Heterogeneity of ouabain binding sites in Schistosoma mansoni. First evidence for the presence of two (Na+ + K+)-ATPase isoforms in platyhelminths.

Binding experiments with [3H]ouabain were performed to investigate the presence of (Na+ + K+)-ATPase (EC3.6.1.3) isoforms in adult male Schistosoma mansoni, the trematode responsible for human schistosomiasis. Non-linear regression analysis of equilibrium experiments performed with homogenates in a Mg-Pi medium indicated the presence of about 10% (Bmax = 223 +/- 67 fmol/mg protein) high-affinity sites (KD = 0.285 +/- 0.045 microM) and 90% (Bmax = 2117 +/- 348 fmol/mg protein) sites with a 20-fold lower affinity (KD = 4.9 +/- 1.28 microM). This was confirmed by their-exponential decay of [3H]ouabain dissociation. Furthermore, determination of association and dissociation rate constants indicated that the two classes of binding sites differed by their dissociation rate constants for ouabain (k-1 = 0.0185 +/- 0.0019 min-1 and 0.0997 +/- 0.0528 min-1 for high- and low-affinity sites, respectively). Surprisingly, the association rate constant measured for ouabain binding to S. mansoni homogenate (0.038 microM-1.min-1) was lower (25- to 80-fold) than the one usually observed for mammalian enzymes. This is the first direct evidence for the existence of (Na+ + K+)-ATPase isoforms in platyhelminths, invertebrates of great importance from the phylogenetic point of view.