Na+-coupled transporters

Tamai I, China K, Sai Y, Kobayashi D, Nezu J, Kawahara E et al. Na(+)-coupled transport of L-carnitine via high-affinity carnitine transporter OCTN2 and its sub-cellular localization in kidney. Biochim Biophys Acta 2001 1512(2) 273-284. 

Table 1. Diversity of Na+-Coupled Transport in Animal Cells and Tissues... 

All models for Na+-coupled transport contain the following concepts ... 

From the earliest proposal for Na+-coupled transport, it was considered likely that K+ participated in the overall reaction (Crane, 1965). With the use of membrane vesicles where the internal and external K+ levels could be manipulated and replaced by other ions (Colombini and Johnstone, 1974 Murer and Hopfer, 1974 Sigrist-Nelson et al., 1975 Hopfer, 1978), it became clear that the apparent K+ requirement in intact cell systems was likely indirect and due to its requirement by the Na+/K+ ATPase to maintain the electrochemical potential difference for Na+. 

The first reported instances using isolated plasma membrane vesicles to study Na+-coupled transport were derived from brush borders of the small intestine (Murer and Hopfer, 1974 Sigrist-Nelson et al., 1975) and Ehrlich cells (Colombini and Johnstone, 1974). In rapid succession a number of other systems were established to study translocation of many solutes in many animal cell systems (Schuld-iner and Kaback, 1975 Lever, 1977 Hammerman and Sacktor, 1978 Wright et al., 1983 Saieret al., 1988 see also Table 1). 

Berry, C. A., and Rector, F. C. (1989). Electroneutral NaCl absorption in the proximal tubule Mechanisms of apical Na-coupled transport. Kidney Int. 36,403-411. 

In this chapter, we have examined coupled transport systems that rely on ATP hydrolysis, on primary gradients of Na or Ff, and on phosphotransferase systems. Suppose you have just discovered an unusual strain of bacteria that transports rhamnose across its plasma membrane. Suggest experiments that would test whether it was linked to any of these other transport systems. 

U Kompella, KJ Kim, MHI Shiue, VHL Lee. (1995). Possible existence of Na+-coupled amino acid transport in the pigmented rabbit conjunctiva. Life Sci 57 1427-1431. 

Figure 8.5 Act/Ve amino add transport into cells. Amino acids are transported into cells against their concentration gradient coupled to Na ion transport down its concentration gradient. The Na ion is transported out in exchange for K -ions, via the NayK ATPase, Chapter 5).

Active transport of a molecule across a membrane against its concentration gradient requires an input of metabolic energy. In the case of ATP-driven active transport, the energy required for the transport of the molecule (Na+, K+, Ca2+ or H+) across the membrane is derived from the coupled hydrolysis of ATP (e.g Na+/K+-ATPase). In ion-driven active transport, the movement of the molecule to be transported across the membrane is coupled to the movement of an ion (either Na+ or H+) down its concentration gradient. If both the molecule to be transported and the ion move in the same direction across the membrane, the process is called symport (e.g. Na+/glucose transporter) if the molecule and the ion move in opposite directions it is called antiport (e.g. erythrocyte band 3 anion transporter). 

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