Conventional and saturation transfer EPR spectroscopy of Na+/K(+)-ATPase modified with different maleimide-nitroxide derivatives.

The membranous Na+/K(+)-ATPase from Squalus acanthias has been covalently modified on either Class I or Class II sulphydryl groups using derivatives of 3-(maleimidomethyl)-1-oxyl-2,2,5,5-tetramethylpyrrolidine with substituents of different charge and hydrophobicity attached at the remaining unsubstituted position of the pyrrolidine ring. The substituent groups used were a methyl and a hexyl ester, and di- and tri-methylammonium ethyl esters, as well as the parent underivatized compound. Additionally, another series of maleimide-nitroxides differing (by zero to seven intervening atoms) in the length of the linking group between the maleimide and the pyrrolidine moieties was used. The sites of attachment have been characterized in terms of the rotational mobility and environmental polarity by using conventional and saturation transfer EPR spectroscopy of these spin-labelled reagents. This provides a further sub-classification of the primary Class I and Class II SH-groups on the alpha-subunit of the enzyme, which differ both in their reactivity and influence on the Na+/K(+)-ATPase activity.