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

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 K m 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 K m 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.