Background
Antiplasmodial activities of angiotensin II (Ang II) and its analogues have been extensively investigated [
1‐
6]. Due to its vasoconstrictor property, Ang II cannot be used as an anti-malarial drug regardless of its antiplasmodial activity (in vitro assays) in
Plasmodium gallinaceum sporozoites [
1] and
Plasmodium falciparum [
6].
It is known that protein or peptide-folding mechanism involves a complex bunch of elementary reactions and the different energies associated with positioning of the different amino acid residues near or far from each other or from solvent enable some structures to be more stable than others [
7,
8]. Likewise, protein or peptide can be made synthetically and it is a tool for different kind of studies [
8,
9].
Chamlian et al. [
2] studied some lactam bridged Ang II analogues, which presented Asp and Lys insertion in order to restrict the peptide. They observed that VC-12 (Asp-Arg-Val-Tyr-Ile-Asp-His-Lys-Pro-Phe) and VC-26 (Asp-Arg-Val-Tyr-Asp-Ile-His-Lys-Pro-Phe) analogues showed relevant antiplasmodial activity against
P. gallinaceum sporozoite, 87 and 73 %, respectively.
In order to understand the role of each amino acid and its side chain, Silva et al. [
3] and Ferreira et al. [
4] proposed different modifications in the Ang II molecule. They replaced each amino acid by Ala [
4] or deleted the residues of the native Ang II molecule [
4]. The biological activities of the analogues Asp-Arg-Val-Tyr-Ala-His-Pro-Phe and Asp-Arg-Val-Tyr-Ile-Ala-Pro-Phe on the
P. gallinaceum sporozoites were equipotent to native Ang II [
4], 75 and 79 % of activity, respectively. The most active analogues studied by Ferreira et al. presented biological activities about 50 % (analogues Asp-Arg-Val-Tyr-Ile-His-Pro and Arg-Val-Tyr-Ile-His-Pro-Phe). They synthesized three short peptides (Val-Tyr-Ile-His-Pro-Phe; Val-Tyr-Ile-His-Pro and Ile-His-Pro-Phe) to verify the importance of hydrophobic cluster studied by Tzakos [
10] and Fermandjian [
11,
12]. The peptides presented antiplasmodial activities about 80 %. These studies were important to understand the position of each amino acid side chain and intra/intermolecular interactions, which play an important role in the native sequence, and that the hydrophobic cluster have significant influence on both cases.
In this work, new linear peptides and Ang II analogues were synthesized and tested in vitro in order to find a short bioactive peptide as well as to verify the hydrophobic cluster’s influence on parasite-membrane interaction on both P. gallinaceum and P. falciparum parasite species, as previously mentioned.
Discussion
Silva et al. observed that when the Arg, Tyr, Pro, and Phe amino acids residues were replaced by Ala, the antiplasmodial activity of these peptides decay 50 % in comparison to the native Ang II [
3]. Ferreira et al. observed that when the same amino acids residues were deleted, the analogues presented similar activity [
4]. Moreover, these authors also observed that ultra-short peptides, designed based on hydrophobic C-terminal extremity of Ang II, increase the antiplasmodial activity [
4].
This leads to two questions: Are these amino acid residues (Arg, Tyr, Pro, and Phe) relevant on Ang II backbone to maintain the antiplasmodial activity? Does the Ang II hydrophobic cluster exert any influence in the peptide-parasite membrane interaction?
Ang II and several analogues tend to β-conformation at studied solvents [
4,
5,
28]. In order to answer these questions there were designed and synthesized six peptides (Table
1, peptides 1–6) in order to evaluate their structuring tendency behaviour in different solvents and their antiplasmodial response.
Firstly, the peptides were tested in vitro on P. gallinaceum sporozoite model. Based on Ang II primary sequence (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), the peptides 1 (Asp-Arg-Val-Tyr-His-Ile-Pro-Phe) and 2 (Asp-Arg-Val-Tyr-Pro-Phe) were designed to verify if the hydrophobic residues exert some influence on parasite membrane. The position of His and Ile amino acid residues in peptide 1 was inverted. This promoted an increase in the antiplasmodial activity (94 %). The same residues (His and Ile) were deleted, in peptide 2. This modification leaded to a decreased antiplasmodial activity of 64 %.
Matsoukas et al. suggested that a π-stacking interaction between His and Phe side chains, on Ang II molecule, is an important aspect of the activation mechanism in its biological activity [
29]. Due to changes in residue position, on peptide 1, both the hydrogen bond between phenol group of Tyr with imidazole group of His [
30], and van der Waals interaction between Ile and Phe residues [
12,
31] could promote the peptide-parasite interaction. It cannot occur in native molecule, because Ile side chain presents a steric hindrance towards Tyr. Besides that, His spatial organization leads to the stability of the Ang II conformation [
12].
The deletion of His and Ile residues, as peptide 2, can be responsible for the biological activity decrease. It was observed in CD spectrum (Fig.
1) that peptide 2 tends to adopt an α-helix structuring (i.e., a positive band near 205 nm and two negative bands near 208 nm and 222 nm) in SDS solvent, even for hexapeptides [
32,
33], while peptide 1 tends to adopt a β-turns structuring (i.e., a positive absorption near 195 nm and a minimum near 225 nm) also in SDS solvent. (See Additional file
2).
Arrighi et al. studied three different conformations (random coil, α-helix and β-sheet) with murine parasite and suggested that only α-helix structuring did not present efficacy with parasite membrane activity [
34]. This information is in accordance with peptide 2, which leads to decreased biological activity. Based on the previous result and considering that the hydrophobic residues are responsible for peptide-parasite membrane interaction, the Ang II derivative (peptide 3, Arg-Tyr-Pro-Phe) was designed to verify if Arg, Tyr, Pro, and Phe residues are relevant for this interaction [
4]. The peptide 4 (Arg-Tyr-His-Ile-Pro-Phe) was designed to verify if His and Ile amino acid residues addition at primary sequence of peptide 3 enhance the biological activity (Figs.
1,
2).
The results showed that, in this case, the activity was equipotent (77 and 76 % antiplasmodial activity, respectively). The effect of the cluster formed by Ile and His inverted residues was not significant in biological activity of the peptide 4, because the interactions between Arg and Tyr residues are predominant when compared to the interactions between Tyr and His residues [
35].
The Arg and Tyr residues are not adjacent in peptide 1. It probably leads to the influence of the cation-π interactions between Tyr and Arg residue side chains contained in the molecule providing greater stability [
36]. This hypothesis can explain the higher activity of peptide 1 than peptide 4 and because peptides 3 and 4 present lower biological activities. CD spectra for both peptides (3 and 4) tend to β-turns conformation.
Silva et al. also suggested that the aromatic or hydrophobic residues can promote peptide-parasite interactions [
4]. Thus, peptides 5 (Tyr-His-Pro-Phe) and 6 (Val-Ile-Pro-Phe) were designed based on this data. The bioactivities observed for both peptides were 89 and 94 %, respectively. It leads to the deduction that short-peptides were able to maintain the antiplasmodial activity of this class of molecules.
Cruzeiro-Silva et al., by NMR studies, analysed possible interactions of the PW2, an antimicrobial synthetic peptide (His-Pro-Leu-Lys-Gln-Tyr-Trp–Trp-Arg-Pro-Ser-Ile), simulating several lipophilic environments [
37]. In those studies, they suggested the aromatic region comprises the primary sequence Trp–Trp-Arg (a slice of PW2 sequence) that is responsible by anchoring the peptide in the membrane interface and these same regions display some degree of conformational order in solution [
37]. This information was relevant to elucidate the decrease of peptide 5 antiplasmodial activity and/or its interaction with the parasite membrane.
Perez-Picaso et al. studied the hydrophobicity of the amino acid residues and associated it with anti-malarial activity [
38]. They suggested that among the residues on peptides that tend to act as anti-malarials are: Val, Ile, Pro, and Phe, the same applied to design peptide 6, besides other bulky or hydrophobic residues such as Trp and Arg [
38]. It is in accordance with the results obtained in this work. Peptides 5 and 6 tend to adopt β-turn structuring. Peptides that tend to adopt β-turn structures present higher interaction with
P. gallinaceum sporozoites membrane (>75 %) [
4,
5]. The same characteristic was observed here.
In order to find a potent antiplasmodial with 100 % of efficacy, the peptide 6 was modified. Each amino acid residues was replaced by 2-Nal. This was chosen because it has the hydrophobic bulky side chains. The 2-Nal scan library of peptide 6 derivatives was realized, obtaining peptides 7 to 10, Table
1. CD studies were performed (Fig.
1) and their activities against mature
P. gallinaceum sporozoites (Fig.
2). Meyer et al. studied the substitution of aromatic residues as Pro and Trp by 2-Nal and 1-Nal residues in a β-hairpin peptide [
39]. They observed that the molecular geometry was maintained intact when Phe was replaced by 2-Nal residue [
39].
It was observed that the substitution in the C-terminal extremity caused a decrease on peptide-parasite interaction (peptides 9 and 10, 70 and 73 %, respectively). The increase of the biological activity was observed by Chamlian et al. [
2] in Ang II analogues, when C-terminal extremity is modified with two amino acids insertion (Asp and Lys). Despite the antiplasmodial activity decay of peptides 7 and 8 when compared to peptide 6, CD spectra suggest that in general these two peptides adopt similar conformational tendency in all solvent systems. A positive band in ~200 and ~230 nm and a negative band in ~218 nm indicates that peptides 7 and 8 tend to adopt β-turn structuring [
40]. Besides that, a sharp positive band at ~230 nm indicates the presence of naphthyl group, as reported by Ueno et al. [
41].
The peptides tend to present antiplasmodial activity when they adopt similar structuring in all solvents studied [
4]. It was observed in peptide 1 in comparison to Ang II or peptide 5 when compared to peptide 6. The CD spectra suggested that β-turn conformations are observed for the most bioactive peptides in aqueous organic means with the presence of SDS. The structuring tendency presented by this class of peptides introduced here is in accordance with the results presented here.
Saraiva et al. described studies that aimed to identify the molecular mechanisms induced by Ang II, which are involved in the modulation of
P. falciparum erythrocytic cycle. The results showed that Ang II had some influence in the parasite ring forms reduction when tested in vitro (47 % at 10
−8 mol L
−1) [
6]. Torres et al. studied the influence of restrict peptides in
P. falciparum red blood cells. Three analogues reduced more than 30 % of the parasitaemia and one of them (
Cys-Arg-Asp-Cys-Val-Tyr-Ile-His-Pro-Phe) was active in two different parasite species (
P. falciparum and
P. gallinaceum) [
5]. In order to verify if some peptides have influence in the same stage, all of them were tested in vitro on red blood cells infected with
P. falciparum species.
Four peptides presented activity in the parasite ring forms reduction between 27 and 53 % (Fig.
3) and this contribution was noticed only on ultra short-peptides containing Val, Ile, Pro, Phe, Tyr, Arg, and/or His residues in their primary sequences. The results presented in this work are also in accordance with Perez-Picaso’s observations [
38].
It was observed that two ultra short-peptides had influence at parasitaemia in both models studied (peptides 5 and 6). IC
50 values of these two peptides were determined by testing seven concentrations resulting in 7–65 % of inhibition (Fig.
4) with 8.9 × 10
−10 mol L
−1 (peptide 5) and 6.2 × 10
−10 mol L
−1 (peptide 6). Saraiva et al. observed that the Ang II concentration to reduce parasite invasion in a dose-dependent manner with the maximum effect was 10
−8 mol L
−1 [
6].
In this work, it was observed that the concentration that has the maximum effect was 10
−4 mol L
−1 for both peptides (5 and 6) with 57 and 65 % of parasitaemia reduction, respectively. To verify if peptides 5 and 6 have the same effect of the Ang II, they were tested at 10
−8 mol L
−1 presenting parasitaemia reductions of 51 and 53 %, respectively (Fig.
4). The results showed that peptides 5 and 6 are equipotent to Ang II [
6].
Haemolysis of erythrocytes and cell toxicity of mammalian somatic cells are often thought to be the major parameters of peptide toxicity toward eukaryotic cells [
42]. Haemolytic activity is positively correlated with the peptides hydrophobicity [
43]. Likewise, it was verified by erythrocytic haemolysis assays that these short peptides did not affect the integrity of the erythrocytes. They presented haemolytic activity under 7 %. It is not statistically significant when compared to control (Fig.
5). Contractile response assays with the most active peptides showed that they did not exhibit significant contractile activity (Fig.
6).