ISSN:
1432-1424
Keywords:
Sodium
;
Urea
;
Symport
;
Cotransport
;
Charophytes
;
Slip model
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Abstract We have previously investigated the electrogenic influx of urea in Chara, and the urea- and sodium-dependent membrane current. We have shown that there is a sodium-stimulated component of urea influx and a urea-stimulated component of sodium influx, and that these are of the same size. We conclude that the electrogenic inward transport of urea, and of its analogues acetamide and acrylamide, is by sodium symport, with a stoichiometric ratio of 1∶1. The kinetics of the fluxes and currents show two different K M values for sodium in different cells and two different kinds of kinetics for the effect of urea on membrane current, one of which fits the Michaelis-Menten equation, while the other shows a maximum and fits the difference of two Michaelis-Menten terms, suggesting a phenomenon like cis-inhibition. Similarities in kinetic characteristics between the inhibitory site and the electrically silent uptake site (System II) lead us to suggest that the same protein may be responsible for both the low-K M, electrogenic influx of urea (System I) and the high-K M, electrically silent influx by System II. We suggest a “slip” model for urea uptake in Chara.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00233665
