12 PRPP AND PURINE NUCLEOTIDE METABOLISM IN HUMAN LYMPHOBLASTS WITH BOTH PRPP SYNTHETASE SUPERACTIVITY AND HGPRT DEFICIENCY

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PRPP and purine nucleotide synthesis and culture growth in response to medium additions were compared in cloned human B lymphoblast lines selected for resistance to growth inhibition by 6-thioguanine (TG) and in the respective normal and PS-superactive parental cell lines. TG-resistant cell lines were severely deficient in HGPRT (<0.2% normal activity). Cells with both HGPRT deficiency and PS superactivity retained the kinetic defect in PS of the parental cells but had PRPP concentration 20-fold greater than normal cells and 6- and 5-fold greater, respectively, than the parental PS-superactive cells and cells defective in HGPRT alone. Nevertheless, PRPP generation in HGPRT-deficient lines was minimally increased (<8%) compared with parental cell lines and purine synthesis de novo in the cell line with both enzyme defects was comparable to that in the PS-superactive parental line.Regardless of the status of PS activity, all HGPRT-deficient cell lines showed growth patterns typical of HGPRT deficiency in HAT medium and in medium with 8-azaguanine. However, growth of HGPRT-defIcient cell lines, including the line with PS superactivity, was more sensitive than that of parental lines to inhibition of purine synthesis de novo by 6-methylthioinosine (MMPR), especially when adenine was provided, most likely reflecting compromised guanine nucleotide synthesis in HGPRT-deficient cells when purine synthesis de novo is blocked and adenine is the sole source of purine for salvage.