title: Calyculin A modulates the kinetic constants for the Na+-coupled taurine transport in Ehrlich ascites tumour cells.
abstract: The effect of the phosphatase inhibitor calyculin A (cal A) on the kinetic parameters of the Na+-coupled taurine uptake via the taurine transporter in the Ehrlich ascites tumour cells has been investigated. Preincubation with cal A (100 nM) reduces the initial taurine influx by about 20%, but has no effect on the diffusional component of the taurine influx or on the taurine release from cells suspended in isotonic or in hypotonic medium. Thus, cal A-sensitive phosphatases only affect taurine transport mediated by the Na+-dependent taurine transporter. Cal A increases the Michaelis-Menten constant for binding of taurine to the transporter from 31+/-6 to 45+/-4 microM and reduces the taurine transport capacity from 210+/-20 to 170+/-10 nmol x g dry wt(-1) x min(-1) [corrected]. The Michaelis-Menten constant for binding of Na+ to the taurine transporter is concomitantly increased from 96+/-11 to 129+/-8 mM and the Na+:taurine coupling ratio for activation of the transport cycle is reduced from 3.3+/-0.6 to 2.4+/-0.2. This suggests that cal A-sensitive phosphatases maintain a high affinity of the taurine transporter towards Na+ and taurine as well as a high taurine transport capacity in unpertubated Ehrlich cells.
authors.affiliation: The August Krogh Institute, Biochemical Department, University of Copenhagen, 13 Universitetsparken, DK-2100 Copenhagen, Denmark. jmollerup@aki.ku.dk
authors.forename: J
authors.initials: J
authors.lastname: Mollerup
authors.forename: I H
authors.initials: IH
authors.lastname: Lambert
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002250:Carbon Radioisotopes; D002273:Carcinogens / Q000494:pharmacology; D002286:Carcinoma, Ehrlich Tumor / Q000201:enzymology / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D048429:Cell Size / Q000187:drug effects; D005260:Female; D007700:Kinetics; D008387:Marine Toxins; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D051379:Mice; D010080:Oxazoles / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors; D012964:Sodium / Q000378:metabolism*; D013654:Taurine / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D000099247:Taurine Transporters
chemical_list: D002250:Carbon Radioisotopes; D002273:Carcinogens; D002352:Carrier Proteins; D008387:Marine Toxins; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D010080:Oxazoles; D010749:Phosphoprotein Phosphatases; D012964:Sodium; D013654:Taurine; D000099247:Taurine Transporters; C059041:calyculin A