Evidence for the regulation of fatty acid utilization in human sperm by docosahexaenoic acid

The effects of stearic (18:0), linolenic (18:3), and docosahexaenoic (22:6) acids on palmitoyl coenzyme A (CoA) formation by a long-chain fatty acid:CoASH ligase (adenosine monophosphate) (E. C. 6.2.1.3-enriched fraction from human spermatozoa were studied. Both 18:0 and 18:3 were competitive inhibitors for palmitic (16:0) acid activation with Kis of 17.7 and 5.7 microM, respectively. In contrast, 22:6 was a noncompetitive inhibitor demonstrating a Ki of 9.5 microM. These data coupled with previous studies support the conclusion that 16:0, 18:0, and 18:3 and other saturated and unsaturated fatty acids are activated by the same ligase enzyme in sperm. Although the kinetics and interactions of 22:6 are unique compared to the other fatty acids found in sperm phospholipids, we cannot discern from our data if it is activated by a separate enzyme. We propose that 22:6, or a metabolite of 22:6, may regulate free fatty acid utilization in human sperm and that this hypothesis may provide an enzymatic explanation for the changes observed in phospholipid-bound fatty acids during the epididymal maturation of sperm.     

Solubilization of the alternative oxidase of cuckoo-pint (Arum maculatum) mitochondria. Stimulation by high concentrations of ions and effects of specific inhibitors.

Selective solubilization of cyanide- and antimycin-insensitive duroquinol oxidase activity from cuckoo-pint (Arum maculatum) mitochondria was achieved using taurocholate. Inhibitor-sensitivities and water-forming DQH2 (tetramethyl-p-hydroquinone, reduced form): O2 stoichiometry were the same for the alternative oxidase of intact Arum mitochondria. Cyanide-insensitive oxidation of DQH2 by intact and solubilized mitochondria was stimulated by up to four-fold by high concentrations of anions high in the Hofmeister series, such as phosphate, sulphate or citrate. Optimal (0.7 M) sodium citrate increased Vmax. for DQH2 oxidation by the solubilized preparation from 450 to 2400 nmol of O2 X min-1 X mg of protein-1 and decreased the apparent Km for DQH2 from 0.53 to 0.38 mM. Inhibition of solubilized DQH2 oxidase activity by CLAM (m-chlorobenzhydroxamic acid) and SHAM (salicylhydroxamic acid) was mixed competitive/non-competitive, with apparent inhibition constants for CLAM of 25 microM (Ki) and 81 microM (KI) and for SHAM of 53 microM (Ki) and 490 microM (KI). Propyl gallate and UHDBT were non-competitive inhibitors with respect to DQH2 (apparent Ki = 0.3 microM and 12 nM respectively). Low concentrations of C18 fatty acids selectively inhibited cyanide-insensitive oxidation by intact and solubilized mitochondria, and inhibition was reversed by 1% (w/v) bovine serum albumin. Inhibition was competitive with DQH2, suggesting that fatty acids interfere reversably with the binding of DQH2 to the oxidase. These results tend to support the view that quinol oxidation by the alternative pathway of Arum maculatum mitochondria is catalysed by a quinol oxidase protein, rather than by a non-enzymic mechanism involving fatty acid peroxidative reaction. [Rustin, Dupont & Lance (1983) Trends Biochem. Sci. 8, 155-157; (1983) Arch. Biochem. Biophys. 225, 630-639].     

Parinaric acids as probes of binding domains in neutrophil elastase

Human neutrophil elastase has an extended hydrophobic substrate binding site which serves as a target for a number of hydrophobic inhibitors. We show here that the parinaric acids, fluorescent-conjugated tetraenoic fatty acids of plant origin, are inhibitors of neutrophil elastase. cis-Parinaric acid (cis-PA) interacts with the enzyme in two inhibitory modes. The high affinity interaction (Ki = 55 +/- 6 nM) results in partial noncompetitive inhibition of amidolytic activity, with 82% residual activity. A lower affinity interaction with cis-PA (Ki = 4 +/- 1 microM) results in competitive inhibition. trans-PA also acts as a high affinity partial noncompetitive inhibitor of elastase with a Ki equal to that for cis-PA but has no low affinity competitive inhibitory action. The endogenous fluorescence from the 3 tryptophan residues in elastase is partially quenched on binding cis- or trans-PA. Dependence of quenching of tryptophan fluorescence on PA concentration is consistent with binding to a single site with an apparent Kd of 26 +/- 3 nM, which may be equivalent to the high affinity partial noncompetitive inhibitory binding mode. Analysis of quenching according to the modified Forster theory of energy transfer developed by Snyder and Freire (Snyder, B., and Freire, E. (1982) Biophys. J. 40, 137-148) leads to an estimate of apparent closest indole-PA distance of 13 +/- 3 A. Fluorescence of either cis- or trans-PA is apparently unperturbed upon binding in the high affinity mode to elastase, but at micromolar cis-PA concentrations, binding to elastase results in a blue shift and 20% increase in intensity of PA emission, suggesting that the lower affinity competitive inhibitory binding mode of binding to elastase provides a hydrophobic environment for cis-PA.     

Inhibitors directed to binding domains in neutrophil elastase

Human neutrophil elastase (HNE) can be inhibited by unsaturated fatty acids, including oleic acid [Ashe, B. M., & Zimmerman, M. (1977) Biochem. Biophys. Res. Commun. 75, 194-199; Cook, L., & Ternai, B. (1988) Biol. Chem. Hoppe-Seyler 369, 627-631], but is not affected by saturated fatty acids. We have shown that the interaction of oleic acid with HNE can be characterized by two apparent inhibitory modes: a high-affinity mode (Ki = 48 +/- 3 nM), resulting in partial noncompetitive inhibition (87% residual activity), and a competitive inhibitory mode of lower affinity (Ki = 16 +/- 1 microM). Binding of oleate in the high-affinity mode induces a blue shift in the endogenous fluorescence arising from the tryptophan residues in HNE. This shift is maximal in the presence of 1 microM oleate; higher concentrations of fatty acid have no further effect on the fluorescence spectrum. The negatively charged fluorescent ester of oleic acid and hydroxypyrenetrisulfonate (HPTSoleate) interacts with HNE at an apparent single site (Ki = 44 +/- 3 nM), resulting in competitive inhibition. A blue shift in the emission maximum of the pyrene fluorescence at 410 nm and a decrease in the ratio of the intensities of the maximum at 388 and 410 nm indicate that upon binding to HNE the environment of the pyrene ring in HPTSoleate becomes more hydrophobic.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermostabilization of porcine kidney D-amino acid oxidase by a single amino acid substitution

D-amino acid oxidase (DAO) is of considerable practical importance, such as bioconversion and enzymatic assay. In this study, we succeeded in obtaining a thermostable mutant DAO from porcine kidney by a single amino acid substitution. This mutant enzyme, F42C, was stable at 55 degrees C, while the wild-type enzyme was stable only up to 45 degrees C. The Km values of F42C for D-amino acids was about half of those of the wild-type enzyme. This mutant DAO with improved stability and affinity for its substrates is advantageous for the determination of D-amino acids.     

The inhibitory effect of polyunsaturated fatty acids on human CYP enzymes

The inhibitory effect of saturated fatty acids (SFAs): palmitic acid (PA), stearic acid (SA) and polyunsaturated fatty acids (PUFAs): linoleic acid (LA), linolenic acid (LN), arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on six human drug-metabolizing enzymes (CYP1A2, 2C9, 2C19, 2D6, 2E1 and 3A4) was studied. Supersomes from baculovirus-expressing single isoforms were used as the enzyme source. Phenacetin O-deethylation (CYP1A2), diclofenac 4-hydroxylation (CYP2C9), mephenytoin 4-hydroxylation (CYP2C19), dextromethorphan O-demethylation (CYP2D6), chlorzoxazone 6-hydroxylation (CYP2E1) and midazolam 1-hydroxylation (CYP3A4) were used as the probes. Results show that all the five examined PUFAs competitively inhibited CYP2C9- and CYP2C19-catalyzed metabolic reactions, with Ki values ranging from 1.7 to 4.7 microM and 2.3 to 7.4 microM, respectively. Among these, AA, EPA and DHA tended to have greater inhibitory potencies (lower IC(50) and Ki values) than LA and LN. In addition, these five PUFAs also competitively inhibited the metabolic reactions catalyzed by CYP1A2, 2E1 and 3A4 to a lesser extent (Ki values>10 microM). On the other hand, palmitic and stearic acids, the saturated fatty acids, had no inhibitory effect on the activities of six human CYP isozymes at concentrations up to 200 microM. Incubation of PUFAs with CYP2C9 or CYP2C19 in the presence of NADPH resulted in the decrease of PUFA concentrations in the incubation mixtures. These results indicate that the PUFAs are potent inhibitors as well as the substrates of CYP2C9 and CYP2C19.     

Investigating the role of active site residues of Rhodotorula gracilis D-amino acid oxidase on its substrate specificity

D-amino acid oxidase (DAAO) is a flavoprotein that catalyzes stereospecifically the oxidative deamination of D-amino acids. The wild-type DAAO is mainly active on neutral D-amino acids, while basic D-amino acids are poor substrates and the acidic ones are virtually not oxidized. To present a comprehensive picture of how the active site residues can modulate the substrate specificity a number of mutants at position M213, Y223, Y238, R285, S335, and Q339 were prepared in the enzyme from the yeast Rhodotorula gracilis. All DAAO mutants have spectral properties similar to those of the wild-type enzyme and are catalytically active, thus excluding an essential role in catalysis; a lower activity on neutral and basic amino acids was observed. Interestingly, an increase in activity and (k(cat)/K(m))(app) ratio on D-aspartate was observed for all the mutants containing an additional charged residue in the active site. The active site of yeast DAAO appears to be a highly evolved scaffold built up through evolution to optimize the oxidative deamination of neutral D-amino acids without limiting its substrate specificity. It is noteworthy, that introduction of a sole, additional, positively charged residue in the active site is sufficient to optimize the reactivity on acidic D-amino acids, giving rise to kinetic properties similar to those of D-aspartate oxidase.     

Dipeptide-hydroxamates are good inhibitors of the angiotensin I-converting enzyme

The inhibition constants (Ki) and modes of inhibition have been determined for a series of dipeptide-hydroxamate compounds with bovine lung parenchyma angiotensin I-converting enzyme (peptidyldipeptide carboxy-hydrolase, E.C. 3.4. 15.1). The hydroxamido function was borne by aspartic, glutamic, or aminoadipic acid and extended by 2, 3 or 4 bond lengths from the proline amide bond. L-glu(NHOH)-L-pro (Ki = 3.4 microM) and D,L-aminoadipicyl (NHOH)-L-pro (Ki = 1.2 microM) were the best competitive inhibitors of the hydrolysis of benzoyl-gly-his-gly but were not effective as affinity ligands for purification of the enzyme.     

Catalytic and structural characteristics of carp hepatopancreas D-amino acid oxidase expressed in Escherichia coli

D-amino acid oxidase of carp (Cyprinus carpio) hepatopancreas was overexpressed in Escherichia coli cells and purified to homogeneity for the first time in animal tissues other than pig kidney. The purified preparation had a specific activity of 293 units mg(-1) protein toward D-alanine as a substrate. It showed the highest activity toward D-alanine with a low Km of 0.23 mM and a high kcat of 190 s(-1) compared to 10 s(-1) of the pig kidney enzyme. Nonpolar and polar uncharged D-amino acids were preferable substrates to negatively or positively charged amino acids. The enzyme exhibited better thermal and pH stabilities than several yeast counterparts or the pig kidney enzyme. Secondary structure topology consisted of 11 alpha-helices and 17 beta-strands that differed slightly from pig kidney and Rhodotorula gracilis enzymes. A three-dimensional model of the carp enzyme constructed from a deduced amino acid sequence resembled that of pig kidney D-amino acid oxidase but with a shorter active site loop and a longer C-terminal loop. Judging from these characteristics, carp D-amino acid oxidase is close to the pig kidney enzyme structurally, but analogous to the R. gracilis enzyme enzymatically in turnover rate and pH and temperature stabilities.     

D-氨基酸氧化酶研究进展

D-氨基酸氧化酶(D-amino acid oxidase:oxidoreductase, DAAO, EC 1.4.3.3)是一种以黄素腺嘌呤(FAD)为辅基的典型黄素蛋白酶类,可氧化D-氨基酸的氨基生成相应的酮酸和氨。在体内D-氨基酸的代谢中起着重要作用。主要介绍了D-氨基酸氧化酶的生理功能和应用、表达条件优化及通过定点突变对酶学性质的研究。     

Dimensional mapping of the active site of cholesterol esterase with alkylboronic acid inhibitors

The cholesterol esterase-catalyzed hydrolysis of the water-soluble substrate p-nitrophenyl butyrate occurs via an acylenzyme mechanism, and is competitively inhibited by boronic acid transition state analog inhibitors. Accordingly, we undertook to dimensionally map the enzyme's active site via synthesis and characterization of a series of n-alkyl boronic acid inhibitors. The most potent of these is n-hexaneboronic acid, with a Ki = 13 +/- 1 microM, since inhibitor potency declines for both longer and shorter boronic acids. No inhibition is observed for methaneboronic acid and n-octaneboronic acid inhibits poorly, with a Ki of 7 mM. These results indicate that the ability of the enzyme to form tight complexes with boron-containing transition state analog inhibitors is sensitive to alkyl chain length. The trend in inhibitor potency is discussed in terms of substrate specificity of and transition state stabilization by cholesterol esterase, and has important implications for the design of optimal reversible inhibitors of the enzyme.     

The primary structure of D-amino acid oxidase from Rhodotorula gracilis

Summary The amino acid sequence of D-amino acid oxidase from Rhodotorula gracilis was determined by automated Edman degradation of peptides generated by enzymatic and chemical cleavage. The enzyme monomer contains 368 amino acid residues and its sequence is homologous to that of other known D-amino acid oxidases. Six highly conserved regions appear to have a specific role in binding of coenzyme FAD, in active site topology and in peroxisomal targeting. Moreover, Rhodotorula gracilis D-amino acid oxidase contains a region with a cluster of basic amino acids, probably exposed to solvent, which is absent in other D-amino acid oxidases.     

Affinity-labelling of the anti-inflammatory drug and prostaglandin-binding site of 3 alpha-hydroxysteroid dehydrogenase of rat liver cytosol with 17 beta- and 21-bromoacetoxysteroids.

The homogeneous 3 alpha-hydroxysteroid dehydrogenase of rat liver cytosol binds prostaglandins with low micromolar affinity at its active site and is competitively inhibited by the non-steroidal and steroidal anti-inflammatory drugs [Penning, Mukharji, Barrows & Talalay (1984) Biochem. J. 222, 601-611]. To examine the portion of this binding site that accommodates the glucocorticoid side chain, we have synthesized 17 beta-bromoacetoxy-5 alpha-dihydrotestosterone (BrDHT) and 21-bromoacetoxydesoxycorticosterone (BrDOC) as affinity-labelling agents. Both these agents promote rapid inactivation of the purified enzyme in a time- and concentration-dependent manner. Analyses of the inactivation progress curves gave estimates of Ki for the inactivators and half-life (t1/2) for the enzyme at saturation (tau) as follows: Ki = 33 microM and tau = 18 s for BrDHT, and Ki = 10 microM and tau = 203 s for BrDOC. Under initial-velocity conditions BrDHT and BrDOC act as competitive inhibitors, yielding Ki values identical with those measured in the inactivation experiments. Both indomethacin and prostaglandin E2 can protect the enzyme from inactivation, yielding Ki values for these ligands consistent with those measured independently by competitive-inhibition studies. These data confirm that the bromoacetoxysteroids label the active site, which is coincident with the prostaglandin- and anti-inflammatory-drug-binding site. Neither gel filtration nor extensive dialysis restores activity to the enzyme inactivated with either affinity-labelling agent. Use of radioactive BrDHT or BrDOC, in which either the steroid portion is labelled with 3H or the bromoacetate portion is labelled with 14C, indicates that inactivation is accompanied by a stoichiometric incorporation of 0.7-1.0 molecules of inhibitor per enzyme monomer. The linkage that forms between the dehydrogenase with either [14C]BrDHT or [14C]BrDOC is stable to acid and base treatment. Complete acid hydrolysis of the enzyme inactivated with [14C]BrDHT, followed by amino acid analyses, indicates that 87% of the radioactivity is eluted with carboxymethylcysteine. An almost identical result is obtained with [14C]BrDOC, where at least 75% of the radioactivity is eluted with this amino acid. Thus BrDHT and BrDOC alkylate at least one reactive cysteine residue at the active site that may be of functional importance in binding the glucocorticoid side chain.     

Stereoselectivity of 1-aminocyclopropanecarboxylate malonyltransferase toward stereoisomers of 1-amino-2-ethylcyclopropanecarboxylic acid

A malonyltransferase isolated from mungbean (Vigna radiata L.) hypocotyls catalyzed the malonylation of both 1-aminocyclopropane-1-carboxylic acid (ACC) and D-amino acids. The possibility that ACC was recognized by the enzyme as a D-amino acid was investigated by examining the efficiencies of the four stereoisomers of 1-amino-2-ethylcyclopropane-1-carboxylic acid (AEC) serving as substrates of malonyltransferase and as inhibitors of ACC malonyltransferase. Although all four isomers were malonylated by the enzyme and competitively inhibited the malonylation of ACC to N-malonyl-ACC, (1R,2S)-AEC and (1R,2R)-AEC, both of which have an R-configuration as a D-amino acid, had lower Km and Ki values (0.1 to 0.2 mM) than their enantiomers, (1S,2R)-AEC (Km and Ki values were about 1 mM) and (1S,2S)-AEC (Km and Ki values were higher than 10 mM), which have an S-configuration as an L-amino acid. Similarly, (R)-isovaline (2-amino-2-methylbutanoic acid), which has an R-configuration as a D-amino acid, inhibited more effectively the enzymatic conversion of ACC to malonyl-ACC than did (S)-isovaline, which has an S-configuration as an L-amino acid. In mungbean hypocotyls (1R,2S)-AEC and (1R,2R)-AEC were also more efficiently converted into malonyl conjugates and more efficiently inhibited the conversion of radioactive ACC into malonyl-ACC than their enantiomers, although the differences in efficiency among stereoisomers were smaller in hypocotyls than in enzymatic reactions. These results suggest that ACC is recognized by the enzyme as a D-amino acid.     

Inhibition in vitro of lipogenic enzymes from bovine (Bos taurus) mammary tissue by methylmalonyl-coenzyme A and coenzyme A

Bovine mammary fatty acid synthetase was inhibited by approximately 50% by 40 microM methylmalonyl-CoA; this inhibition was competitive with respect to malonyl-CoA (apparent Ki = 11 microM). Similarly, 6.25 microM coenzyme A inhibited the synthetase by 35% and this inhibition was again competitive (apparent Ki = 1.7 microM). Apparent Km for malonyl-CoA was 29 microM. The short-chain dicarboxylic acids malonic, methylmalonic and ethylmalonic at high concentrations (160-320 microM) and ATP (5 mM) enhanced the synthetase activity by about 50% respectively; the activating effects of methylmalonic acid and ATP on the synthetase were additive. Methylmalonyl-CoA at 50 microM concentration inhibited the partially purified acetyl-CoA carboxylase uncompetitively by 10% and the propionyl-CoA carboxylase activity of the enzyme preparation competitively (apparent Ki = 21 microM) by 40%. Malonyl-CoA also inhibited the acetyl-CoA carboxylase activity competitively (apparent Ki = 7 microM) by 35% and the propionyl-CoA carboxylating activity of the preparation competitively (apparent Ki = 4 microM) by 82%. The possibility that methylmalonyl-CoA may be a causal factor in the aetiology of the low milk-fat syndrome in high yielding dairy cows is discussed.     

Properties of the cyclic GMP phosphodiesterase from rat lung. Inhibition by unsaturated fatty acids

Catalytic and regulatory properties of the major form of cyclic GMP phosphodiesterase (3':5'-cyclic-GMP 5'-nucleotidohydrolase, EC 3.1.4.35) from rat lung were studied. The enzyme partially purified by a DEAE-Sepharose chromatography displayed a much higher affinity toward cyclic GMP than toward cyclic AMP, the apparent Km values being 5.7 microM and 482 microM for the guanylic and the adenylic cyclic nucleotide, respectively. In contrast, the V value for cyclic AMP was about 3-times higher than the V value for cyclic GMP. Linear double reciprocal plots of initial velocity were observed with each cyclic nucleotide. From 10(-8) to 3.3 X 10(-6) M, cyclic GMP did not change the hydrolysis of 1 or 10 microM cyclic [3H]AMP, while it became inhibitory at higher concentrations. In contrast with a calmodulin-sensitive phosphodiesterase prepared from rat brain, the lung enzyme was not stimulated by a heat-stable Ca2+-dependent factor from rat lung or by rat brain calmodulin or by lipids including fatty acids and lysophosphatidylcholine. Various unsaturated 18- and 20-carbon fatty acids inhibited at varying degrees the cyclic GMP phosphodiesterase from rat lung. The inhibitory potency increased with the number of double bonds in the hydrocarbon chain. In contrast, the methyl esters of the unsaturated fatty acids and the saturated fatty acids of variable hydrocarbon chain lengths had no appreciable effects. A linear Hill plot of phosphodiesterase inhibition with a slope of unity was obtained with arachidonic acid up to 30 microM, suggesting only one type of inhibitory site. In this range of concentrations the inhibition was entirely reversible. Kinetics analysis demonstrated that up to 30 microM arachidonic acid was a purely competitive inhibitor with an apparent Ki of 20 microM. Over 30 microM, the Hill coefficient increased progressively, indicating the binding to other inhibitory sites, while the reversibility disappeared.     

Characterization of the substrate specificity of alpha1,3galactosyltransferase utilizing modified N-acetyllactosamine disaccharides.

alpha1,3galactosyltransferase (alpha1,3GalT) catalyzes the synthesis of a range of glycoconjugates containing the Galalpha1,3Gal epitope which is recognized by the naturally occurring human antibody, anti-Gal. This enzyme may be a useful synthetic tool to produce a range of compounds to further investigate the binding site of anti-Gal and other proteins with a Galalpha1,3Gal binding site. Thus, the enzyme has been probed with a series of type 2 disaccharide-C8(Galbeta1-4GlcNAc-C8) analogs. The enzyme tolerated acceptors with modifications at C2 and C3 of the N-acetylglucosamine residue, producing a family of compounds with a nonreducing alpha1,3 linked galactose. Compounds that did not serve as acceptors were evaluated as inhibitors. Interestingly, the type 1 disaccharide-C8, Galbeta1-3GlcNAc-C8, was a good inhibitor of the enzyme (Ki = 270 microM vs. Km = 190 microM for Galbeta1-4GlcNAc-C8). A potential photoprobe, based on a modified type 2 disaccharide (octyl 3-amino-3-deoxy-3-N-(2-diazo-3, 3, 3-trifluoropropionyl-beta-D-galactopyranosyl-(1, 4)-2-acetamindo-2-deoxy-beta-D-glycopyranoside, (DTFP-LacNAc-C8)), was evaluated as an inhibitor of alpha1,3GalT. alpha1,3GalT bound DTFP-LacNAc-C8 with an affinity (Ki = 300 microM) similar to that displayed by the enzyme for LacNAc-C8. Additional studies were done to determine the enzyme's ability to transfer a range of sugars from UDP-sugar donors. The results of these experiments demonstrated that alpha1,3GalT has a strict specificity for UDP-Gal. Finally, inactivation studies with various amino acid modifiers were done to obtain information on the importance of different types of amino acids for alpha1,3GalT activity.     

The mechanism of action of dipeptidyl aminopeptidase. Inhibition by amino acid derivatives and amines; activation by aromatic compounds.

A variety of amino acid and peptide amides have been shown to be inhibitors of dipeptidyl aminopeptidase. Among these compounds derivatives of strongly hydrophobic amino acids are the strongest inhibitors (Phe-NH2, Ki = 1.0 +/- 0.2 mM), while amides of basic amino acids were somewhat less effective (Lys-NH2, Ki = 36 +/- 3 mM). Short chain amino acid amides are notably weaker inhibitors (Gly-NH2, Ki = 293 +/- 50 mM). The interaction of the side chains of compounds with the enzyme appears to be at a site other than that at which the side chain of the amino-penultimate residue of the substrate interacts since the specificity of binding is different. Primary amines have been shown to inhibit, e.g., butylamine, Ki = 340 +/- 40 mM, and aromatic compounds have been shown to stimulate activity toward Gly-Gly-NH2 and Gly-Gly-OEt (phenol, 35% stimulation of activity at a 1:1 molar ratio with the substrate). The data suggest that inhibition involves binding at the site occupied by the free alpha-amino group and the N-terminal amino acid.     

Inhibition of angiotensin converting enzyme by phosphoramidates and polyphosphates

N alpha-Phosphoryl-L-alanyl-L-proline is a reversible competitive inhibitor of angiotensin converting enzyme with a Ki of 1.4 nM. Alkylation of one phosphate oxygen with methyl, ethyl, or benzyl does not change the Ki. The high activity of the O-alkylated inhibitors demonstrates that the two phosphate oxygen anions do not constitute a bidentate ligand of the active site zinc ion. Substitution of valyltryptophan, glycylglycine, or delta-aminovaleric acid for alanylproline in the phosphoramidate raises the Ki to 12 nM, 25 microM, and 178 microM, respectively. Methylation of the alanine nitrogen in phosphorylalanylproline raises the Ki to 29 microM. Polyphosphates inhibit converting enzyme with the following Ki's: phosphate, approximately 300 mM; pyrophosphate, 2 mM; tripolyphosphate, 18 microM; tetrapolyphosphate, 150 microM. The inhibition by tripolyphosphate appears to be competitive and is unaffected by the addition of excess zinc ion. Since the Ki of tripolyphosphate is nearly 10-fold lower than that of N-phosphoryl-delta-aminovaleric acid and is near that of N alpha-phosphorylglycylglycine, its terminal phosphates may bind the zinc site and the cationic site on the enzyme, thus spanning the S1' and S2' sites.