| Popis: |
Human Na(+)-K(+)-ATPase alpha(1)beta(1), alpha(2)beta(1), and alpha(3)beta(1) heterodimers were expressed individually in yeast, and ouabain binding and ATP hydrolysis were measured in membrane fractions. The ouabain equilibrium dissociation constant was 13-17 nM for alpha(1)beta(1) and alpha(3)beta(1) at 37 degrees C and 32 nM for alpha(2)beta(1), indicating that the human alpha-subunit isoforms have a similar high affinity for cardiac glycosides. K(0.5) values for antagonism of ouabain binding by K(+) were ranked in order as follows: alpha(2) (6.3 +/- 2.4 mM)alpha(3) (1.6 +/- 0.5 mM) approximately alpha(1) (0.9 +/- 0.6 mM), and K(0.5) values for Na(+) antagonism of ouabain binding to all heterodimers were 9.5-13.8 mM. The molecular turnover for ATP hydrolysis by alpha(1)beta(1) (6,652 min(-1)) was about twice as high as that by alpha(3)beta(1) (3,145 min(-1)). These properties of the human heterodimers expressed in yeast are in good agreement with properties of the human Na(+)-K(+)-ATPase expressed in Xenopus oocytes (G Crambert, U Hasler, AT Beggah, C Yu, NN Modyanov, J-D Horisberger, L Lelievie, and K Geering. J Biol Chem 275: 1976-1986, 2000). In contrast to Na(+) pumps expressed in Xenopus oocytes, the alpha(2)beta(1) complex in yeast membranes was significantly less stable than alpha(1)beta(1) or alpha(3)beta(1), resulting in a lower functional expression level. The alpha(2)beta(1) complex was also more easily denatured by SDS than was the alpha(1)beta(1) or the alpha(3)beta(1) complex. |
