Characterization of intestinal Na+-K+-ATPase in the gilthead seabream (Sparus aurata L.). Evidence for a tissue-specific heterogeneity

Gilthead seabream intestine contains both a Mg2+ -dependent Na+-K+-ATPase w hich is completely inhibited by 1 x 10(-3) M ouabain, and also a residue-AT Pase activity that is entirely ouabain-insensitive. The maximal activity of intestinal Na+-K+-ATPase (35.15 mu mol inorganic phosphate (P-i) mg protei n(-1) h(-1)) was observed in the microsomal fraction at 35 degrees C, pH 7. 5, 2-5 mM MgCl2, 5 mM ATP, 10 mM K+ and 200 mM Na+. The intestinal Na+-K+-A TPase of gilthead seabream exhibits similar characteristics to other teleos t Na+-K+-ATPases regarding pH dependence, Mg2+/ATP optimal ratios and ouaba in sensitivity, but exhibits unusual sensitivity to ionic strength and cati on promoted cooperative activation and higher affinity for magnesium and AT P when compared to other marine and euryhaline teleosts. The Arrhenius plot for intestinal Na+-K+-ATPase showed a break point at 15.41 degrees C, with similar activation energies above and below the discontinuity point. The a nalysis of polar lipid fatty acid composition was correlated to the break p oint in the Arrhenius plot, suggesting a regulatory role of the lipid micro environment on the enzyme activity. Finally, the kinetic characteristics, t emperature-activity relationship, fatty acid composition and substrate depe ndence of intestinal Na+-K+-ATPase are compared with literature data and di scussed on the basis of the differences between species and also between os moregulatory tissues. It is concluded that the Na+-K+-ATPase of Sparus aura ta is differently and specifically expressed between the osmoregulatory org ans. (C) 1998 Elsevier Science Inc. All rights reserved.