title: Concentrative transport of adenosine in murine splenocytes: limitation by an ecto-adenosine deaminase.
abstract: Coincident with studies of the transport of (3H]adenosine in murine splenocytes, we have evidence for the extracellular degradation of adenosine. A Na(+)-dependent active transport system for nucleosides exists in splenocytes, but no intracellular concentration gradient of adenosine was observed. Inhibition of adenosine transport across the plasma membrane by dipyridamole and a Na(+)-free medium did not prevent the deamination of extracellular adenosine by what has been generally considered to be a cytosolic enzyme. This failure to achieve an adenosine concentration gradient appears to be consequent to the action of a very active ecto-adenosine deaminase. Inhibition of the adenosine deaminase by deoxycoformycin permits a 6-fold increase in intracellular adenosine concentration relative to the medium by the Na(+)-dependent process. Rapid inhibition of adenosine deaminase by deoxycoformycin occurs even in the presence of dipyridamole which prevents the entry of deoxycoformycin as well as adenosine into the cells in a Na(+)-free medium. These results further support the view that this is an ectoenzyme activity. The kinetics of active adenosine transport were Km = 7.8 +/- 1.1 microM with Vmax = 8.2 +/- 2.8 microM/s in a Na+ medium and much less efficiently in a Li+ medium (Km = 250 +/- 50 microM,Vmax = 7.8 +/- 1.3 microM/s). Inhibition of adenosine transport by other nucleosides suggests a single Na(+)-dependent nucleoside transport system in murine splenocytes with narrow substrate specificity.
authors.affiliation: Department of Physiology, National University of Singapore.
authors.forename: C W
authors.initials: CW
authors.lastname: Lee
authors.forename: R E
authors.initials: RE
authors.lastname: Handschumacher
mesh_terms: D000241:Adenosine / Q000378:metabolism*; D000243:Adenosine Deaminase / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002794:Choline / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D004176:Dipyridamole / Q000494:pharmacology; D005260:Female; D007288:Inosine / Q000378:metabolism; D051379:Mice; D008815:Mice, Inbred Strains; D015649:Pentostatin / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D013154:Spleen / Q000166:cytology* / Q000201:enzymology; D014316:Tritium; D014529:Uridine / Q000378:metabolism
chemical_list: D014316:Tritium; D015649:Pentostatin; D007288:Inosine; D004176:Dipyridamole; D012964:Sodium; D000243:Adenosine Deaminase; D000241:Adenosine; D002794:Choline; D014529:Uridine