title: Purine and pyrimidine transport by cultured Novikoff cells. Specificities and mechanism of transport and relationship to phosphoribosylation.
abstract: Adenine, guanine, and hypoxanthine were rapidly incorporated into the acid-soluble nucleotide pool and nucleic acids by wild type Novikoff cells. Incorporation followed normal Michaelis-Menten kinetics, but the following evidence indicates that specific transport processes precede the phosphoribosyltransferase reactions and are the rate-limiting step in purine incorporation by whole cells. Cells of an azaguanine-resistant subline of Novikoff cells which lacked hypoxanthine-guanine phosphoribosyltransferase activity and failed to incorporate guanine or hypoxanthine into the nucleotide pool, exhibited uptake of guanine and hypoxanthine by a saturable process. Similarly, wild type cells which had been preincubated in a glucose-free basal medium containing KCN and iodoacetate transported guanine and hypoxanthine normally, although a conversion of these purines to nucleotides did not occur in these cells. The mutant and KCN-iodoacetate treated wild type cells also exhibited countertransport of guanine and hypoxanthine when preloaded with various purines, uracil, and pyrimidine nucleosides. The cells also possess a saturable transport system for uracil although they lack phosphoribosyltransferase activity for uracil. In the absence of phosphoribosylation, none of the substrates was accumulated against a concentration gradient. Thus transport is by facilitated diffusion (nonconcentrative transport). Furthermore, the apparent Km values for purine uptake by untreated wild type and azaguanine-resistant cells were higher and the apparent Vmax values were lower than those for the corresponding phosphoribosyltransferases...
mesh_terms: D000225:Adenine / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D002151:Calorimetry; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002460:Cell Line; D002499:Centrifugation, Density Gradient; D002729:Chloromercuribenzoates / Q000494:pharmacology; D003571:Cytochalasin B / Q000494:pharmacology; D003596:Cytosine / Q000378:metabolism; D004176:Dipyridamole / Q000494:pharmacology; D004229:Dithiothreitol / Q000494:pharmacology; D006147:Guanine / Q000378:metabolism; D007041:Hypoxanthine Phosphoribosyltransferase / Q000378:metabolism; D007042:Hypoxanthines / Q000378:metabolism; D007700:Kinetics; D008113:Liver Neoplasms; D009374:Neoplasms, Experimental / Q000378:metabolism; D010430:Pentosyltransferases / Q000378:metabolism*; D011687:Purines / Q000378:metabolism* / Q000494:pharmacology; D011743:Pyrimidines / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats; D013816:Thermodynamics; D013997:Time Factors; D014498:Uracil / Q000378:metabolism
chemical_list: D002729:Chloromercuribenzoates; D007042:Hypoxanthines; D011687:Purines; D011743:Pyrimidines; D003571:Cytochalasin B; D014498:Uracil; D006147:Guanine; D004176:Dipyridamole; D003596:Cytosine; D010430:Pentosyltransferases; D007041:Hypoxanthine Phosphoribosyltransferase; D000225:Adenine; D004229:Dithiothreitol