Mechanism of Uptake of l‐Arginine by Sugar‐Cane Cells

Abstract

Suspension cells of sugar cane were used as a model system for cells of higher plants to study the mechanism of l‐arginine uptake. The uptake system is specific for the l‐arginine molecule in the fully ionized state, i.e. δ‐guanidine group and α‐amino group positively charged and carboxyl group negatively charged. This was concluded because the Km value for uptake increased strongly for: (a) l‐arginine analogues which lack the charged carboxyl group (l‐arginine methyl ester, agmatin); (b) l‐arginine analogues, which lack the charged α‐amino group (l‐arginine acid, γ‐guanidinobutyric acid); (c) l‐arginine analogues which lack the charged δ‐guanidine group or γ‐guanidinoxy group (l‐citrulline, l‐canavanine at neutral and alkaline pH‐values). The importance of the positive charge at the δ‐guanidine group or γ‐guanidinoxy group was further documented by Km values for l‐arginine and l‐canavanine uptake at different pH values. Only at pH values where the γ‐guanidinoxy group is protonated, was there an effective uptake of l‐canavanine and effective competition of l‐canavanine with l‐arginine. The length of the l‐arginine molecule was less important: slightly larger l‐homoarginine) or shorter analogues (l‐lysine) were taken up rather well. A spatial rearrangement at the α‐carbon (d‐arginine) was, however, not tolerated. The uptake of l‐arginine proceeds by electrogenic uniport, there is no evidence for symport or antiport of another molecule (though l‐canavanine uptake at neutral pH value causes a transient alkalinization of the suspension medium). Charge equilibration is brought about by efflux of protons and potassium ions. Copyright © 1981, Wiley Blackwell. All rights reserved