The half-lives of rabbit kidney and dog brain (Na+ + K+) adenosine triphosphatase (ATPase)-ouabain complexes were 10 and 89 min respectively. The ratios between these half-lives and those previously reported for guinea-pig kidney and rat brain enzyme-ouabain are more than sufficient to account for the differences in the K0.5 values for ouabain and those enzymes reported in the literature. The K0.5 values for the ouabain-enzyme interaction could be reduced up to 12-fold by pre-incubating the enzyme with Na+, Mg2+, ATP and ouabain prior to the addition of K+ to start the enzyme assay. These differences show that the K0.5 values are not obtained under equilibrium conditions and suggest that the true equilibrium position is intermediate between the measured K0.5 values. Ki values estimated from the dissociation rates of enzyme-ouabain fall between the measured K0.5 values. It appears that the differences in the dissociation rates alone are sufficient to account for the observed differences in the K0.5 values. The experiments also show a marked difference in glycoside sensitivity between rat brain and heart (Na+ + K+) ATPase.
|Number of pages||8|
|State||Published - Jun 1 1972|
Bibliographical noteFunding Information:
Acknowledgements-Thisw ork was supportedb y grantsf rom the Michigan Heart Association, the National Institute of Mental Health (M.H. 12783-05a)n d a General ResearchS upport grant (NIH RR 05623-04t)o the cOlle8eo f Veterinary Medicine, Michigan State University.T he authors wish to thank Mr. Roxy So for excellentt echnicala ssistance.
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