TRANSPORT OF INORGANIC PYROPHOSPHATE ACROSS THE SPINACH CHLOROPLAST ENVELOPE

Spinach-leaf chloroplasts take up PP(i) at a rate of 1.9 mumol/h per mg of chlorophyll (Chl) in the dark and 1.6 mumol/h per mg of Chl in the light. The K(m) for PP(i) transport is 32 muM in the dark and 6 muM in the light. Uptake is inhibited by pyridoxal phosphate, 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid and imidodiphosphate, but not by NaF or EDTA. PP(i) does not appear to cross the chloroplast envelope in exchange for P(i), suggesting that it is not transported by the phosphate translocator. Exchange of PP(i) and adenine nucleotides across the chloroplast envelope is very slow and PP(i) does not competitively inhibit ATP uptake, suggesting that little, if any, PP(i) is transported by the adenine-nucleotide translocator. These results are consistent with the presence of a specific, high-affinity PP(i) translocator in the spinach chloroplast envelope. It is proposed that in vivo PP(i) is taken up into the chloroplast from the cytosol to replenish the P(i) pool in the stroma.