Is Na + K ATPase a Myelin‐Associated Enzyme?

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Abstract
On the basis of marker enzyme studies, the Na + K ATPase activity of myelin was higher than could be accounted for by microsomal contamination. Fractions prepared from white matter-enriched areas of rat brain showed a 3-fold enrichment in Na + K ATPase activity in myelin as compared with the white matter homogenate. The ATPase activity in myelin was stimulated 4-fold by treatment with sodium deoxycholate, but the activity in the whole brain homogenate and the microsomal fraction was only doubled. The discontinuity temperature for Na + K ATPase activity was significantly higher for the myelin fraction (29.degree. C) than for the microsomal fraction (21.degree. C), but the energies of activation, both above and below the discontinuity temperature, were the same for both fractions. Myelin Na + K ATPase had a lower affinity for strophanthidin than the microsomal enzyme, but both fractions were inhibited to the same extent by 10-3 M-strophanthidin. Much of the ATPase activity of myelin apparently is not the result of microsomal contamination. Although the possibility of axolemmal contamination cannot be ruled out conclusively, indirect evidence suggests that this is not a significant factor and that Na + K ATPase may be a myelin-associated enzyme.