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

Abstract

Suspension cells of sugarcane 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., .delta.-guanidino group and .alpha.-amino group positively charged and carboxyl group negatively charged. This was concluded because the Km value for uptake increased strongly for: L-arginine analogs which lack the charged carboxyl group (L-arginine methyl ester, agmatin); L-arginine analogs, which lack the charged .alpha.-amino group (L-arginine acid, .gamma.-guanidinobutyric acid); L-arginine analogs which lack the charged .delta.-guanidino group or .gamma.-guanidinoxy group (L-citrulline, L-canavanine at neutral and alkaline pH-values. The importance of the positive charge at the .delta.-guanidino group or .gamma.-guanidinoxy group was documented by Km values for L-arginine and L-canavanine uptake at different pH values. Only at pH values where the .gamma.-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 analogs (L-lysine) were taken up. A spatial rearrangement at the .alpha.-C (D-arginine) was 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 alkaliniation of the suspension medium). Charge equilibration is brought about by efflux of protons and K ions.