral intermediate formed upon addition of water to the scissile carbonyl group. The inhibitors containing (O) Phe at the P1' position proved to be the most potent reversible inhibitors of carboxypeptidase A reported to date: the dissociation constants of ZAFP(O)F, ZAAP(O)F, and ZFAP(O)F are 4, 3, and 1 pM, respectively. Because of the high affinity of these inhibitors, their dissociation constants could not be determined by steady-state methods. Instead, the course of the association and dissociation processes was monitored for each inhibitor as its equilibrium with the enzyme was established in both the forward and reverse directions. A phosphonamidate analogue, ZAAPF, in which the peptide linkage is replaced with a -PO2-NH- moiety, was prepared and shown to hydrolyze rapidly at neutral pH (t1/2 = 20 min at pH 7.5). This inhibitor is bound an order of magnitude less tightly than the corresponding phosphonate, ZAAP(O)F, a result that contrasts with the 840-fold higher affinity of phosphonamidates for thermolysin [Bartlett, P. A., & Marlowe, C. K. (1987) Science 235, 569-571], a zinc peptidase with a similar arrangement of active-site catalytic residues.     

A brain protein (P30) that immunoreacts with a polyclonal anti-pancreatic carboxypeptidase A antibody shows properties that are shared with tubulin carboxypeptidase

A preparation of tubulin carboxypeptidase partially purified from bovine brain was found to contain a protein of molecular mass 30 kDa (P30) as determined by SDS-PAGE, that is recognized by a polyclonal anti-bovine pancreatic carboxypeptidase A. However, this protein is different from pancreatic carboxypeptidase A as judged by the isoelectric point and the pattern of peptides produced by trypsin digestion. The isoelectric point of P30 was similar to that found for tubulin carboxypeptidase (9 ± 0.2). When the tubulin carboxypeptidase preparation was subjected to gel filtration chromatography under low salt concentration, P30 behaved as a protein of molecular mass 38 kDa whereas tubulin carboxypeptidase eluted at a position of 75 kDa molecular mass. However, when the chromatography was performed at relatively high salt concentration they behaved as proteins of 49 and 56 kDa, respectively. We considered that P30 may be an inactive monomeric form of the dimeric tubulin carboxypeptidase. However we can not rule out the possibility that it represents another carboxypeptidase not yet described.     

Inhibition of carboxypeptidase A catalyzed peptide hydrolysis by 3-phenylpropanoate at activating and nonactivating substrate concentrations.

The carboxypeptidase A catalyzed hydrolyses of five structurally related dipeptide substrates in the presence of the inhibitor 3-phenylpropanoate have been studied. At nonactivating substrate concentrations, 3-phenylpropanoate is a mixed inhibitor of carbobenzoxyglycyl-L-phenylalanine hydrolysis and a noncompetitive inhibitor of the hydrolyses of benzoylglycyl-L-phenylalanine, cinnamoylglycyl-L-phenylalanine, hydrocinnamoylglycyl-L-phenylalanine, and acetylglycyl-L-phenylalanine. When carbobenzoxyglycyl-L-phenylalanine and benzoylglycyl-L-phenylalanine exhibit substrate activation, inhibition by 3-phenylpropanoate is mixed but appears to be mostly competitive. Proposed here is a site for the binding of 3-phenylpropanoate along with a kinetic mechanism consistent with these data.     

Catalytic activity of carboxypeptidase B and of carboxypeptidase Y with anisylazoformyl substrates

Anisylazoformyllysine (CH3OC6H4-N = N-CO-Lys-OH) is rapidly hydrolyzed at the acyl-lysine linkage by the zinc-enzyme porcine carboxypeptidase B. The catalytic reaction is readily monitored spectrophotometrically by disappearance of the intense absorption (348.5 nm, epsilon 18400) of the azo chromophore, which chemically fragments after substrate cleavage. Carboxypeptidase Y has no activity toward this type of substrate.     

Human carboxypeptidase M. Purification and characterization of a membrane-bound carboxypeptidase that cleaves peptide hormones

A membrane-bound neutral carboxypeptidase B-like enzyme was solubilized from human placental microvilli with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) and purified to homogeneity by ion-exchange chromatography and affinity chromatography on arginine-Sepharose. It gave a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent Mr of 62,000 with or without reduction. The enzyme is a glycoprotein as shown by its high affinity for concanavalin A-Sepharose and reduction in mass to 47,600 daltons after chemical deglycosylation. It has a neutral pH optimum, is activated by CoCl2, and inhibited by o-phenanthroline, 2-mercaptomethyl-3-guanidinoethylthiopropanoic acid, or cadmium acetate, indicating it is a metallopeptidase. The enzyme cleaves arginine or lysine from the COOH terminus of synthetic peptides (e.g. Bz-Gly-Arg, Bz-Gly-Lys, Bz-Ala-Lys, dansyl-Ala-Arg, where Bz is benzoyl and dansyl is 5-dimethylaminonaphthalene-1-sulfonyl) as well as from several biologically active substrates: dynorphin A(1-13), Met5-Arg6-enkephalin (Km = 46 microM, kcat = 934 min-1), bradykinin (Km = 16 microM, kcat = 147 min-1), Met5-Lys6-enkephalin (Km = 375 microM, kcat = 663 min-1), and Leu5-Arg6-enkephalin (Km = 63 microM, kcat = 106 min-1). Although the enzyme shares some properties with other carboxypeptidase B-like enzymes, it is structurally, catalytically, and immunologically distinct from pancreatic carboxypeptidase A or B, human plasma carboxypeptidase N, and carboxypeptidase H ("enkephalin convertase"). To denote that the enzyme is membrane-bound, and to distinguish it from other known carboxypeptidases, we propose the name "carboxypeptidase M." Because of its localization on the plasma membrane and optimal activity at neutral pH, carboxypeptidase M could inactivate or modulate the activity of peptide hormones either before or after their interaction with plasma membrane receptors.     

Chromophoric and fluorophoric peptide substrates cleaved through the dipeptidyl carboxypeptidase activity of cathepsin B

The action of bovine spleen cathepsin B as a dipeptidyl carboxypeptidase on newly synthesized substrates of the type peptidyl-X-p-nitrophenylalanyl (Phe(NO2))-Y (X,Y = amino acid residue) or 5-dimethylaminonaphthalene-1-sulfonyl (Dns)-peptidyl-X-Phe(NO2)-Y was investigated. The kinetic parameters of hydrolysis of the X-Phe(NO2) bond were determined by difference spectrophotometry (delta epsilon 310 = 1600 M-1 cm-1) or by spectrofluorometry by following the five- to eightfold increase of Dns-group fluorescence with excitation at 350 nm and emission at 535 nm. The substrates were moderately sensitive to cathepsin B; kcat varied from 0.7 to 4 s-1 at pH 5 and 25 degrees C; Km varied from 6 to 240 microM. The very acidic optima of pH 4-5 are characteristic for dipeptidyl carboxypeptidase activity of cathepsin B. Bovine spleen cathepsins S and H had little and no activity, respectively, when assayed with Pro-Glu-Ala-Phe(NO2)-Gly. These peptides should be a valuable tool for routine assays and for mechanistic studies on cathepsin B.     

Inhibition of epoxide metabolism by alpha,beta-epoxyketones and isosteric analogs

Chalcone oxides and several isosteric compounds have been prepared to examine the importance of the alpha,beta-epoxyketone moiety in the inhibition of the hydrolysis of [3H]-trans-stilbene oxide to its meso-diol by mouse liver cytosolic epoxide hydrolase (cEH). Inhibition of microsomal EH and glutathione S-transferase were also examined. For cEH, replacement of the carbonyl by methylidene reduces inhibitor potency by a factor of 44, while replacement of the epoxide ring with a cyclopropyl ring reduces inhibition by a factor of 450. A 2'-hydroxyl also reduces cEH inhibition by 100 times. These observations are consistent with a model of the active site in which the carbonyl is hy-hydrogen-bonded to an acidic site presumed to be involved in initiating epoxide hydrolysis. The chalcone oxides thus bind tightly but are not readily turned over as substrates.     

Hydrolysis of ester substrates of trypsin and chymotrypsin by barley carboxypeptidase

Purified barley carboxypeptidase exhibits high activity against a number of N-substituted amino acid esters, which are commonly used as synthetic substrates for mammalian and microbial proteinases. The proteinases of barley, on the contrary, do not hydrolyse these compounds. Because many other plants contain carboxypeptidases closely resembling the barley enzyme, we conclude that synthetic ester substrates should not be used to detect proteinase activity in extracts of higher plants. Plant carboxypeptidases also liberate C-terminal tryptophan from α-casein. Therefore, casein also is an unreliable substrate for plant proteinases.     

Inhibition of carboxypeptidase A by D-penicillamine: mechanism and implications for drug design

Zinc metalloprotease inhibitors are usually designed to inactivate the enzyme by forming a stable ternary complex with the enzyme and active-site zinc. D-Cysteine inhibits carboxypeptidase, ZnCPD, by forming such a complex, with a K(i) of 2.3 microM. In contrast, the antiarthritis drug D-penicillamine, D-PEN, which differs from D-Cys only by the presence of two methyl groups on the beta-carbon, inhibits ZnCPD by promoting the release of the active-site zinc. We have given the name catalytic chelator to such inhibitors. Inhibition is a two-step process characterized by formation of a complex with the enzyme (K(i(initial)) = 1.2 mM) followed by release of the active-site zinc at rates up to 420-fold faster than the spontaneous release. The initial rate of substrate hydrolysis at completion of the second step also depends on D-PEN concentration, reflecting formation of a thermodynamic equilibrium governed by the stability constants of chelator and apocarboxypeptidase for zinc (K(i(final)) = 0.25 mM). The interaction of D-PEN and D-Cys with the active-site metal has been examined by replacing the active-site zinc by a chromophoric cobalt atom. Both inhibitors perturb the d-d transitions of CoCPD in the 500-600 nm region within milliseconds of mixing but only the CoCPD.D-Cys complex displays a strong S --> Co(II) charge-transfer band at 340 nm indicative of a metal-sulfur bond. While the D-Cys complex is stable, the CoCPD.D-PEN complex breaks down to apoenzyme and Co(D-PEN)(2) with a half-life of 0.5 s. D-PEN is the first drug found to inhibit a metalloprotease by increasing the dissociation rate constant of the active-site metal. The ability of D-PEN to catalyze metal removal from carboxypeptidase A and other zinc proteases suggests a possible mechanism of action in arthritis and Wilson's disease and may also underlie complications associated with its clinical use.     

Use of circular dichroism to study the interaction of carboxypeptidase A with substrates and inhibitors

The phenylalanyl circular dichroism (CD) bands of peptides were used to assay peptidase activity of carboxypeptidase A (EC.3.4.12.2.). Hippuryl-L-phenylalanine has a sharp, negative CD band at 254 nm whilst L-phenylalanine (the optically active product) has positive CD. Thus the hydrolysis of this substrate as well as the inhibition effect of dipeptides, may be measured from the CD change at 254 nm. The addition of the dipeptide GLy-Tyr to carboxypeptidase A makes the CD spectrum more positive in the region from 270-295 nm. This alteration can result from the tyrosyl and tryptophanyl CD bands of the protein as well as from the tyrosyl CD band of the inhibitor.     

Avian retroviral protease and cellular aspartic proteases are distinguished by activities on peptide substrates

The avian sarcoma/leukemia virus protease (PR), purified from avian myeloblastosis virus has a native molecular mass of 26 kDa, suggesting a dimer structure. The enzymatic activity of PR has been characterized using synthetic peptide substrates. PR is most active at pH 5.5, 35 degrees C and 2-3 M NaCl. Under these conditions PR cleaves decapeptides which are resistant in low ionic strength. This high, nonphysiological, salt concentration also increases the proteolytic activity of a cellular aspartic protease, pepsin. PR and pepsin show additional similarities: they both cleave a synthetic decapeptide at the same Tyr-Pro bond in low and high salt, while the cleavage site preferences of human renin and cathepsin-D in this substrate are altered at high salt concentrations. In addition, iodination of the tyrosine residue in this decapeptide causes an increase in the rates of hydrolysis by both PR and pepsin. However, Km values are too high to be estimated accurately for PR using Tyr-Pro and Tyr(I)-Pro decapeptides as substrates. Comparison of the digestion products of two additional decapeptides, altered in a single amino acid residue, shows that PR cleaves at fewer sites than all three cellular enzymes. Furthermore pepstatin, a strong inhibitor of pepsin, renin, and cathepsin-D has little effect on PR.     

Oxidation of secondary alcohols to methyl ketones by yeasts.

Cell suspensions of yeasts, Candida utilis ATCC 26387, Hansenula polymorpha ATCC 26012, Pichia sp. NRRL-Y-11328, Torulopsis sp. strain A1, and Kloeckera sp. strain A2, grown on various C-1 compounds (methanol, methylamine, methylformate), ethanol, and propylamine catalyzed the oxidation of secondary alcohols to the corresponding methyl ketones. Thus, isopropanol, 2-butanol, 2-pentanol, and 2-hexanol were converted to acetone, 2-butanone, 2-pentanone, and 2-hexanone, respectively. Cell-free extracts derived from methanol-grown yeasts catalyzed an oxidized nicotinamide adenine dinucleotide-dependent oxidation of secondary alcohols to the corresponding methyl ketones, Primary alcohols were not oxidized. The effect of various environmental factors on the production of methyl ketones from secondary alcohols by methanol-grown Pichia sp. was investigated.