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  • Title: Purine and pyrimidine transport by cultured Novikoff cells. Specificities and mechanism of transport and relationship to phosphoribosylation.
    Author: Zylka JM, Plagemann PG.
    Journal: J Biol Chem; 1975 Aug 10; 250(15):5756-67. PubMed ID: 168203.
    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...
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