THE UPTAKE AND METABOLISM OF UREA BY CHARA AUSTRALIS .4. SYMPORT WITHSODIUM - A SLIP MODEL FOR THE HIGH AND LOW-AFFINITY SYSTEMS

We have previously investigated the electrogenic influx of urea in Cha ra, 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 current s 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 sho ws a maximum and fits the difference of two Michaelis-Menten terms, su ggesting a phenomenon like cis-inhibition. Similarities in kinetic cha racteristics between the inhibitory site and the electrically silent u ptake 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 sugg est a ''slip'' model for urea uptake in Chara.