title: Characterization of a phosphate transport system in human fibroblast lysosomes.
abstract: The uptake of [32P]KH2PO4 by Percoll-purified human fibroblast lysosomes at pH 7.0 was investigated to determine if lysosomes contain a transport system recognizing phosphate. Lysosomal phosphate uptake was linear for the first 2 min, attained a steady state by 8-10 min at 37 degrees C, and was not Na+ or K+ dependent. Upon entering lysosomes, [32P]phosphate was rapidly metabolized to trichloroacetic acid-soluble and trichloroacetic acid-insoluble products. After 1-min incubations, 50% of the radioactivity recovered from lysosomes was in the form of inorganic phosphate; and after a 2.5-min incubation, 27% of the radioactivity was recovered as inorganic phosphate. When lysosomes are loaded with radioactivity by incubation with 0.03 mM [32P]KH2PO4 for 25 min and then washed at 4 degrees C, lysosomes fail to release the accumulated radioactivity during a subsequent incubation at 37 degrees C. Lysosomal phosphate uptake gave linear Arrhenius plots (Q10 = 1.8) and was inversely proportional to medium osmolarity. Phosphate uptake was maximal at pH 5-6, half-maximal at pH 7.1, with little transport activity at pH greater than 8, suggesting that the transport system recognizes the monobasic form of phosphate. Lysosomal phosphate uptake is saturable, displaying a Km of 5 microM at pH 7.0 and 37 degrees C. High specificity for phosphate is demonstrated since large concentrations of Na2SO4, NaHCO3, KCl, NaCl, 5'-AMP, or the anion transport inhibitor, 4,4'-diisothiocyanatostilbene-2,2'-disulfonate, have no effect on lysosomal phosphate transport. In contrast, the phosphate analog, arsenate, strongly inhibits lysosomal phosphate uptake in a competitive manner with a Ki of 7 microM. Pyridoxal phosphate, CTP, adenosine 5'-(beta,gamma-imino)triphosphate (AMP-PNP), and glucose 6-phosphate were found to be noncompetitive inhibitors of lysosomal phosphate uptake displaying Ki values of 80-250 microM. When lysosomes are incubated with [gamma-32P]ATP, the lysosomal membrane ATPase hydrolyzes the ATP to form inorganic phosphate which then enters lysosomes by this lysosomal phosphate transport route.
authors.affiliation: Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor 48109-2029.
authors.forename: R L
authors.initials: RL
authors.lastname: Pisoni
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology; D001692:Biological Transport; D002478:Cells, Cultured; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008247:Lysosomes / Q000378:metabolism*; D010710:Phosphates / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011732:Pyridoxal Phosphate / Q000494:pharmacology
chemical_list: D010710:Phosphates; D011732:Pyridoxal Phosphate; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases