CARRIER-MEDIATED TRANSPORT OF SULFATE THROUGH THE ENVELOPE OF SPINACH CHLOROPLAST

Abstract

Evidence is presented for low rates of carrier-mediated uptake of sulfate, thiosulfate and sulfite into the stroma of the C3 plant Spinacia oleracea. Uptake of sulfate in the dark was followed using 2 techniques: uptake of 35S as determined by silicon oil centrifugal filtration and uptake as indicated by inhibition of CO2-dependent O2 evolution rates after addition of sulfate. Sulfate, thiosulfate and sulfite were transported across the envelope leading to an accumulation in the chloroplasts. Sulfate transport had saturation kinetics of the Michaelis-Menten type (Vmax: .apprx. 25 .mu.mol mg-1 chlorophyll .cntdot. h-1 at 22.degree. C; Km: 2.5 mM). The rate of transport for sulfate was not influenced either by illumination or pH change in the external medium. Phosphate was a competitive inhibitor of sulfate uptake by chloroplasts (Ki: 0.7 mM). The rate of transport for phosphate appeared to be much higher than for sulfate. When the chloroplasts were preloaded with labeled sulfate, radioactivity was rapidly released after addition of phosphate into the external medium. Consequently, the transport of sulfate occurs by a strict counter-exchange: for each molecule of sulfate entering the chloroplast, one molecule of phosphate leaves the stroma, and vice-versa. The uptake of sulfate by isolated intact chloroplasts exchanging for internal free phosphate induced a lower lower rate of photophosphorylation, which in turn inhibited CO2-dependent O2 evolution. The presence on the inner membrane of the chloroplast envelope of a specific sulfate carrier, distinct from the phosphate translocator, is discussed.