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Mechanism of Partial Reactions in the Cardiac Na + ‐Ca 2+ Exchange Systema a
Author(s) -
KHANANSHVILI DANIEL
Publication year - 1991
Publication title -
annals of the new york academy of sciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.712
H-Index - 248
eISSN - 1749-6632
pISSN - 0077-8923
DOI - 10.1111/j.1749-6632.1991.tb17291.x
Subject(s) - annals , citation , library science , chemistry , classics , computer science , history
The Na+-Ca2+ exchange system is a typical representative of a large family of antiporters (otherwise known as “exchange-only” systems). lz2 In general, antiporters modulate primary ion gradients, created by ion pumps, and can be regulated by numerous regulatory modes, including membrane potential. However, an understanding of cellular regulatory mechanisms is seriously hampered because very little is known about the basic mechanism of ion transport. Most antiporters (especially cation antiporters), are low-abundance proteins ( lo00 sec-1).1-3 This presents serious problems for obtaining reasonable quantities of purified proteins for structural studies and for observing pre-steady-state kinetics on the level of a single catalytic cycle. The exchange-only systems can expose ion-binding sites to the opposite sides of the membrane at different stages of the catalytic c y ~ l e . ~ . * ~ ~ The ion pumps can also exhibit an alternative exposure of ion-binding sites, but in order to provide this, they have to undergo additional steps of phosphorylation and dephosphorylation. I s 2 Two different basic mechanisms of ion transport can be considered for the Na+-Ca2+ exchanger; the ping-pong (consecutive) and the sequential (simultaneous) mechanisms.’r5f’ In the ping-pong mechanism, translocation of Na+ and CaZ+ ions are separate events, while in the sequential mechanism the exchanger binds Na+ and Ca2+, and translocates both ions in a single step. A distinction between the two basic mechanisms was a difficult issue for many years (for review see Refs. 3-5). By using a semi-rapid mixing device, we have recently measured the V,,, K,, and V,,/K,,, parameters of Na,-dependent 45Ca uptake in EGTA-entrapped proteoliposomes (zero-trans conditions).6 On the basis of these measurements, we have concluded that the experimental data are consistent with the ping-pong mechanism. A strong support for the ping-pong mechanism has been very recently obtained in heart cells by using a combination of “caged-Ca” and patch-clamp techniques.’ Additional evidence for the ping-pong mechanism has come from the measurement of transient charge movements of Na+ half-cycle via the exchanger (Hilgemann & Philipson, personal communication).