Binding of divalent metal ions to 1–25 β-caseinophosphopeptide: An isothermal titration calorimetry study

Albumin Promotes the Absorption of Pb in the Intestine: A Possible Pathway Linking to the High Absorption of Pb in Adolescence

High blood lead (Pb) levels have long been a significant environmental issue affecting the health and development of adolescents. However, the main reasons behind this phenomenon, especially the absorption and transportation processes of Pb in the intestine, have not been elucidated, which is the major barrier to reducing blood Pb levels in the human body, especially in adolescents. In this study, we explored the intestinal absorption process of Pb in rats of different ages at environmentally relevant concentrations. The results indicated that albumin serves as a crucial mediating role in the process of gradually decreasing Pb levels with age (aging). Next, experiments with induced senescence and the addition of albumin confirmed that binding to albumin facilitates the absorption and transportation of Pb. Furthermore, a low-protein diet can reduce Pb levels by approximately 50%, possibly due to a decrease in the synthesis of albumin resulting from reduced raw materials. Collectively, these findings reveal the potential reasons for high Pb levels in adolescents, elucidate the influence of albumin on Pb absorption and transportation, and provide an alternative pathway for regulating intestinal Pb absorption in adolescents at ambient concentrations.

Isolation and differential structure characteristics of calcium-binding peptides derived from Pacific cod bones by hydroxyapatite affinity

Calcium-chelating peptides were found in Pacific cod bone, but their binding structure and properties have not been elucidated. Novel calcium-binding peptides were isolated by hydroxyapatite affinity chromatography (HAC), and their binding structure and properties were investigated by isothermal titration calorimetry (ITC), multispectral techniques, and mass spectrometry. Based on multiple purifications, the calcium binding capacity (CBC) of Pacific cod bone peptides (PBPs) was increased from 1.71 ± 0.15 μg/mg to 7.94 ± 1.56 μg/mg. Peptides with a molecular weight of 1-2 kDa are closely correlated with CBC. After binding to calcium, the secondary structure of peptides transitioned from random coil to β-sheet, resulting in a loose and porous microstructure. Hydrogen bonds, electrostatic interaction, and hydrophobic interaction contribute to the formation of peptide‑calcium complexes. The F21 contained 42 peptides, with repeated "GE" motif. Differential structure analysis provides a theoretical basis for the targeted preparation of high CBC peptides.

Hemoglobin Hydrolyzate Promotes Iron Absorption in the Small Intestine through Iron Binding Peptides

Hemoglobin is an excellent source of iron supplements, and its hydrolyzate spontaneously binds iron during digestion and promotes iron absorption in vivo. However, the underlying mechanisms of what peptides bind and how they bind iron ions remain unclear. This study prepared the porcine hemoglobin hydrolyzate through enzymatic hydrolysis and acid treatment and investigated the mechanisms of hemoglobin hydrolyzate on iron absorption through the determination of iron levels in dietary intervention mice, iron binding site analyses, peptide digestion analyses, molecular simulation docking, and INT407 cell validation. The results showed that ingestion of the hemoglobin hydrolyzate diets increased iron levels in the blood of mice, accompanied by the upregulation of duodenal iron circulation-related genes such as ferritin, PCBP1, and HP. Carboxyl, imidazole groups, and aromatic amino acid residues were iron binding sites of hemoglobin hydrolyzate during digestion. VDEVGGEA and VDEVGGE were found to involve the spontaneous and efficient binding of hemoglobin hydrolyzate to iron ions in the intestinal cavity. In particular, the DEVGGE peptide was the typical sequence for hemoglobin hydrolytic peptides to exert iron binding activity.

Screening, separation and identification of metal-chelating peptides for nutritional, cosmetics and pharmaceutical applications

Metal-chelating peptides, which form metal-peptide coordination complexes with various metal ions, can be used as biofunctional ingredients notably to enhance human health and prevent diseases. This review aims to discuss recent insights into food-derived metal-chelating peptides, the strategies set up for their discovery, their study, and identification. After understanding the overall properties of metal-chelating peptides, their production from food-derived protein sources and their potential applications will be discussed, particularly in nutritional, cosmetics and pharmaceutical fields. In addition, the review provides an overview of the last decades of progress in discovering food-derived metal-chelating peptides, addressing several screening, separation and identification methodologies. Furthermore, it emphasizes the methods used to assess peptide-metal interaction, allowing for better understanding of chemical and thermodynamic parameters associated with the formation of peptide-metal coordination complexes, as well as the specific amino acid residues that play important roles in the metal ion coordination.

Structural identification and combination mechanism of iron (II)-chelating Atlantic salmon (Salmo salar L.) skin active peptides

In order to utilize salmon skin for high value, and investigate the structural identification and combination mechanism of iron (II)-chelating peptides systemically, Atlantic salmon (Salmo salar L.) skin, a by-product of Atlantic salmon processing, was treated by two-step enzymatic hydrolysis to obtain salmon skin active peptides (SSAP). Then they reacted with iron (II) to obtain iron (II)-chelating salmon skin active peptides (SSAP-Fe) with a high iron (II) chelating ability of 98.84%. The results of Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD) spectroscopy, 8-anilino-1-naphthalenesulfonic acid ammonium salt hydrate (ANS) combined fluorescence measurement, isothermal titration calorimetry (ITC) and full wavelength ultraviolet (UV) scanning showed that the structural characteristics of SSAP changed before and after chelating iron (II). Reverse phase high performance liquid chromatography (RP-HPLC) and mass spectrometry were used to identify and quantify the peptides in SSAP-Fe. Four peptide sequences (STEGGG, GIIKYGDDFMH, PGQPGIGYDGPAGPPGPPGPPGAP and QNQRESWTTCRSQSSLPDG) were identified. The content of PGQPGIGYDGPAGPPGPPGPPGAP was the highest, at 25.17 μg/mg. The pharmacokinetic and pharmacodynamic properties of these four peptides were also investigated, and the results indicated that they have satisfactory predicted ADMET properties. Molecular docking technology was used to analyze the binding sites between iron (II) and SSAP, and it was found that PGQPGIGYDGPAGPPGPPGPPGAP had the lowest predicted binding energy with iron (II) and the most stable predicted binding energy with iron (II). This results showed that the stability of SSAP-Fe were closely related to the number of covalent bonds and the types of amino acids. This study revealed the structure and combination mechanism of SSAP-Fe, and indicated that SSAP-Fe prepared by chelation may be used as a Fe supplement that can be applied in functional foods or ingredients.

Interaction mechanism of egg derived peptides RVPSL and QIGLF with dipalmitoyl phosphatidylcholine membrane: microcalorimetric and molecular dynamics simulation studies

BACKGROUND

Egg-derived peptides are becoming increasingly popular due to their biological activity and non-toxic effects. The egg-derived peptides Arg-Val-Pro-Ser-Leu (RVPSL) and Gln-Ile-Gly-Leu-Phe (QIGLF) display strong angiotensin-converting enzyme inhibitory activity and they can be taken up by intestinal epithelial cells. The interaction of the egg-derived peptides RVPSL and QIGLF with the membrane remains unclear.

RESULTS

The position and structure of the peptides in the membrane were calculated. The maximum density values of RVPSL and QIGLF were 2.27 and 1.22 nm from the center of the 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) membrane, respectively, indicating that peptides penetrated the membrane-water interface and were embedded in the membrane. The interaction of RVPSL and QIGLF with the DPPC membrane did not affect the average area per lipid or the lipid sequence parameters. The thermodynamic parameters ΔH, ΔG, and ΔS of the interaction between the peptide RVPSL with the DPPC membrane were 17.91 kJ mol-1 , -17.63 kJ mol-1 , 187.5 J mol-1 ·k-1 , respectively. The thermodynamic parameters ΔH, ΔG, and ΔS of the interaction between peptide QIGLF with DPPC membrane were 17.10 kJ mol-1 , -17.12 kJ mol-1 , 114.8 J mol-1 ·k-1 , respectively.

CONCLUSION

The results indicated that the binding of peptides RVPSL and QIGLF to DPPC is an endothermic, spontaneous, and entropy-driven reaction. The results of the study are relevant to the problem of the low bioavailability of bioactive peptides (BP). © 2023 Society of Chemical Industry.

Effect of Dietary Casein Phosphopeptide Addition on the Egg Production Performance, Egg Quality, and Eggshell Ultrastructure of Late Laying Hens

(1) Objective: This study aimed to assess the effects of dietary casein phosphopeptide (CPP) supplementation on the egg production performance of late laying hens and the resulting egg quality and eggshell ultrastructure. (2) Methods: A total of 800 laying hens aged 58 weeks were randomly assigned into 5 groups with 8 replicates of 20 hens each. The hens were fed a basal diet supplemented with 0 (control, T1), 0.5 (T2), 1.0 (T3), 1.5 (T4), and 2.0 (T5) g/kg CPP for 9 weeks. (3) Results: Dietary CPP supplementation was found to be beneficial for improving eggshell quality. The spoiled egg rate of the experimental groups was lower than that of the control group (linear and quadratic effect, p < 0.05). The yolk color in the T2, T3, and T4 groups was higher than that in the T1 group (quadratic effect, p < 0.05). The shell thickness in the T4 group was higher than that in the T1 and T2 groups (linear effect, p < 0.05). The shell color in the experimental groups was higher than that in the control group (linear and quadratic effect, p < 0.05). The effective thickness in the T3-T5 groups (linear and quadratic, p < 0.05) and the number of papillary nodes in the T2 and T3 groups were higher than those in the T1 group (quadratic, p < 0.05). The calcium content in the T2 and T3 groups was higher than that in the T1 group (quadratic effect, p < 0.05). The iron content in the T2 and T3 groups was higher than that in the T1 group (p < 0.05). (4) Conclusion: In summary, 0.5-1.0 g/kg CPP supplementation reduced the spoiled egg rate, enhanced the yolk and eggshell colors, increased the thickness of the effective layer, and the calcium and iron contents in the eggshell.

Development, characterization and in vitro bile salts binding capacity of selenium nanoparticles stabilized by soybean polypeptides

The paper presents the positive effect of soybean polypeptides (SP) on the stability and the potential hypolipidemic effect of selenium nanoparticles (SeNPs). After preparing SeNPs, SP with different molecular weight were introduced to stabilize SeNPs. We found that the SP with molecular weight >10 kDa (SP5) had the best stabilizing effect on SeNPs. We inferred that the steric resistance resulting from the long chains of SP5 protected SeNPs from collision-mediated aggregation, and the electrostatic repulsions between SP5 and SeNPs also played a positive role in stabilizing SeNPs. The as-prepared SP5-SeNPs were spherical, amorphous and zero valent. It was proved that SeNPs were bound with SP5 through O- and N- groups in SP5, and the main forces were hydrogen bonds and van der Waals forces. The bile salts binding assay showed that the SP5-SeNPs exhibited a high binding capacity to bile salts, which indicated their potential in hypolipidemic application.

A comprehensive review on preparation, structure-activities relationship, and calcium bioavailability of casein phosphopeptides

Calcium is one of the important elements for human health. Calcium deficiencies can lead to numerous diseases. Calcium chelating peptides have shown potential application in the management of calcium deficiencies. Casein phosphopeptides (CPP) are phosphoseryl-containing fragments of casein by enzymatic hydrolysis or fermentation during manufacture of milk products as well as during intestinal digestion. An increasing number of CPP with the ability to facilitate and enhance the bioavailability of calcium are being discovered and identified. In this review, 249 reported CPP derived from four types of bovine casein (αs1, αs2, β and κ) were collected, and the amino acid sequence and phosphoserine group information were sorted out. This review outlines the current enzyme hydrolysis, detection methods, purification, structure-activity relationship and mechanism of intestinal calcium absorption in vitro and in vivo as well as application of CPP.

Interaction between pH-shifted ovalbumin and insoluble neohesperidin: Experimental and binding mechanism studies

In this study, ovalbumin (OVA) formed a complex with neohesperidin (NH) via a pH-shifting method. The NH-OVA complex self-assembled into NH-OVA nano-particles, which were then characterized and whose binding mechanism was evaluated by using multi-spectroscopic, thermodynamics, and molecular docking simulation methods. Fluorescence intensity decreased after OVA was complexed with NH. The binding constant of the OVA-NH complex was in the order of 6.32 × 10⁵ M^-1 suggesting that the complex is stable. Circular dichroism (CD) analysis showed that α -helix content increased, β-folding, β -turning, and irregular crimp content decreased after OVA and NH binding. Isothermal titration calorimetry results showed that hydrophobic interactions and hydrogen bonds made an important impact in the complex formation. The molecular docking results revealed that Van der Waals forces and hydrogen bonds contributed to the free binding energy of the complex. There were multiple possible surface binding sites between OVA with NH. The obtained results provide new insights into the interaction mechanism of OVA and NH, and as a vehicle for NH, the OVA has shown promising applications in functional foods.

Characterization of Chelation and Absorption of Calcium by a Mytilus edulis Derived Osteogenic Peptide

In a previous study, the peptide LGKDQVRT, which was identified by enzymatic hydrolysis, released during the proteolysis of Mytilus edulis, had potential osteogenic activity. In this study, the octapeptide LGKDQVRT was able to spontaneously bind calcium in a 1:1 stoichiometric ratio, and the calcium-binding site likely involves calcium and amino acid VAL6 in the LGKDQVRT peptide to form a metal-donor to metal acceptor complex. The peptide LGKDQVRT has the activity of promoting the proliferation and differentiation of osteoblasts. The results of this study suggest that hydrolyzed peptides from Mytilus edulis protein can be used as a dietary supplement to improve calcium absorption and prevent osteoporosis.

Calcium Delivery Systems Assembled using Antarctic Krill Derived Heptapeptides: Exploration of the Assembly Mechanism, In Vitro Digestion Profile, and Calcium Absorption Behavior

A novel heptapeptide QEELISK derived from Antarctic krill was used to assemble a calcium delivery system, of which the calcium binding mechanism of QEELISK, in vitro digestion kinetics, and calcium absorption behaviors were explored. QEELISK with continuous Glu possessed higher calcium binding capacity than that of QELEISK and QAALISK. Ca²⁺ bound to the carboxyl oxygen of Glu at position 3 of the QEELISK peptide at a stoichiometric ratio of 1:1 through charge-charge interaction; the formed QEELISK-Ca showed superior stability. Moreover, QEELISK-Ca underwent disaggregation and self-assembly during in vitro digestion reflected by visualization of calcium ions and circular dichroism spectra. QELEISK was partially stable during gastrointestinal digestion, and calcium chelation improved the digestive stability of QELEISK. In addition, a significant enhancement of calcium absorption with QELEISK-Ca occurred in the duodenum and ileum when compared to CaCl₂ absorption, which indicated that QEELISK might carry calcium ions through the gastrointestinal tract.

Antarctic krill-derived peptides with consecutive Glu residues enhanced iron binding, solubility, and absorption

Three peptides containing three glutamic acid (Glu) residues at different positions derived from Antarctic krill were obtained to investigate their iron-binding properties, digestive stability, and effectiveness on enhancing iron solubility and absorption. Results indicated that Fe2+ bound to the carbonyl, carboxyl, or hydroxyl groups of DELEDSLER, EEEFDATR, and DTDSEEEIR at stoichiometric ratios of 0.453, 0.466, and 0.490, respectively. DTDSEEEIR with three consecutive Glu in the middle of the sequence possessed higher iron-binding ability and iron release potential than EEEFDATR with three consecutive Glu in the N-terminal, and DELEDSLER with three discontinuous Glu showed the lowest values. Although EEEFDATR showed remarkably lower digestion stability than DTDSEEEIR, the effect of EEEFDATR-iron on iron solubility and absorption was comparable to that of DTDSEEEIR-iron, but better than that of DELEDSLER-iron and FeSO4. Thus, peptides with consecutive Glu have the potential as an effective iron carrier to improve iron absorption.

Deciphering calcium-binding behaviors of casein phosphopeptides by experimental approaches and molecular simulation

Casein phosphopeptides (CPPs) as premium additives in functional foods can facilitate the transport and adsorption of calcium. The atomic resolution decipherment of calcium-CPP binding behaviors is critical for understanding the calcium bioavailability enhancement potential of CPPs. In the present study, the experimental methods (UV-vis, FTIR and isothermal titration calorimetry) and molecular dynamics simulation were combined to reveal the calcium-binding behaviors of β-casein phosphopeptides (1-25) (P5) with the best capability in carrying calcium ions. We found that it could carry approximately six calcium ions, and the calcium-binding sites were primarily located at the carbonyl group of Glu-2 and the phosphate group of phosphorylated Ser-15, Ser-18, and Ser-19. An interesting finding was that calcium ions could be bound by three coordinated modes, including unidentate, bidentate and tridentate geometries, resulting in the strong binding abilities. The binding process of calcium ions to P5 was spontaneous with the binding free energies of -5.2 kcal mol-1. Hydrophobic interactions were considered to be the major driving force for the calcium ion binding. The present study provides novel molecular insights into the binding process between Ca2+ and calcium-binding peptides.

The influence of temperature- and divalent-cation-mediated aggregation of β-casein on the physical and microstructural properties of β-casein-stabilised emulsions

The objective of this study was to assess the influence of self-association of β-casein (β-CN) induced by both increasing temperature (5-55 °C) and divalent cation addition (Ca²⁺ or Mg²⁺) on the properties of β-CN-stabilised emulsions. The particle size of 0.5% (w/w) β-CN in 10 mM imidazole/HCl buffer (pH 6.8) was determined as a function of temperature and addition of divalent cations. Addition of CaCl₂ caused a greater increase in protein particle size than MgCl₂. Oil-in-water emulsions stabilised with 0.5% (w/w) β-CN, β-CN with added CaCl₂ or MgCl₂ (β-CN/Ca and β-CN/Mg, respectively) were also investigated as a function of temperature using light scattering, analytical centrifugation, rheology and confocal laser scanning microscopy (CLSM). Emulsions prepared with β-CN/Ca flocculated after incubation at 55 °C for 20 min and displayed significantly different physical properties (p < 0.05) compared to emulsions stabilised with β-CN or β-CN/Mg in the temperature range 5-55 °C. Based on CLSM analysis and analysis of the interfacial protein load, this flocculation was attributed to the interaction of adsorbed β-CN between droplets and the interaction of adsorbed and non-adsorbed β-CN aggregates in the aqueous phase via calcium bridges. Furthermore, the flocculation of β-CN/Ca emulsions was reversible upon cooling, which is similar to that of β-CN/Ca in solution. In conclusion, the temperature-dependent behaviour of β-CN-stabilised emulsions correlated to the temperature-induced aggregation of β-CN, particularly in the presence of Ca²⁺. Hence, the stability of β-CN-stabilised emulsions can be predicted from the extent of β-CN aggregation in aqueous solution (i.e., aggregate size).

Isolation and identification of zinc-chelating peptides from sea cucumber (Stichopus japonicus) protein hydrolysate

BACKGROUND

Zinc is known to play an essential role in the biological activities in the human body. In this study, a zinc-chelating peptide (ZCP) produced by Alcalase-assisted hydrolysis of the body wall of sea cucumber was isolated and identified. The ZCP was purified stepwise by ultrafiltration, anion-exchange chromatography, and gel filtration chromatography, in conjunction with ultraviolet-visual (UV-visual) spectrophotometry, which was used to analyze each purified fraction.

RESULTS

Analysis of the purified ZCP revealed that its zinc-chelating ability was 33.31%. Analysis of isothermal titration calorimetry suggested that the binding of ZCP and zinc (N ≈ 2) was endothermic, with weak binding affinity. Fourier transform infrared spectroscopy spectra (FTIR) indicated that carboxylic and amide groups in ZCP were the primary binding sites of Zn. Sequencing the result by ultra-performance liquid chromatography-quadrupole/time of flight mass spectrometry (UPLC-Q-TOF-MS/MS) showed that a representative ZCP had the sequence WLTPTYPE with a molecular weight of 1005.5 Da.

CONCLUSION

These results provide a promising foundation for the production of zinc supplements from sea-cucumber-derived ZCPs. © 2019 Society of Chemical Industry.

Zinc-Chelating Mechanism of Sea Cucumber (Stichopus japonicus)-Derived Synthetic Peptides

In this study, three synthetic zinc-chelating peptides (ZCPs) derived from sea cucumber hydrolysates with limited or none of the common metal-chelating amino-acid residues were analyzed by flame atomic absorption spectroscopy, circular dichroism spectroscopy, size exclusion chromatography, zeta-potential, Fourier transform infrared spectroscopy, Raman spectroscopy and nuclear magnetic resonance spectroscopy. The amount of zinc bound to the ZCPs reached maximum values with ZCP:zinc at 1:1, and it was not further increased by additional zinc presence. The secondary structures of ZCPs were slightly altered, whereas no formation of multimers was observed. Furthermore, zinc increased the zeta-potential value by neutralizing the negatively charged residues. Only free carboxyl in C-terminus of ZCPs was identified as the primary binding site of zinc. These results provide the theoretical foundation to understand the mechanism of zinc chelation by peptides.

Interaction landscape of a 'CαNN' motif with arsenate and arsenite: a potential peptide-based scavenger of arsenic

Arsenic (As) is a toxic metalloid that has drawn immense attention from the scientific community recently due to its fatal effects through its unwanted occurrence in ground water around the globe. The presence of an excess amount of water soluble arsenate and/or arsenite salt (permissible limit 10 μg L-1 as recommended by the WHO) in water has been correlated with several human diseases. Although arsenate (HAsO4 2-) is a molecular analogue of phosphate (HPO4 2-), phosphate is indispensable for life, while arsenic and its salts are toxic. Therefore, it is worthwhile to focus on the removal of arsenic from water. Towards this end, the design of peptide-based scaffolds for the recognition of arsenate and arsenite would add a new dimension. Utilizing the stereochemical similarity between arsenate (HAsO4 2-) and phosphate (HPO4 2-), we successfully investigated the recognition of arsenate and arsenite with a naturally occurring novel phosphate binding 'CαNN' motif and its related designed analogues. Using computational as well as biophysical approaches, for the first time, we report here that a designed peptide-based scaffold based on the 'CαNN' motif can recognize anions of arsenic in a thermodynamically favorable manner in a context-free system. This peptide-based arsenic binding agent has the potential for future development as a scavenger of arsenic anions to obtain arsenic free water.

The purification, identification and bioactivity study of a novel calcium-binding peptide from casein hydrolysate

In this study, a novel calcium-binding peptide from casein hydrolysate was purified using reversed-phase high performance liquid chromatography and sequenced by high-performance liquid chromatography-mass spectrometry (MS)/MS.

Cryptides: latent peptides everywhere

Proteomic surveys with top-down platforms are today revealing thousands of naturally occurring fragments of bigger proteins. Some of them have not functional meaning because they derive from pathways responsible for protein degradation, but many have specific functions, often completely different from that one of the parent proteins. These peptides encrypted in the protein sequence are nowadays called cryptides. They are frequent in the animal and plant kingdoms and represent a new interesting -omic field of investigation. To point out how much widespread is their presence, we describe here the most studied cryptides from very common sources such as serum albumin, immunoglobulins, hemoglobin, and from saliva and milk proteins. Given its vastness, it is unfeasible to cover the topic exhaustively, therefore only several selected examples of cryptides from other sources are thereafter reported. Demanding is the development of new -omic platforms for the functional screening of new cryptides, which could provide suggestion for peptides and peptido-mimetics with variegate fields of application.

Protein degradation and peptide release from milk proteins in human jejunum. Comparison with in vitro gastrointestinal simulation

Human jejunal digests after oral ingestion of casein and whey protein were collected by a nasogastric tube and protein degradation and peptide release was compared with that found in the digests of the same substrates using a standardised protocol. No intact casein was detected in the jejunal nor in the in vitro samples taken during the intestinal phase, while β-lactoglobulin was found in one hour-jejunal samples in agreement with the in vitro digestion. In vivo and in vitro digests showed comparable peptide profiles and high number of common sequences. A selective precipitation step was used to strengthen the identification of phosphorylated peptides. Most of the sequences found in jejunum, some of them not previously described, were also identified in the simulated digests. Common resistant regions to digestion were identified, revealing that the in vitro protocol constitutes a good approximation to the physiological gastrointestinal digestion of milk proteins.

Surface plasmon resonance analysis of the binding mechanism of pharmacological and peptidic inhibitors to human somatic angiotensin I-converting enzyme

Somatic angiotensin I-converting enzyme (ACE) possesses two catalytic domains and plays a major role in the regulation of blood pressure, thus representing a therapeutic target for the treatment of hypertension. We present a comprehensive surface plasmon resonance (SPR) study of the interaction of human somatic ACE with the pharmacological inhibitors captopril and lisinopril, the bradykinin potentiating peptide BPP-11b, and the food peptidic inhibitors from bovine αs2-casein, F(174)ALPQYLK(181) and F(174)ALPQY(179). SPR binding curves recorded with the high potency inhibitors captopril, lisinopril, and BPP-11b were evaluated both by regression analysis and by kinetic distribution analysis. The results indicated that captopril and lisinopril bound ACE with two K(D)'s differing by a factor 10-20 and >30, respectively (lowest K(D) = 0.1-0.3 nM for both inhibitors). This shows, for the first time in a direct binding assay with the two-domain enzyme, the existence of two binding modes of the pharmacological inhibitors, presumably with the two ACE domains. The BPP-11b-ACE binding curves were complex but showed a predominant interaction with K(D) in the nanomolar range. The caseinopeptides, known to inhibit ACE with an IC₅₀ of 4.3 μM, bound to ACE with K(D) = 3-4 μM. Mapping of the F(174)ALPQY(179) binding site on ACE by sequential binding studies using captopril or BPP-11b indicated that it bound to (or near) the two active sites of ACE, in agreement with the stoichiometry of 2 determined from data fitting. Our results provide a detailed characterization of ACE-inhibitor binding modes and validate SPR for predicting the inhibitory potential of new compounds.