Effect of alcalase on calcium-binding activity
Enzymatic protein hydrolysis is based on the use of commercial alcalase to generate short bioactive peptides. Some research reveals that alcalase treatment can more efficiently increase the metal-binding characteristics of defatted wheat germ protein compared to flavorzyme, papain, neutrase, and protamex15,24. Several parameters can influence the metal-binding capabilities of hydrolyzed protein, including the degree of hydrolysis (DH) and the side chains of amino acid residues. In our previous investigation of the relationship between DH and calcium binding activity which occurred in a competition assay against calcium phosphate precipitation, we discovered that the best condition with the maximum DH had a calcium binding activity of 60.39 ± 1.545% with a final protein content of 0.1455 ± 0.0015 mg/mL23. This result was consistent with a study of Ke et al.25 who discovered that DH had a significant impact on zinc binding activity. Proper hydrolysis may expose specific groups which enhance calcium coordination26. Consequently, the molecular masses and side chains of the amino acid residue of DLBS were examined and debated further.
Calcium-binding peptide purification
In this study, the DLBS protein hydrolysates first underwent purification via ultrafiltration before the peptides were classified by their MWCO. This is important because the chelating abilities of the peptides may be dependent upon molecular weight. Previous studies suggest that higher binding activity is a feature of peptides which have a lower molecular weight, as in the case of one particular peptide obtained from wheat germ protein hydrolysates (579.34 Da)17 and another peptide obtained from the Schizochytrium protein (291.15 Da and 328.17 Da)27. Figure 1a presents the results, indicating that calcium-binding activity increased as the MWCO decreased. The sample measuring 3–0.65 kDa showed the highest activity level. This stood in contrast to the case of soybean protein hydrolysate, whose calcium-binding activity declined with a lower MWCO28. This occurrence matches earlier studies linking peptide size with binding activity. Accordingly, the best ligand binding with calcium ions will result from an ideal size, which is neither too large nor too small16,28,29. After using the ultrafiltration technique, we found that the greatest calcium-binding activity peptide was presented in the 3–0.65 kDa fraction.
(a) Percentage of calcium-binding activity for each of the five fractions of the Alcalase hydrolysate following ultrafiltration; (b) Fractions obtained via RP-HPLC and the calcium-binding activity of the MWCO 3–0.65 kDa fraction; (c) calcium-binding activity of the four fractions (F1, F2, F3, and F4) separated via RP-HPLC. The values are shown as percentage of calcium-binding activity ± SD, and all testing was carried out in triplicate with a same protein concentration. Where the means is shown using a different letter, this is indicative of a significant difference in Duncan’s multiple range test (p < 0.05).
To study the effects of structure on the activities of calcium-binding peptides, amino acid sequences were first identified after separation. RP-HPLC was then used for the calcium-binding peptide and protein selection process. Separation via RP-HPLC is based upon the hydrophobicity of the solute, whereby a solute with high polarity will have correspondingly minimized hydrophobicity, and will therefore be less likely to combine with stationary phases. The active fraction with molecular weight 3–0.65 kDa was selected to be lyophilized and subsequently dissolved in distilled water before loading into the C18 RP-HPLC. This fraction was then further separated to create four new fractions: F1 (14.80 min); F2 (19.35 min); F3 (34.08 min); and F4 (42.85 min). Figure 1b illustrates the chromatographic profile. The fractions were then pooled to evaluate calcium binding activity, as shown in Fig. 1c. The greatest calcium-binding activity was exhibited by the F4 fraction. While this F4 fraction resulting from the RP-HPLC process had a lower activity level than the 3–0.65 kDa fraction resulting from the ultrafiltration stage, the activity of the 3–0.65 kDa fraction resulted from the activity of its combined peptides within the fraction. The peptide within the F4 fraction was then identified. It could be suggested that the calcium binding activity of the 3–0.65 kDa fraction occurred from the combined activity of peptides in its fraction. After finishing the RP-HPLC process, some active peptides may be eliminated from this fraction and the calcium binding activity was reduced. The peptide within the F4 fraction was then identified.
Calcium-binding peptide identification
Identification of the peptide in the F4 fraction after RP-HPLC was performed via mass spectrometry with de novo sequencing applied for data analysis. The findings indicate the presence of a pair of peptides: AFNRAKSKALNEN; Basil-1, (Fig. 2a) and YDSSGGPTPWLSPY; Basil-2 (Fig. 2b). Confirmation of the amino acid sequences these peptides was conducted by matching with Ocimum’s sequence using the National Center for Biotechnology Information (NCBI) database. Basil-1 exhibited a 100% (4/4) match to an amino acid sequence in the NADH dehydrogenase subunit F of O. americanum (SwissProt accession number AFP73817) as well as MAP-kinase of O. bacillicum (SwissProt accession number AMR58300.1). Meanwhile, the similarity exhibited by the Basil-2 sequence also represented a 100% (4/4) match with the amino acid sequence of NADH dehydrogenase subunit F, but there was a slight difference with the SwissProt accession number AFP73817.1. Furthermore, certain parts of the amino acid in Basil-2 show 100% (4/4) similarity to an amino acid sequence found in the cinnamyl alcohol dehydrogenase sequence of O. tenuiflorum (SwissProt accession number ADO16245.1).
The MS/MS spectrum analysis and details of the amino acid sequence of the F4 fraction calcium-binding peptide obtained from the RP-HPLC process: (a) Basil-1, and (b) Basil-2.
Table 1 presents the characteristics of the two DLBS peptides, including water solubility which was obtained via the Innovagen server. The water solubility of Basil-1 was good but for Basil-2 it was poor. This finding matches the results from an in vitro study where both peptides were shown to be soluble when immersed in water, although Basil-2 dissolved much more slowly. Toxicity was analyzed via the ToxinPred server, with non-toxicity reported. Finally, sensory qualities were assessed via the BIOPEP-UWM server. In the case of Basil-1, the reported flavors from the amino acids in the sequence were listed as umami, sweet, bitter, sour, astringent, bitterness suppressing, and salt enhancing. Meanwhile, Basil-2 can be described as umami, sweet, bitter, sour, and salty. The identified peptides were then synthesized and tested to determine the calcium-binding activity; for Basil-1, the calcium-binding activity was 38.62 ± 1.33%, while for Basil-2 the activity slightly increased in comparison at 42.19 ± 2.27%, although the difference was not determined to be of significance (P > 0.05). It is commonly reported that the activity of short peptides exceeds that of long peptides, but this would imply that Basil-1which has a lower amino acid content than Basil-2 would show a higher level of activity, which was not the case. This result might be explained as a consequence of other effects such as the type or position of the amino acids within the sequence, or the influence of the net charge of the peptide30,31.
Earlier research demonstrates that amino acid residues, and in particular their sequences and composition, can have a critical effect on the peptide in terms of its calcium-binding abilities15,32. It has been suggested that chelation with calcium ions may arise for amino, carboxyl, and phosphoric groups12,33. Additionally, the hydrophobic amino acid residue in the sequence was investigated for its interaction with calcium ions. It was noted that that the presence of Leu at the C- and N- terminals and in the peptide sequence leads to superior binding of metal ions compared to Leu occurring at both the C- and N- terminals. Since Leu appears in the Basil-1 and Basil-2 sequences, this may lead to improved calcium-binding activity in those particular peptides. The R group of Leu has a carbon atom which can interact with the calcium ions to create carbon–calcium bonds16. Further investigations were conducted on acidic amino acids and their correlation with calcium binding, and it was shown that as the acidic amino acid content increases in a peptide, the calcium-binding activity also increases. This may be due to the carboxyl groups in the acidic amino acids which provide both an oxygen atom and a suitable binding site34,35,36,37. In general, the interactions of metal ions and ligands occur in line with acid–base theory. The dissociation of a hydrogen ion from the ligand acid groups leads to the metal ion and the ligand combining. A significant number of peptides with metal-chelating properties have been isolated from hydrolysates, a majority of which are rich in Asp and Glu. Potential binding sites which may have a role in the coordination of chelation complexes include the COO- groups which are negatively charged as well as the NH2 or NH groups of amido bonds10. Asp and Glu amino acid residues can also be found in calcium-binding peptides which are obtained from the protein hydrolysates of cheese whey34, hoki (Johnius belengerii) bone35, and porcine blood plasma37. In this research, Basil-1 and Basil-2 contained Asp and Glu acidic amino acids. In addition, Ser, Arg, and Lys appeared in the peptide sequence and could have been involved in the calcium chelation due to the combination of calcium ions with various C = O bonds, C–OH bonds, NH2 bonds, and C = NH bonds. Many recent studies have sought to identify and isolate the calcium-binding peptides as well as to examine the particular amino acids and groups which are involved in the process of calcium binding.
Fluorescence assay
It was possible for the aromatic groups within the amino acid structure to produce endogenous fluorescence at suitable excitation wavelengths due to the presence of Phe, Tyr, and Typ. For this reason, structural transformations which arise as the peptide undergoes calcium binding could be detected via examination of the fluorescence spectrum38. The maximum absorption peak was reported at 315 nm, although the binding of calcium ions with the Basil-1 structure then resulted in increased intensity as the peak made a shift to 316 nm, as shown in Fig. 3a. This outcome concurs with earlier accounts of the reaction taking place between β-lactoglobulin hydrolysate and iron ions, whereby the maximum peak shifted to a lower absorption peak during chelation, with a change in intensity39. The maximum absorption peak for Basil-2 occurred at 358 nm. Following chelation with calcium ions, this value shifted to 356 nm while the intensity also declined, as can be seen in Fig. 3b. Knappskog et al.40 and Miranda et al.41 reported that during the metal-binding reaction, three aromatic amino acids moved to the peptide structure surface, which caused the alteration of the maximum absorption peak. Uppal et al.42 argued that fluorescence quenching might be a consequence of a heightened concentration of metal ions, as indicated by a decline in the intensity. Zhou et al.39 produced similar results, observing that fluorescence quenching occurred as the calcium ions entered into combination with the calcium-chelating peptide. Furthermore, Wu et al.43 demonstrated that the reduction in the intensity of fluorescence commonly resulted from peptide folding as ferrous ions underwent chelation with peptides from sturgeon proteins, since during the folding process, those ferrous ions closed to tryptophan residues. The findings might suggest that the binding process between these peptides and the calcium ions could cause a blue shift at the maximum fluorescence peak, and is indicative of a clear difference between the original peptide and the new calcium-peptide complex.
The fluorescence spectra of: (a) Basil-1 and the Basil-1-calcium complex; (b) Basil-2 and the Basil-2-calcium complex.
FTIR measurement
FTIR allows the detection of peptide changes after calcium binding occurs. FTIR works by identifying the functional groups which occur within the sequence. Metal ions will usually form bonds with carbonyl or carboxyl groups or the amide group of amino acid residues found within the peptide segment. Accordingly, there is a shift in the calcium-peptide complex absorption peaks when compared to those of the initial peptide39. Figure 4a presents the different absorption peaks of the Basil-1 and Basil-1-Ca complexes. Following chelation of the Basil-1 sequence with calcium ions, there was a shift in the absorption peak for Basil-1 from 3274.05 cm-1 to 3262.48 cm-1, while three of the peaks completely disappeared. The wavenumbers represent the amide A band (3500–2800 cm-1), since it is possible for the nitrogen atom in the N–H bonds to coordinate with the calcium ions as an electron pair is offered44,45. The amide-I and amide-II vibrations were of particular significance; C = O bond stretching leads to the amide-I vibration (1700–1600 cm-1), while C–N bond stretching and N–H bond deformation lead to the amide-II vibration (1600–1500 cm-1)46,47. Moreover, there was shift in the amide-I band of Basil-1 from 1624.34 to 1626.85 cm-1, and in the amide-II band of Basil-1 from 1534.34 to 1543.40 cm-1 after chelation with calcium ions. The amide-I and amide-II bands of Basil-1 decreased sharply in absorption intensity once the complex had been formed with the calcium ions. A pair of small peaks arising in the range of 1500 cm-1–1400 cm-1 appeared after chelation with calcium ions, potentially due to the oxygen atom in the carboxylate group which then encounters the calcium ions to form COO–Ca. The final wavenumber which is seen from 800 to 500 cm-1 may result from the O–Ca vibrational mode arising when metal ions and ligand atoms (nitrogen, oxygen, sulfur) combine33,36 which can be observed as the wavenumber alters from 517.55 to 504.14 cm-1.
The FTIR spectra of: (a) Basil-1 and the Basil-1-calcium complex; (b) Basil-2 and the Basil-2-calcium complex.
Figure 4b presents the contrasting absorption peaks for Basil-2 and the Basil-2-Ca complex. For amide-I, there were four peaks in the band wavenumber of the original peptide, but the first of these absorption peaks at 3273.10 cm-1 shifted to a lower frequency region around 3252.77 cm-1 and exhibited some different intensities, whereas the second peak at 3079.11 cm-1 shifted to a much greater absorption wavenumber of 3124.89 cm-1, while the other peaks disappeared as a consequence of the interaction between the calcium ions and the nitrogen atom from the N–H bonds. In the amide-I band, there was a shift in the absorption peak from 1632.51 to 1644.25 cm-1. However, as a consequence of the coordinate binding involving calcium ions and the C–N and N–H bonds of the amide-II vibration, the absorption peak at 1514.35 cm-1 disappeared, suggesting the creation of C–Ca and N–Ca within the amide-II wavenumber. Furthermore, there is an influence upon the absorption peak of the original peptide at 1441.40 cm-1 exerted by the –COO–Ca combination, which induces a shift to 1456.70 cm-1 and a change in the intensity. In addition, there were two other Basil-2 absorption peaks at 745.66 cm-1 and 513.42 cm-1 which underwent shifts to 761.63 cm-1 and 503.49 cm-1 as the ligand atom interacted with the calcium ion. It can thus be argued that there was interaction between the calcium ions and the C = O bonds, –COO– bonds, N–H bonds, and C–N bonds as the peptide calcium complex was formed. The chelation produces a different absorption peak for the peptide-calcium complex when compared to the original peptide, and thus the new peaks confirm the existence of a new substance. This matches the findings for FTIR spectra of metal ions chelated with original peptides, and was confirmed in the works of Zhang et al.33 and Chen et al.48, while Wang et al.17 observed that when metal ions and peptide functional groups interact, the outcome can be a wider or weaker absorption peak, or in some cases it disappears.
Presently, a computer simulation method was used to explore the binding position and the interaction between ligand and receptor. Numerous studies have argued that the binding of calcium and peptides depends upon the particular groups that are found in the peptide sequence. The groups that most frequently appear in the literature in the context of calcium binding are phosphate and carboxyl groups49,50. In the human diet, milk casein from cows and egg yolk casein contain phosphorylated groups, including CPPs and phosvitin phosphopeptides (PPPs), which are able to aggregate calcium via highly polar acidic motifs. These phosphorylated groups are ideal for calcium binding because of their phosphoserine residues, which produce amorphous Ca3(PO4)2 nanoparticles when exposed to calcium51. Various peptides which do not feature such phosphorylated residues are also known to bind calcium through the presence of the carboxyl group, Glu, and Asp. These include peptides derived from whey proteins52, bovine serum proteins53, soybean proteins54, and tilapia scale proteins26. The reason for the success of carboxylic groups in creating these complexes lies in the presence of carbonyl oxygen, which offers a non-bonding free electron pair which then performs the chelation of metal ions that have empty electron shells. In addition, sulfhydryl groups, the nitrogen-rich group of the His imidazole, hydrophilic groups, and negatively charged groups play significant roles in calcium-binding55. This part might be supported by the result of the FTIR assay because the prediction of calcium binding sites was obtained at carbonyl group, carboxyl group, C–H bond, C–C bond, and N–H bond which are correlated with the change in FTIR spectra when the peptide is formed into the calcium-peptide complex. Otherwise, some metal ions have been reported to interact with these function groups, which has a major role in metal ion chelation56,57.
Calcium uptake and cell viability of Caco-2 cells
Human Caco-2 cells are adenocarcinoma cells found in the intestines and offer biochemical and morphological properties similar to enterocytes. As such, they have been shown to play a key role in studies of calcium absorption along with other minerals when examined in vitro28,58. Accordingly, the researchers sought to investigate the influence of Basil-1 and Basil-2 on the calcium uptake of Caco-2 cell monolayers. Initially, the MTT assay was performed to determine the potential range of concentrations for the samples. Figure 5a-c shows cell viability over 90% following treatment with original peptides, peptide-calcium complexes, and calcium chlorine solution at various concentrations.
Based on the MTT results, the concentration of Basil-1-Ca complex and Basil-2-Ca complex at 250 µg/mL exhibited 0.2 µg/mL and 0.24 µg/mL calcium concentration, respectively. These calcium concentrations in the complexes were the maximum concentrations that did not cause toxicity and were selected to determine calcium absorption. Caco-2 cells were cultured with varying quantities of calcium ions from the peptide-calcium complex, while free calcium from CaCl2 was used as a comparison. Figure 6 shows the effect of free calcium ions and calcium ions from the complex in the Caco-2 cell monolayer model. The calcium ions in the Basil-1-Ca complex were shown to have a raised absorption when the calcium concentration in the complex was increased until it reached 0.2 µg/mL, which was the maximum absorption (Fig. 6a). At this concentration, Caco-2 cells absorb 77% of the calcium ion when compared to the control (cells without calcium ion). Moreover, the uptake-enhanced effects of the Basil-1-Ca complex were over three times greater than those of CaCl2 when the calcium concentration was maintained fixed at 0.2 µg/mL. In contrast to the results obtained with free calcium ions from calcium chloride solution, there was no significant difference in the results obtained with calcium uptake when the concentrations of free calcium ions from calcium chloride solution were varied (P < 0.05). Although the range of calcium chloride solutions in the calcium absorption study did not inhibit the growth of Caco-2 cells, all Caco-2 cells were active cells that had a maximum calcium absorption rate, so the increase of free calcium ions did not promote calcium absorption.
(a) Effects of Basil-1-calcium complex (
), CaCl2 (
) solution and (b) Basil-2-calcium complex (
) on the uptake of calcium in Caco-2 cell monolayers at different CaCl2 concentrations. The values are shown as percentage of calcium-binding activity ± SD, while all testing was carried out in triplicate. Where the mean is shown with a different letter, this indicates a significant difference in Duncan’s multiple range test (p < 0.05).
When considering calcium ions transported from the Basil-2-Ca complex, absorption increased slightly as the calcium concentration rose from 0.03 to 0.24 µg/mL, leading to a maximum absorption of 66.78% at that final concentration of 0.24 µg/mL. In contrast, the maximum free calcium ion absorption from calcium chloride solution occurred at 0.03 µg/mL, differing significantly from the absorption at 1.95 µg/mL of calcium but with no significant difference from other concentrations (P < 0.05), as shown in Fig. 6b. This result indicates that the calcium uptake efficiency of the Basil-2-Ca complex was also remarkably higher than free calcium. In the past, research has been published on the promotion of calcium uptake from calcium-peptide complexes, such as the fish scales protein hydrolysate-calcium complex (FSPH-ca)59, desalted duck egg white peptide-calcium (DPs-calcium)60, Ca-casein phosphopeptides (Ca-CPPs)14, and bovine serum protein hydrolysate-calcium complex (BSP-ca)53 by acting as calcium carriers and affecting the calcium receptor on the membrane of intestinal cells to promote calcium uptake.
Calcium ions are normally transported into cells via both passive and active transportation. Earlier studies have found that the chelation of calcium ions with peptides or protein hydrolysates may support the transportation of calcium since the peptide is able to carry the calcium and overcome the obstacle of the plasma membrane, thus allowing the calcium ions to reach the Caco-2 cells13,28,61. It may be possible for the peptide-calcium complex to prevent the precipitation of calcium in the intestines, thus boosting calcium ion concentrations in the extracellular cytoplasm. This can effectively diffuse calcium or allow intracellular passive transportation. Earlier research indicates that an increase in the uptake of calcium correlates if Glu and Asp are present within the sequence62. Moreover, the basic amino acids (Lys, His, and Arg) are positively charged and can therefore influence calcium transportation as they interact with the cell membrane and its negative charge63. For this reason, it is possible that the presence Arg, Glu, and Lys in the Basil-1-Ca complex and of Asp in the Basil-2-Ca complex could support the transportation of calcium into Caco-2 cells. It may further be possible to conclude that calcium absorption is promoted by Basil-1-Ca and the Basil-1-Ca complex through the prevention of calcium precipitation, which leads to increased transportation of calcium through the cells of the intestines.
