Kinetic characterization of the perturbation by dodecylmaltoside of sarcoplasmic reticulum Ca 2+ ‐ATPase

We investigated the functional aspects of the interaction between the sarcoplasmic reticulum (SR) membranous Ca 2+ ‐ATPase and the non‐ionic detergent dodecylmaltoside, using detergent concentrations allowing perturbation of the membrane but not its solubilization. At pH 7.5, the effects of dodecylmaltoside on ATPase activity and delipidation had previously been shown to resemble, in some respects, those of octa(ethylene glycol) monododecylether (C 12 E 8 ), an appropriate detergent for ATPase studies. Our aim here was to explore the specific effects of dodecylmaltoside on the different steps in the ATPase catalytic cycle, which may owe their specificity to the difference between the polar head groups of dodecylmaltoside and C 12 E 8 . This was done at 20 °C, both at pH 6 in the absence of KCl and at pH 7.5 in the presence of 100 mM KCl, two conditions under which the characteristics of unperturbed ATPase have already been well defined. Preliminary estimation of dodecylmaltoside partition between water and SR membranes at pH 6 yielded a partition coefficient K close to 4 × 10 5 (ratio of the molar fraction of dodecylmaltoside in the lipid to that in the aqueous phase at a low detergent concentration, assuming that most of this detergent was present in the lipid phase). At near saturation of SR membranes, bound dodecylmaltoside was roughly equimolar with the constituent phospholipids. Non‐solubilizing concentrations of dodecylmaltoside inhibited SR ATPase activity by up to 65–70% at pH 7.5, but not at pH 6, unlike the results of similar experiments with C 12 E 8 . The rates of the four main steps in the ATPase catalytic cycle were measured by fast kinetic techniques; they were similarly modified at both pH. Dodecylmaltoside slowed down both the rate of calcium‐saturated ATPase phosphorylation and the rate of ATPase isomerization after phosphorylation, two steps which were not targets of perturbation by C 12 E 8 . The slowing down of the isomerization step by dodecylmaltoside might well explain why it inhibited overall ATPase activity at pH 7.5. In contrast to C 12 E 8 , dodecylmaltoside did not affect the dephosphorylation step, which was the main target of inhibition by C 12 E 8 and the main rate‐limiting step at pH 6. However, like C 12 E 8 , dodecylmaltoside accelerated the calcium binding‐induced transition of nonphosphorylated ATPase. Another striking feature of the perturbation induced by dodecylmaltoside was that it significantly altered the binding of 45 Ca 2+ to the ATPase and the corresponding conformational changes. At pCa 5–5.5, it almost halved calcium binding to the ATPase but ATPase phosphorylation was unimpaired. These results establish dodecylmaltoside as a potentially useful tool for studying the interaction between the two Ca 2+ binding sites of membranous SR ATPase and their coupling with the catalytic site of ATPase.