Sulphydryl groups of (Na+ + K+)-ATPase from rectal glands of Squalus acanthias: Detection of ligand-induced conformational changes

1. Modification of the Class II sulphydryl groups on the (Na⁺ + K⁺)-ATPase from rectal glands of Squalus acanthias with N-ethylmaleimide has been used to detect conformational changes in the protein. The rates of inactivation of the enzyme and the incorporation of N-ethylmaleimide depend on the ligands present in the incubation medium. With 150 mM K⁺ the rate of inactivation is largest (k₁ = 1.73 mM^?1 · min^?1) and four SH groups per α-subunit are modified. The rate of inactivation in the presence of 150 mM Na⁺ is smaller (k₁ = 1.08 mM^?1 · min^-1) but the incorporation of N-ethylmaleimide is the same as with K⁺. 2. ATP in micromolar concentrations protects the Class II groups in the presence of Na⁺ (k₁ = 0.08 mM^?1 · min^?1 at saturating ATP) and the incorporation id drastically reduced. ATP in millimolar concentrations protects the Class II groups partially in the presence of K⁺ (k₁ = 1.08 mM^?1 · min^?1) and three SH groups are labelled per α subunit. 3. The K⁺ -dependent phosphatase is inhibited in parallel to the (Na⁺ + K⁺)-ATPase under all conditions, and the ligand-dependent incorporation of N-ethylmaleimide was on the α-subunit only. 4. It is shown that the difference between the Na⁺ and K⁺ conformations sensed with N-ethylmaleimide depends on the pH of the incubation medium. At pH 6 there is a very small difference between the rates of inactivation in the presence of Na⁺ and K⁺, but at higher pH the difference increases. It is also shown that the rate of inactivation has a minimum at pH 6.9, which suggests that the conformation of the enzyme changes with pH. 5. Modification of the Class III groups with N-ethylmaleimide-whereby the enzyme activity is reduced from about 16% to zero-shows that these groups are also sensitive to conformational changes. As with the Class II groups, ATP in micromolar concentrations protects in the presence of Na⁺ relative to Na⁺ or K⁺ alone. ATP in millimolar concentrations with K⁺ present increases the rate of inactivation relative to K⁺ alone, in contrast to the effect on the Class II groups. 6. Modification of the Class II groups with a maleimide spin label shows a difference between Class II groups labelled in the presence of Na⁺ (or K⁺) and Class II groups labelled in the presence of K + ATP, in agreement with the difference in incorporation of N-ethylmaleimide. The spectra suggest that the SH group protected by ATP in the presence of K⁺ is buried in the protein. 7. The results suggest that at least four different conformations of the (Na⁺ + K⁺)-ATPase can be sensed with N-ethylmaleimide: (i) a Na⁺ form of the enzyme with ATP bound to a high-affinity site (E₁-Na-ATP); (ii) a Na⁺ form without ATP bound (E₁-Na); (iii) a K⁺ form without ATP bound (E₂-K); and (iv) an enzyme form with ATP bound to a low-affinity site in the presence of K⁺, probably and E₁-K-ATP form.     

Sulphydryl groups of (Na+ + K+)-ATPase from rectal glands of Squalus acanthias. Titrations and classification

1. (Na+ + K+)-ATPase from rectal glands of Squalus acanthias contains 34 SH groups per mol (Mr 265000). 15 are located on the alpha subunit (Mr 106000) and two on the beta subunit (Mr 40000). The beta subunit also contains one disulphide bridge. 2. The reaction of (Na+ + K+)-ATPase with N-ethylmaleimide shows the existence of at least three classes of SH groups. Class I contains two SH groups on each alpha subunit and one on each beta subunit. Reaction of these groups with N-ethylmaleimide in the presence of 40% glycerol or sucrose does not alter the enzyme activity. Class II contains four SH groups on each alpha subunit, and the reaction of these groups with 0.1 mM N-ethylmaleimide in the presence of 150 mM K+ leads to an enzyme species with about 16% activity. The remaining enzyme activity can be completely abolished by reaction with 5-10 mM N-ethylmaleimide, indicating a third class of SH groups (Class III). This pattern of inactivation is different from that of the kidney enzyme, where only one class of SH groups essential to activity is observed. 3. It is also shown that N-ethylmaleimide and DTNB inactivate by reacting with the same Class II SH groups. 4. Spin-labelling of the (Na+ + K+)-ATPase with a maleimide derivative shows that Class II groups are mostly buried in the membrane, whereas Class I groups are more exposed. It is also shown that spin label bound to the Class I groups can monitor the difference between the Na+- and K+-forms of the enzyme.     

Solubilization and molecular weight determination of the (Na+ + K+)-ATPase from rectal glands of Squalus acanthias.

The membrane-bound (Na+ + K+)-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3) system was treated with the nonionic detergent octaethylene-glycoldodecyl ether, yielding a transparent supernatant after centrifugation. The supernatant was highly active with both ATPase and p-nitrophenylphosphatase, with initial specific activities of 2300 mumol Pi released . mg-1 protein. h-1 and 350 mumol p-nitrophenol released.mg-1 protein.h-1, respectively. The supernatant was purified to 95--100%, with respect to the 96 000 dalton and the 56 000 dalton peptides. The solubilized enzyme was gel filtered in Sepharose 4B-Cl and displayed 2 peaks, both with catalytic activity. The low molecular weight particles eluted at Kav = 0.54, corresponding to a molecular weight of approximately 500 000 daltons and the particles had a specific activity of 2100 mumol Pi.mg-1 protein.h-1. Both peaks contained phospholipid with 60 mol phospholipid bound per 300 000 g protein. The low molecular weight particles had a molecular weight of 276 000 as determined by sedimentation equilibrium analysis.     

Preparation of membrane-bound and of solubilized (Na+ + K+)-ATPase from rectal glands of Squalus acanthias. The effect of preparative procedures on purity, specific and molar activity.

The effect of spectrin on the polymerization of muscle actin has been investigated by hydrodynamic methods and electron microscopy. Spectrin markedly accelerated polymerization of actin. The effect was more easily observed in lower concentrations of KCl (e.g. 24 mM) where spontaneous polymerization was negligibly small. Similarly large acceleration was observed for polymerization in MgCl2 or CaCl2. The rate of polymerization of actin was proportionally increased with the concentration of spectrin added to a fixed concentration of action. The stationary level of specific viscosity also increased with the spectrin concentration, but at larger concentrations it became smaller. The flow birefringence and electron microscope measurements indicated that actin polymers formed under the influence of spectrin were shorter than those of control F-actin filaments. The structural viscosity and electron microscope observations suggested that the interaction between F-actin fibers was not increased by spectrin. These data strongly suggest a seeding role of spectrin in the polymerization of actin. Spectrin accelerates formation of the nuclei for polymerization. The more the nuclei are formed, the larger the number of the grown polymers are and this leads to rapid formation of shorter polymers since the amount of actin is limited. The acceleration activity was found only in freshly prepared spectrin from fresh ghosts taken from freshly drawn blood.