Abstract
Uptake of system L amino acid substrates into isolated placental plasma membrane vesicles in the absence of opposing side amino acid (zero-trans uptake) is incompatible with the concept of obligatory exchange, where influx of amino acid is coupled to efflux. We therefore hypothesized that system L amino acid exchange transporters are not fully obligatory and/or that amino acids are initially present inside the vesicles. To address this, we combined computational modeling with vesicle transport assays and transporter localization studies to investigate the mechanisms mediating [14C]l-serine (a system L substrate) transport into human placental microvillous plasma membrane (MVM) vesicles. The carrier model provided a quantitative framework to test the 2 hypotheses that l-serine transport occurs by either obligate exchange or nonobligate exchange coupled with facilitated transport (mixed transport model). The computational model could only account for experimental [14C]l-serine uptake data when the transporter was not exclusively in exchange mode, best described by the mixed transport model. MVM vesicle isolates contained endogenous amino acids allowing for potential contribution to zero-trans uptake. Both L-type amino acid transporter (LAT)1 and LAT2 subtypes of system L were distributed to MVM, with l-serine transport attributed to LAT2. These findings suggest that exchange transporters do not function exclusively as obligate exchangers.—Widdows, K. L., Panitchob, N., Crocker, I. P., Please, C. P., Hanson, M. A., Sibley, C. P., Johnstone, E. D., Sengers, B. G., Lewis, R. M., Glazier, J. D. Integration of computational modeling with membrane transport studies reveals new insights into amino acid exchange transport mechanisms.
Highlights
We recently developed a computational amino acid transport model that describes the possibility for the uptake of L-serine into human placental microvillous plasma membrane (MVM) vesicles by both obligate exchange and nonobligate transport mechanisms (Fig. 1A) [19]
The mixed model predicted that the presence of a high outwardly directed L-serine concentration gradient would lead to an initial overshoot of [14C] L-serine accumulation into MVM vesicles above equilibrium concentrations by a dominant fast exchange mechanism followed by attenuation to apparent equilibrium by a slower facilitated transport mechanism (Fig. 2B)
Our studies reveal that exchangers, believed to be obligatory, may exhibit other patterns of transport behavior relating to facilitated transport
Summary
TRANSPORT OF AMINO ACIDS ACROSS an epithelium is a complex process mediated by a broad array of amino acid transporters expressed in the plasma membrane [1, 2] These various transport proteins are classified according to their transport mechanism [2, 3]. The interaction of all 3 mechanisms is required for net directional transfer of amino acids across absorptive epithelia such as intestine, kidney, and placenta [5, 6]. These systems have been characterized at the molecular level [2, 4], their functional coordination and interaction in mediating transepithelial transport is poorly defined.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
