Turbinaria conoides, (J. Agardh) Kutzing (J. Agardh) Kutzing

Chakraborty, Kajal & Dhara, Shubhajit, 2021, Conoidecyclics A-C from marine macroalga Turbinaria conoides: Newly described natural macrolides with prospective bioactive properties, Phytochemistry (112909) 191, pp. 1-14 : 6-8

2.4. Bioactive potential of the conoidecyclics isolated from T. conoides

Conoidecyclic A exhibited dual attenuation property against inducible inflammatory enzymes COX-2 and 5-LOX (IC 50 1.75 and 4.24 mM, respectively). The anti-inflammatory activities of conoidecyclic A were higher than those displayed by conoidecyclic B (IC 50> 1.9 mM) and conoidecyclic C (IC 50 5-LOX 5.07 mM) ( Table 2 ). The anti-inflammatory selectivity index (SI) was greater for conoidecyclic A (1.79) than those displayed by conoidecyclic B and C (1.65–1.68) as well as synthetic anti-inflammatory agents (ibuprofen, 0.44 and sodium salicylate, 0.73) ( Table 2 ). The lesser selectivity ratio of synthetic anti-inflammatory agents specified the selective inhibition towards COX-1, leading to several side effects ( Laneuville et al., 1994). Therefore, it could possibly be concluded that conoidecyclic A, with higher SI and greater specificity towards COX-2 was noteworthy towards the development of selective anti-inflammatory therapeutic lead ( Spangler, 1996). Conoidecyclic A exhibited significantly greater attenuation properties against ACE-I and PTP-1B (IC 50 1.23 and 1.39 mM, respectively) as compared to other studied conoidecyclics (IC 50> 1.80 mM) ( Table 2 ). The radical scavenging activities (IC 50DPPH 1.20 and IC 50ABTS 1.48 mM) exhibited by conoidecyclic A were greater compared to those displayed by conoidecyclic B (IC 50 1.35–1.54 mM), conoidecyclic C (IC 50 1.54–1.81 mM) and commercially available standards (IC 50 1.46–1.69 mM).

2.5. Structure-activity correlation study analysis of conoidecyclic analogues isolated from T. conoides

The steric factors of the studied compounds might play pivotal roles towards their potential bioactivities. Notably, the electronic properties of conoidecyclic B and C were higher than those of conoidecyclic A ( Table 2 ), even though the bioactivities of the latter were greater. This could be explained by the comparatively lesser steric bulkiness of conoidecyclic A (P 1092.7 cm 3, MV 447.4 cm 3) than those recorded for conoidecyclic B (P 1316.5 cm 3, MV 534.9 cm 3) and C (P 1198.4, MV 522.0 cm 3) owing to the presence of bulkier side chain in the latter. These inferences were appropriately corroborated by the efficiency of conoidecyclic A towards the conformationally favorable interaction with the active binding sites of the target enzymes. Notably, the hydrophobicity of conoidecyclic A and B (log POW 3.13–3.83) were found to reside within the permissible limit of hydrophobic-lipophilic threshold ( Lipinski, 2004), which could attribute to their prospective bioactive properties. An earlier report of literature inferred that the effective permeability in the cellular network (through inter membrane barrier) along with the radical scavenging activities of the pharmacophore agents might result in their potential bioactivities ( Ishige et al., 2001).

2.6. ADME and other physicochemical parametrs

Swiss ADME tools were used ( Daina et al., 2017) for the estimation of different physicochemical parameters, drug-likeness, solubilities and ADME behaviors of the isolated compounds (conoidecyclics A-C). Based upon the specific physicochemical parameters, the qualitative prediction was performed, and only conoidecyclic A passed the filter of Lipinski’ s rule without any violation, whereas conoidecyclic B and C had one violation (MW> 500) ( Table 3 ). Therefore, conoidecyclic A could possess greater oral bioavailability. Notably, all the three compounds could pass the filter of Veber rule without any violations ( Table 3 ) ( Daina et al., 2017). In addition to that, predicted bioavailability score for the studied compounds were comparable (0.55) with that of ibuprofen (0.55), which apparently recognized at least 10% oral bioavailability in rat and permeability towards Caco-2 cell lines ( Daina et al., 2017) ( Table 3 ). For the rapid estimation of drug-likeness, the bioavailability radar plot was adopted, and six physicochemical parameters (size, lipophilicity, polarity, flexibility, solubility and saturation) were taken into account (Fig. S34). The optimum range for each parameter was shown by a pink area. Evidently, conoidecyclic B and C displayed a deviation including larger size (MW> 500), even though no eccentricity was apparent for conoidecyclic A, and all the six values (comparing to ibuprofen, conoidecyclic A exhibited lesser flexibility) led to optimal physicochemical attributes leading to an acceptable oral bioavailability. Solubility is considered to be one of the vital parameters for drug development activities, and also related to absorption ( Daina et al., 2017). Estimated aqueous solubility (Log S; ESOL and SILICOS-IT) for three isolated compounds based on molecular structure were in a range of moderately soluble to soluble ( Table 3 ). Logarithm value of skin permeability coefficient Kp (regarding pharmacokinetics) was calculated for the isolated compounds including the standard (ibuprofen), whereas more negative value of Kp inferred lesser skin permeability. The latter could be linearly correlated with the molecular size, lipophilicity, and it was observed that conoidecyclic A exhibited a Kp value (- 6.51 cm /s) closer to that exhibited by conoidecyclic B and C (- 6.56 and - 7.25 cm /s, respectively) ( Table 3 ).

2.7. Kinetic properties of ACE-I, PTP-1B and 5-LOX inhibition

Kinetic studies were performed to determine the mode of inhibition of conoidecyclics A-C, and the inhibition constants (Ki) were determined by Lineweaver-Burk and Dixon plots. Conoidecyclics were found to inhibit ACE-I, PTP-1B and 5-LOX enzymes, in a non-competitive fashion as determined by the Lineweaver-Burk plot (Fig. S35). Increase of substrate concentrations could result in non-intersect series of line on Y-axis in the Lineweaver-Burk plot (Fig. S35) but intersected on the negative X axis (Ki) in the Dixon plots ( Fig. 6 ). Conoidecyclic A exhibited lesser inhibition constant towards inhibition of ACE-I (1.1 mM), PTP-1B (1.2 mM) and 5-LOX (4.0 mM) than those displayed by other studied metabolites ( Fig. 6 ). An inverse relation of Vmax with various concentrations of conoidecyclics A-C inferred the non-competitive inhibition of the target enzymes ( Blat, 2010). Among the studied metabolites, conoidecyclic A exhibited lesser apparent Vmax (0.31–0.14, 0.29–0.17 and 0.32–0.17 ΔA min 1 for ACE-I, PTP-1B and 5-LOX inhibition, respectively) ( Table S2 ) than other studied macrolides, which implied that the former could efficiently bind with targeted enzyme to diminish the reaction velocity.

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2.8. In silico molecular modeling analysis of conoidecyclics isolated from T. conoides

The macrocyclic derivatives (conoidecyclics A-C) were subjected to in-silico molecular modeling studies against pro-inflammatory 5-LOX and COX-2 enzymes, and the results were obtained with the help of RMSD data. Conoidecyclic A, on molecular modeling with COX-2 and ACE-I exhibited three hydrogen bonding interactions with the enzyme active site, whereas five hydrogen bonds were apparent between the ligand and the active site amino acyl residues of PTP-1B ( Fig. 3 , Table 4 ). In comparison, conoidecyclic B and C exhibited lesser number of hydrogen bonding interactions with the active site of targeted enzymes ( Figs. 4–5 , Table 4 ). Likewise, conoidecyclic A recorded least binding energy (13.34, 14.51, 13.87 and 11.27 kcal mol 1 with 5-LOX, COX-2, PTP-1B and ACE-I respectively) and docking score (~ 12 to 15 kcal mol 1) than those displayed by conoidecyclics B and C ( Table 4 ). Likewise, the constant of enzyme inhibition, Ki upon interaction with COX-2 and 5-LOX were lesser for conoidecyclic A (23.20 and 33.23 pM, respectively) followed by those of conoidecyclic B and C ( Table 4 ). The lowest docking score as well as binding energy of conoidecyclic A described its greater attenuation potential against 5-LOX and COX-2 enzymes, which were reported to produce inflammatory prostaglandins (PGE 2, PGG 2, PGI 2, PGF 2 etc), thromboxane (TXA 2) and leukotrienes (such as LTB 4) causing the development of inflammation ( Hanna and Hafez, 2018).

Figures

Fig. 6. Kinetic studies of the pharmacologic response with regard to inhibition mode of ACE-I (A–C), PTP-1B (D–F) and 5-LOX (G–I), respectively to the studied conoidecyclics A-C. Representation of Dixon plots for conoidecyclics A-C, for the determination of the inhibition constant Ki. The Ki value was determined from the negative X-axis value at the point of the intersection of the four lines. The data were expressed as the mean reciprocal of initial velocity for triplicates (n = 3) at each substrate concentration. Different concentrations of isolated compounds were used, and the inhibitory potentials were expressed in mM.

Fig. 3. (A1-A2) Representative hydrogen binding interactions between conoidecyclic A and the amino acyl residues in the catalytic sites of COX-2; (A3-A4) Representative hydrogen binding interactions between conoidecyclic A and the amino acyl residues in the catalytic sites of 5-LOX; (A5-A6) Representative hydrogen binding interactions between conoidecyclic A and the amino acyl residues in the catalytic sites of PTP-1B; (A7-A8) Representative hydrogen binding interactions between conoidecyclic A and the amino acyl residues in the catalytic sites of ACE as obtained from in silico molecular docking analysis.

Fig. 4. (A1-A2) Representative hydrogen binding interactions between conoidecyclic B and the amino acyl residues in the catalytic sites of COX-2; (A3-A4) Representative hydrogen binding interactions between conoidecyclic B and the amino acyl residues in the catalytic sites of 5-LOX; (A5-A6) Representative hydrogen binding interactions between conoidecyclic B and the amino acyl residues in the catalytic sites of PTP-1B; (A7-A8) Representative hydrogen binding interactions between conoidecyclic B and the amino acyl residues in the catalytic sites of ACE as obtained from in silico molecular docking analysis.

Fig. 5. (A1-A2) Representative hydrogen binding interactions between conoidecyclic C and the amino acyl residues in the catalytic sites of COX-2; (A3-A4) Representative hydrogen binding interactions between conoidecyclic C and the amino acyl residues in the catalytic sites of 5-LOX; (A5-A6) Representative hydrogen binding interactions between conoidecyclic C and the amino acyl residues in the catalytic sites of PTP-1B; (A7-A8) Representative hydrogen binding interactions between conoidecyclic C and the amino acyl residues in the catalytic sites of ACE as obtained from in silico molecular docking analysis.