Laurent Counillon - LP2M UMR7370 28 Avenue de Valombrose 06107 Nice France - Keywords: Ionic transport, lithium isotopes

Laurent Counillon - LP2M UMR7370 28 Avenue de Valombrose 06107 Nice France - Keywords: Ionic transport, lithium isotopes

Contribution title: Lithium Isotopic Fractionation by human Na+/H+ exchangers

Ion transport across biological membranes creates electrochemical gradients, which are crucial for all biological functions, ranging from ATP synthesis to nerve conduction. The underlying molecular mechanisms involve a combination of ion selection and translocation by ion channels and transporters. The discrimination between ions of different nature is ensured by sequential electrostatic interactions between polar and/or charged groups of the proteins and partially dehydrated ions. In all known mechanisms the ionic radius, which reflects the electronic occupancy of the element, is the key parameter that determines the optimal coordination, thus explaining why some ion flows at extremely fast rates compared to others.
In this study, we provide precise measurements of lithium stable isotope variations during its incorporation in cells expressing Na+/H+ exchangers, which are also extremely efficient lithium transporters. Using ion fluxes coupled to MC-ICP-MS Mass spectrometry, we show, for the first time, that lithium isotopes are significantly fractionated during transport by several Na+/H+ exchanger isoforms. . This novel use of lithium isotopes has deep implications for characterizing its active role in ionic transport mechanisms in general since our results evidence the strong impact of vibrational energies
We will then discuss how measurement of lithium isotopic fractionation by Na+/H+ exchangers can yield insights into the transport rate limiting steps and also provide clues for the numerous findings showing isotopic fractionation in cells organs and tissues. Since Na-H exchangers are expressed in all cells and phyla and have appeared at the early stages of life evolution, a better understanding of lithium isotope variations exhibited by carbonate fossils over geological timescales can also be inferred.