Rapid kinetic studies of active Ca2+ transport in sarcoplasmic reticulum
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Active uptake of 29 μM free Ca2+ in the presence of 5mm KCl, initiated by the addition of 10−4m Mg2+ and 2×10−4m ATP, occurs with at1/2 of ca. 9 sec. The process results in an internal free Ca2+ concentration of 13mm.
Preincubation with 50mm KCl and 5mm MgCl2, followed by initiation of active uptake by the addition of ATP to give a final concentration of ca. 2.5mm Mg2+ and ca. 2.5mm MgATP, gave faster and larger uptakes. Thet1/2 for the reaction was ca. 600 msec and the internal free Ca2+ concentration was 70±20mm. The turnover number of 7.1±0.8 sec−1 was calculated for the enzyme at mid-reaction under the assumption of a stoichiometry of 2 Ca2+ per cycle.
The accelerative effects of Mg2+ andM+ on the rate of transport were investigated. Experiments in which the cations were added to or omitted from the incubation medium showed that the presence of both classes of activator in the internal aqueous space was necessary for maximal activation of the transport system. The concentration dependencies of these effects were investigated. Analysis shows that the monovalent cation effect is probably based on the countertransport according to the model of Kanazawa et al. (1971) while the Mg2+ effect referable to the inside surface is primarily catalytic. No Mg2+ counter-transport could be demonstrated under conditions in which the internal monovalent cation concentration was adequate.
Under conditions in which the Mg2+ concentration is adequate for stimulation but theM+ concentration is not (and vice versa), the active uptake can be resolved into two phases. The rapid phase is complete within the first 50 msec and corresponds to 0.26–2.06 Ca2+ released to the internal phase per Ca2+-ATPase. These results correspond closely to those of published studies measuring the rate at which Ca2+ becomes inaccessible to the external solution. The comparison shows that Ca2+ is released to the internal aqueous phase almost as rapidly as it becomes inaccessible to the outside phase. Analysis of the concentration dependencies shows that K+/Ca2+ or Mg2+/Ca2+ competition for occupation of the inwardly-oriented translocator (of the phosphorylated enzymes) is involved in the fast phase of Ca2+ release. When the internal concentrations of both K+ and Mg2+ are adequate, the slow phase is speeded up to such an extent that the first partial turnover can no longer be kinetically isolated from the subsequent turnovers. Under these conditions, the rate of enzyme dephosphorylation, the binding of K+ to the translocator, and its return to an outward orientation are no longer rate limiting. The rate constant for the outward-to-inward reorientation of this translocator is ca. 13.8 sec−1. The average turnover number for the first several turnovers, obtained under conditions of maximal stimulation, is ca. 7.1. The latter value was somewhat influenced by trans-inhibition by internal Ca2+. It is concluded that this outward-to-inward transition of the Ca2+-laden translocator is rate-limiting to the first turnover and that the rate of the inward-to-outward transition of the K+-laden translocator becomes limiting in the final phases of the transport process.
Two major lines of evidence against electrogenic models of pump function were the stimulatory effect of internal K+ on the transport reaction and the lack of a stimulatory effect as an inwardly-directed Cl− gradient. Also the mechanism of the reponse of the KCl impermeable vesicles to valinomycin was investigated. Those findings also run counter to the expectations of electrogenic pump mechanisms.
KeywordsSarcoplasmic Reticulum Active Uptake Monovalent Cation Valinomycin Turnover Number
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