Two different modes of inhibition of the rat brain Na⁺, K⁺-ATPase by triterpene glycosides, psolusosides A and B from the holothurian Psolus fabricii

Effects of two triterpene glycosides, isolated from the holothurian Psolus fabricii, on rat brain Na⁺, K⁺-ATPase (Na, K-pump; EC 3.6.1.3) were investigated. Psolusosides A and B (PsA and PsB) inhibited rat brain Na⁺, K⁺-ATPase with I₅₀ values of 1 x 10^-4 M and 3 x 10^-4 M, respectively. PsA significantly stimulated [³H]ATP binding to Na⁺, K⁺-ATPase, weakly increased [³H]ouabain binding to the enzyme, and inhibited K⁺-phosphatase activity to a smaller degree than the total reaction of ATP hydrolysis. In contrast, PsB decreased [³H]ATP binding to Na⁺, K⁺-ATPase, and had no effect on [³H]ouabain binding to the enzyme. K⁺-Phosphatase activity was inhibited by PsB in parallel with Na⁺, K⁺-ATPase activity. The fluorescence intensity of tryptophanyl residues of Na⁺, K⁺-ATPase was increased by PsA and decreased by PsB in a dose-dependent manner. The excimer formation of pyrene, a hydrophobic fluorescent probe, was decreased by PsA only. The different characteristics of inhibition mode for these substances were explained by peculiarities of their chemical structures and distinctive affinity to membrane cholesterol.