Effect of nitroxyalkyl derivatives of fullerenylproline on the activity of CA³⁺-ATPase of sarcoplasmic reticulum

The effect of nitroxyalkyl derivatives of fullerenylproline methyl ester on the enzymatic activity of Ca²⁺-ATPase of sarcoplasmic reticulum (SR) has been studied. It is shown that hybrid derivatives of C₆₀ fullerene are capable of inhibiting the activity of Ca²⁺-ATPase of SR. The mononitrate inhibits the hydrolytic activity of the enzyme with K _i = 1.92 × 10⁻⁶ M; active Ca²⁺ transport, with K _i = 3.79 × 10⁻⁶ M. The dinitrate inhibits ATP hydrolysis with K _i = 2.38 × 10⁻⁸ M; Ca²⁺ transport, with K _i  = 3.08 × 10⁻⁸ M. Fullerenylproline methyl ester does not affect the enzymatic activity of Ca²⁺-ATPase. Based on these data it is possible to predict the possible fields of application for hybrid fullerene derivatives as potential drugs.

Nanotechnological inventions and functional implementation of nanocarbon materials has led to the creation of novel pharmacologically active compounds based on C₆₀ fullerene. Amphiphilic derivatives of C₆₀ are highly membrane active as a result of the unique nanocarbon spheroid incorporated into them. This determines their pharmacokinetic properties, primarily their permeability through the lipid bilayer of biological membranes and also the altered activity of membrane-bound enzymes. The goal of our work was to study modulation of the activity of Ca²⁺-ATPase of sarcoplasmic reticulum (SR) as affected by recently synthesized hybrid fullerene derivatives.

The molecular mechanisms of drug resistance [1] and the antimetastatic activity of many cytostatics [2–5] are known to involve Ca²⁺-ATPase.

Experimental part

We used human albumin, imidazole, ouabain, cGMP, nucleotidase (cobra venom), ATP (Sigma), histidine, DMSO, EDTA, trichloroacetic acid (TCA), saccharose, MgCl₂, NaCl, KCl, CaCl₂, Na oxalate, and ammonium molybdate (Reakhim, Russia) that were purified before use.

The fullerene derivatives were synthesized stepwise by equimolar nucleophilic addition of proline to fullerene to form fullerenylproline (I) and subsequent electrophilic substitution of the hydrofullerenyl proton by nitroxyalkyl halides [6].

The enzyme Ca²⁺-ATPase of SR was isolated from hind paw muscle of white rabbits [7]. Muscles were placed in ice-cold physiological solution (0.5 L solution per 100 g muscle) with EDTA (10 mM) at pH 7.5. Muscles were ground, placed in medium containing histidine (10 mM), EDTA (0.1 mM), and saccharose (10%) at pH 7.0, and homogenized in a Potter homogenizer. The resulting homogenate was centrifuged at 10,000 rpm for 20 min. The supernatant liquid was filtered through burlap (six layers). The filtrate was centrifuged at 36,000 rpm for 60 min. The precipitate was suspended in medium containing KCl (0.6 M) and histidine (10 mM) at pH 7.0 – 7.2, treated with human albumin (100 mg), incubated for 8 h at 4 – 8°C with stirring, and centrifuged at 40,000 rpm for 90 min. The middle gelatinous layer was collected from the centrifuge tubes and suspended in medium containing histidine (10 mM), EDTA (0.1 mM), and saccharose (30%) at pH 7.0, The enzyme preparation obtained in this manner was frozen in liquid N₂ and used in the work.

The enzyme activity was determined by the literature method [6]. The reaction medium contained MgCl₂ (4 mM), imidazole (2.5 mM), NaCl (100 mM), Na oxalate (5 mM), protein (0.04 mg), and ATP (3 mM) at pH 7.2. The reaction was induced by adding CaCl₂ (0.1 mM). The heterolytic activity of Ca²⁺-ATPase was calculated from the slope of the initial portion of the kinetic curve for ATP hydrolysis. The specific hydrolysis rate of Ca²⁺-ATPase was 15,000 nM Pi/mg protein/min. The rate of change of [Ca²⁺] was estimated from Ca²⁺ absorption (from dilute 0.1 mM CaCl₂) by SR vesicles during the ATP hydrolysis.

Inhibition of the enzyme hydrolytic activity was calculated using the formula:

$$ I = {{{100\left( {{A_0} - A} \right)}} \left/ {{{A_0}}} \right.}, $$

where I is the inhibition index in percent; A ₀, the specific content of inorganic phosphate in the control; and A, the specific content of inorganic phosphate in the test sample.

The protein concentration was determined by a modified Lowry method.

The kinetics of Ca²⁺-ATPase of SR inhibition were studied using the rate of the enzymatic reaction as a function of substrate (ATP) concentration in the presence and absence of the mono- and dinitrates at concentrations of 5 × 10⁻⁶ M and 4 × 10⁻⁸ M, respectively.

The reversibility of the action of the studied compounds was determined by dialysis of an aqueous solution of Ca²⁺-ATPase of SR containing II or III (1 μM). The dialysis was carried out against a 100-fold excess of incubation medium without the complexes for 24 h at 4 – 5°C.

Results and discussion

Table 1 shows that fullerenyl mononitrate II and fullerenyl dinitrate III without starting fullerenylproline I had pronounced inhibitory effects on the functioning of Ca²⁺-ATPase of SR. Thus, III at a concentration of 1 μM almost completely (97%) inhibited active Ca²⁺ transport and ATP hydrolysis (87%); at a concentration of 0.01 μM, by 60 and 50%, respectively. Compound II at a concentration of 1 μM inhibited the hydrolytic and transport functions of the enzyme by 56 and 44%, respectively.

Table 1 Effect of Fullerene Derivatives on Activity of Ca²⁺-ATPase of Rabbit Muscle Sarcoplasmic Reticulum

An important characteristic of the inhibition mechanism of the studied compounds is the reversibility of their effect on Ca²⁺-ATPase activity.

Table 2 indicates that the enzyme transport function after dialysis as affected by II and III was partially restored. This indicated that these compounds were partially reversible inhibitors of Ca²⁺-ATPase functioning. This was consistent with their non-covalent binding to the enzyme active site.

Table 2 Effect of Fullerene Derivatives on Hydrolytic and Transport Functions of Ca²⁺-ATPase of SR Before and After Dialysis

A kinetic method for studying enzymatic reactions that could suggest the nature of the enzyme–inhibitor binding gave a more complete description of the Ca²⁺-ATPase inhibition mechanism. The effect of the inhibitor on the enzyme activity is determined from the inverse rate of the enzymatic reaction (1/V) as a function of the inverse substrate concentration (1/S) in the presence of the inhibitor.

The numerical values of the maximum ATP hydrolysis rate and active Ca²⁺ transmembrane transport rate were used to calculate the corresponding inhibition constants (K _i) as affected by II and III. The calculations used the slopes in Lineweaver–Burke coordinates (Figs. 1 and 2), which were (1 + [I]/K _i)-times greater with inhibition than without inhibitor [8]. For ATP hydrolysis, K _i = 1.92 × 10⁻⁶ M; for active Ca²⁺ transport, K _i = 3.79 × 10⁻⁶ M.

Fig. 1

Change of active Ca²⁺ transport rate as a function of its concentration as affected by II and III in Lineweaver–Burke coordinates: control (1), in the presence of fullerenyl mononitrate, 5 × 10⁻⁶ M (2), in the presence of fullerenyl dinitrate, 4 × 10⁻⁸ M (3).

Fig. 2

Change of ATP hydrolysis rate as a function of substrate concentration as affected by II and III in Lineweaver–Burke coordinates: control (1), in the presence of fullerenyl mononitrate, 5 × 10⁻⁶ M(2), in the presence of fullerenyl dinitrate, 4 × 10⁻⁸ M(3).

Figures 1 and 2 show that II was a non-competitive inhibitor of the hydrolytic and transport functions.

Fullerenyl dinitrate III differed from II by K _i values that were two orders of magnitude smaller although it was a close analog of II and, like it, inhibited non-competitively both enzyme functions. The values were K _i = 3.08 × 10⁻⁸ M for ATP hydrolysis; 2.38 × 10⁻⁸ M, for transmembrane Ca²⁺ transport. This indicated that the enzyme hydrolytic function had increased sensitivity to the action of III and may have indicated that the Ca²⁺-ATPase functions were partially decoupled. Therefore, attention should be paid to the noticeable reduction of the [Ca²⁺]/[ATP] ratio compared with the control, which is theoretically equal to 2. This ratio without the studied nitrates (control) was acceptable for [Ca²⁺]/[ATP] ? 1.8, which corresponds with Table 3. This parameter was markedly decreased by fullerenyl mononitrate II whereas fullerenylproline (I) had no effect on the enzyme activity. The effect was practically constant for II at concentrations in the range 10⁻⁶ – 10⁻⁸ M. In contrast with this, the [Ca²⁺]/[ATP] ratio decreased smoothly as the concentration of III increased from 10⁻⁸ to 10⁻⁶ M.

Table 3 Effect of Fullerene Derivatives on [Ca²⁺]/[ATP] Ratio

Thus, II and III, although not bonded to the active site of Ca²⁺-ATPase, could induce certain structural and functional changes in the enzyme that affected the hydrolytic and transport functions of Ca²⁺-ATPase.

The results on the induced change of Ca²⁺-ATPase activity that was related to a change in the ratio of extra- and intracellular Ca²⁺ suggested that the studied C₆₀ fullerene derivatives may exhibit antimetastatic properties.