Ronzani N, Migala A, Hasselbach W
Eur J Biochem. 1979 Nov;101(2):593-606. doi: 10.1111/j.1432-1033.1979.tb19754.x.
The study deals with the interrelationship of the phosphate-transferring activities of the calcium-transporting sarcoplasmic reticulum membrane vesicles: the phosphate exchange between nucleoside triphosphate (NTP) and nucleoside diphosphate (NDP) (NTP-NDP exchange), the calcium-dependent NTase, and the phosphorylation of NDP by inorganic phosphate in the presence of NTP (NTP-Pi exchange). Different nucleotides were used as phosphate donors and acceptors. It is demonstrated for the phosphate transfer from ITP to GDP that the NTP-NDP exchange exhibits ping-pong kinetics with Mg-ITP and unliganded GDP as substrates. The apparent affinities of the enzyme for the nucleoside diphosphate and triphosphate species are deduced according to this mechanism. The enzyme's affinity for the nucleoside triphosphates and diphosphates depends on its functional state being considerably lower under conditions of NTP-NDP exchange than during NTP splitting or NTP synthesis. ATP and GTP are split with the same low rates when calcium-activated NTPase is inhibited by high internal calcium concentrations after calcium transport has reached steady state. The rates of the NTP-NDP exchange reactions, however, differ by a factor of about 10 being approximately equal to 3 mumol . mg-1 . min-1 for ATP-ADP and only approximately equal to 0.3 mumol . mg-1 . min-1 (22 degrees C) for GTP-GDP. When the sarcoplasmic reticulum vesicles are made calcium-permeable, the calcium transport ATPase is turned on and the rates of GTP and ATP splitting increase about tenfold. Yet, while the rate of ATP-ADP exchange is little reduced, the rate of GTP-GDP exchange drops by approximately 50%. The persisting exchange activity of calcium-permeable vesicles demonstrates that high internal calcium concentrations are not required for the transfer of the protein-bound phosphoryl group to NDP during NTP-NDP exchange.
核苷三磷酸（NTP）与核苷二磷酸（NDP）之间的磷酸交换（NTP-NDP交换）、钙依赖性核苷酸转移酶以及在NTP存在下无机磷酸对NDP的磷酸化作用（NTP-磷酸交换）。使用了不同的核苷酸作为磷酸供体和受体。对于从肌苷三磷酸（ITP）到鸟苷二磷酸（GDP）的磷酸转移，已证明以Mg-ITP和未结合配体的GDP为底物时，NTP-NDP交换呈现乒乓动力学。根据该机制推导了该酶对核苷二磷酸和三磷酸物种的表观亲和力。该酶对核苷三磷酸和二磷酸的亲和力取决于其功能状态，在NTP-NDP交换条件下比在NTP裂解或NTP合成过程中要低得多。当钙转运达到稳态后，高内部钙浓度抑制钙激活的NTP酶时，ATP和GTP以相同的低速率裂解。然而，NTP-NDP交换反应的速率相差约10倍，ATP-ADP交换约为3 μmol·mg-1·min-1，而GTP-GDP交换在22℃时仅约为0.3 μmol·mg-1·min-1。当肌浆网囊泡变得对钙通透时，钙转运ATP酶被激活，GTP和ATP的裂解速率增加约10倍。然而，虽然ATP-ADP交换速率几乎没有降低，但GTP-GDP交换速率下降了约50%。钙通透囊泡持续的交换活性表明，在NTP-NDP交换过程中，将蛋白质结合的磷酰基转移到NDP并不需要高内部钙浓度。
