Purine and phosphoribosylpyrophosphate synthesis in differentiating murine virus-induced erythroleukemic cells in vitro

Purine biosynthesis in mutant mammalian cells

De novo purine biosynthesis has been studied in lymphocyte cell lines established from Lesch-Nyhan patients deficient in hypoxanthine-guanine phosphoribosyltransferase (HGPRT), in in vitro differentiating erythroleukaemic cell lines cloned from cells charactistic of virus-induced murine leukaemia, and in mutant hamster cells deficient in amidophosphoribosyltransferase. The relationship between cellular phosphoribosylpyrophosphate (PP-ribose-P) metabolism and the activity of the enzymes which catalyse the early steps of de novo purine biosynthesis has been explored. It was found that hamster cells deficient in amidophosphoribosyltransferase did not accumulate PP-ribose-P as do HGPRT-deficient cells. In these model systems, an accelerated rate of de novo purine biosynthesis tended to be associated with an increase in cellular PP-ribose-P cotent, but decreases in this rate results from the reduction in the activity of amidophosphoribosyltransferase. Regulation of ammonia-dependent de novo purine biosynthesis was similar to that of glutamine-dependent purine biosynthesis.

Phosphoribosyl pyrophosphate and phosphoribosyl pyrophosphate synthetase in rat mammary gland. Changes in the lactation cycle and effects of diabetes, insulin and phenazine methosulphate

Changes in the tissue content of phosphoribosyl pyrophosphate (PPRibP), glucose 6-phosphate, ribose 5-phosphate (Rib5P), RNA and DNA, of the activity of PPRibP synthetase (EC 2.7.6.1) and the conversion of [1-14C]- and [6-14C]-glucose into 14CO2 were measured at mid-lactation in the normal and diabetic rat and in pregnancy, lactation and mammary involution in the normal rat. The PPRibP, glucose 6-phosphate and Rib5P contents increase during pregnancy and early lactation to reach a plateau value at mid-lactation, before falling sharply during weaning. The PPRibP content, PPRibP synthetase activity and flux of glucose through the oxidative pentose phosphate pathway (PPP) all change in parallel during the lactation cycle. Similarly, after 3 and 5 days duration of streptozotocin-induced diabetes, ending on day 10 of lactation, there were parallel declines in PPRibP content, PPRibP synthetase and PPP activity. The effect of streptozotocin was prevented by pretreatment with nicotinamide and partially reversed by insulin administration. Addition of insulin to lactating rat mammary-gland slices incubated in vitro significantly raised the PPRibP content (+47%) and the activity of the PPP (+40%); phenazine methosulphate, which gives a 2-fold increase in PPP activity, raised the PPRibP content of lactating mammary gland slices by approx. 3-fold. It is concluded that Rib5P, generated in the oxidative segment of the PPP, is an important determinant of PPRibP synthesis in the lactating rat mammary gland and that insulin plays a central role in the regulation of the bioavailability of this precursor of nucleotide and nucleic acid synthesis.

Concentration of phosphoribosyl pyrophosphate in the kidney during development and in experimental diabetic hypertrophy

The effect of developmental growth on the kidney content of phosphoribosyl pyrophosphate PPRibP was studied in rats at ages between the foetal animal and up to 100 days of age. In addition, the effect of short-term diabetes (up to 14 days) on the renal content of PPRibP was studied in immature rats and in adults aged approx. 60 days. The developmental pattern of PPRibP is such that the PPRibP content is lowest in the young rat and increases as the rate of kidney growth slows. In the adult rat, the early kidney hypertrophy of diabetes is accompanied by a fall in PPRibP content and, again, the PPRibP content returns to normal as the rate of kidney hypertrophy diminishes. Induction of diabetes in the immature rat causes a lesser degree of kidney hypertrophy and also a smaller depression of renal PPRibP content. The activity of PPRibP synthetase (EC 2.7.6.1) is not significantly affected by age or diabetes. The changes in PPRibP content are discussed in relation to the generation of ribose 5-phosphate by the pentose phosphate pathway and the utilization of PPRibP for nucleotide synthesis via the 'de novo' and salvage pathways.

Regulation of nucleotide and pentose synthesis in resting and stimulated 3T6 fibroblasts

A two-step procedure has been used to follow the activation of one metabolic system involved in the return of cells to a proliferative state after resting in a Go state as a result of serum limitation. One feature of the resting state is a limited capacity to synthesize nucleotides. The limitation apparently is in the rate of synthesis of 5-phosphoribosylpyrophosphate from glucose and indirectly in the capacity of the resting cells to turn over the triphosphopyridine nucleotide pair, NADPH:NADP+. A reaction utilizing NADPH is apparently greatly diminished in resting cells and is substantially increased by only brief contact of cells with the hormonal elements in dialyzed calf serum. Insulin together with platelet-derived growth factor can substitute for calf serum. Aside from stimulating the turnover of the pyridine nucleotide coenzyme pair, serum also stimulates the utilization and reformation of ATP, principally from AMP. Among the NADPH-linked reactions that have been examined for their physiological significance in the initiation of growth stimulation are two steps in the conversion of glutamate to proline in the cytoplasm. Pyrroline 5-carboxylate, an intermediate in this metabolic pathway, has been shown to stimulate PRPP synthesis when added to cultures of resting 3T6 cells. Proline, the product of the reduction of this 5-membered heterocycle is also a stimulant of PRPP synthesis. In addition, dehydroascorbic acid is a potent stimulant of PRPP synthesis. As a working hypothesis, we are exploring the role of a series of reactions that form a pyrroline 5-carboxylate/proline cycle operating between the cytoplasm and mitochondria. The net result is the oxidation of NADPH by molecular oxygen to yield NADP+ and water. The NADP+ is then used in the hexose monophosphate pathway for the conversion of glucose to PRPP. We wish to determine whether dehydroascorbate is operating in this cycle as an oxidant of proline in the mitochondria or whether it participates in some other reaction in the cell that redistributes the ratio of NADPH to NADP.

Effects of cyclic AMP on the growth of differentiating and undifferentiated Friend erythroleukemic cells

Elevated concentrations of cyclic AMP elicit only minor reductions in growth rate and saturation density in undifferentiated Friend erythroleukemic cells. During the course of dimethylsulfoxide (DMSO)-induced differentiation, Friend cells convert from a cyclic AMP-tolerant state to a phenotype characterized by a high degree of sensitivity to cyclic AMP-mediated growth arrest. Conversion to cyclic AMP sensitivity is detectable after 30 hours growth in medium containing 2% DMSO, and either 0.5 mM 8-Br-cyclic AMP or 5 nM cholera toxin. Cultures of differentiating Friend cells achieved a stationary phase density that was approximately 8-fold higher than the cell density observed in parallel, differentiating cultures treated with 0.5 mM 8-Br-cyclic AMP. Temporally, the appearance of cyclic AMP-sensitivity corresponds to the early expression of in vitro erythroid differentiation (Ross et al., '74), but growth arrest does not alter the subsequent accumulation of hemoglobin in non-dividing DMSO-induced cells. Since growth arrest is preceded by a round of cell division, these observations are consistent with the concept that DMSO must be present during DNA replication for the subsequent expression of hemoglobin synthesis (McClintock and Papaconstantinou, '74; Levy et al., '75; Harrison, '76).