Guanosine triphosphate catabolism in human and rabbit erythrocytes: role of reductive deamination of guanylate to inosinate

The basis of echinocytosis of the erythrocyte by glucose depletion

Echinocytosis of erythrocytes by glucose depletion is attributed to adenosine triphosphate depletion, but its process still remains unknown. A mechanism of control of the erythrocyte shape has been previously proposed in which the anion exchanger Band 3, linked to flexible membrane skeleton, has a pivotal role. Recruitments of its inward facing (Band 3(i) ) and outward facing (Band 3(o) ) conformations contract and relax the membrane skeleton, thus promoting echinocytosis and stomatocytosis, respectively. The Band 3(o) /Band 3(i) equilibrium ratio increases with the increase of the Donnan equilibrium ratio, and preferential inward and outward transport by Band 3 of substrates slowly transported are echinocytogenic and stomatocytogenic, respectively. The mechanism suggests the following process. The major organic phosphate 2,3-bisphosphoglycerate is catabolized to lactate to form inorganic phosphate, 3-phosphoglycerate, and adenosine triphosphate. The last two products can be reversibly transformed into 1,3-bisphosphoglycerate and adenosine diphosphate by the glycolytic enzyme phosphoglycerate kinase, thus allowing 2,3-bisphosphoglycerate formation by 2,3-bisphosphoglycerate synthase/phosphatase. The catabolic and cyclic processes initially oppose echinocytosis by increasing the Donnan ratio and outward transport of slowly transported inorganic phosphate by Band 3 (its basic form is transported with a hydrogen ion). Echinocytosis occurs when inward transport of this product becomes predominant. This process can rationalize direct and indirect observations.

Metabolism of guanine and guanine nucleotides in primary rat neuronal cultures

The metabolic fate of guanine and of guanine ribonucleotides (GuRNs) in cultured rat neurons was studied using labeled guanine. 8-Aminoguanosine (8-AGuo), an inhibitor of purine nucleoside phosphorylase, was used to clarify the pathways of GMP degradation, and mycophenolic acid, an inhibitor of IMP dehydrogenase, was used to assess the flux from IMP to GMP and, indirectly, the activity of the guanine nucleotide cycle (GMP----IMP----XMP----GMP). The main metabolic fate of guanine in the neurons was deamination to xanthine, but significant incorporation of guanine into GuRNs, at a rate of approximately 8.5-13.1% of that of the deamination, was also demonstrated. The turnover rate of GuRNs was fast (loss of 80% of the radioactivity of the prelabeled pool in 22 h), reflecting synthesis of nucleic acids (32.8% of the loss in radioactivity) and degradation to xanthine, guanine, hypoxanthine, guanosine, and inosine (49.3, 4.3, 4.1, 1.1, and 0.5% of the loss, respectively). Of the radioactivity in GuRNs, 7.9% was shifted to adenine nucleotides. The accumulation of label in xanthine indicates (in the absence of xanthine oxidase) that the main degradative pathway from GMP is that to xanthine through guanosine and guanine. The use of 8-AGuo confirmed this pathway but indicated the operation of an additional, relatively slower degradative pathway, that from GMP through IMP to inosine and hypoxanthine. Hypoxanthine was incorporated mainly into adenine nucleotide (91.5%), but a significant proportion (6%) was found in GuRNs.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanine ribonucleotide metabolism in human red blood cells: evidence for a high rate of GMP dephosphorylation

The flux rates through the metabolic pathways affecting the maintenance of GuRN pool in intact human RBC were studied. Normal RBC, incubated in KRBB, exhibited a markedly higher accumulation in nucleotides of Gu than of Hx. Addition of 8-AGuo, a potent inhibitor of PNP, resulted in a marked increase in the accumulation of label in the nucleosides, in Ino following incubation with Hx, and in Guo following incubation with Gu, indicating a very high rate of IMP and GMP degradation to bases through their respective nucleosides. Most of the degradation of GMP is by dephosphorylation to Guo, rather than through reductive deamination to IMP. The ultimate fate of IMP in RBC is its degradation to Ino and consequently to Hx. The contribution of AdRN or of IMP to the GuRN pool is negligible. The results indicate that concerning IMP and GMP, human RBC contain very active futile cycles, nucleotide----nucleoside----base----nucleotide, catalyzed by 5'-nucleotidase, PNP, and HGPRT. The operation of the complete cycles is essential for the maintenance of GuRN and the IMP pool size. These results may explain the finding of reduced GTP content in RBC from patients with an inborn deficiency of PNP or of HGPRT.

Purine and pyridine nucleotides in rabbit red blood cells of different maturity

Using reversed-phase high-performance liquid chromatography purine nucleotides, nucleosides and nucleobases as well as pyridine nucleotides were determined in extracts of reticulocytes and mature red blood cells of rabbits. The concentrations of almost all compounds measured decrease during the last phase of red blood cell maturation. These changes were interpreted with respect to the loss of mitochondria, accompanied by shifting the energy production from the preferentially oxidative mode to the exclusively glycolytic one and variations in the concentrations of purine compounds in blood plasma during reticulocytosis.

Metabolism of guanosine in human erythrocytes

The metabolism of guanosine in human erythrocytes has been studied in two different experimental systems--direct incubation and dialysis incubation--the latter allowing continuous addition and removal of substances. Intra- and extracellular purine compounds were analyzed using high-performance liquid chromatography (HPLC). At 37 degrees C, a normal pH (7.4) and a favorably high concentration of inorganic phosphate, guanine nucleotides were synthesized at a substance rate of about 0.17 mmol . h-1 (calculated per liter erythrocytes) when guanosine was kept at a concentration of 25 mumol . l-1. At a higher guanosine concentration the rate of synthesis increased only moderately. Erythrocytes loaded with guanylates lost these nucleotides at a rate of 0.023 mmol . h-1 at a normal phosphate concentration and somewhat slower at a higher phosphate concentration. The metabolism kept the guanylates in an equilibrium that was similar to the equilibrium between the adenylates.

Nucleotide catabolism and nucleoside cycles in human thymocytes. Role of orthophosphate

The catabolism of GTP and ATP in intact human thymocytes was studied, and the effect of intracellular Pi concentrations on various catabolic reactions was evaluated. Induction of nucleotide catabolism was performed with either NaN3 or deoxyglucose. NaN3 induced an increase in intracellular Pi, whereas deoxyglucose induced a decrease in Pi in human thymocytes. At elevated Pi concentrations, an intracellular accumulation of GMP, IMP and AMP was observed, and the entry of these nucleotides into the nucleoside cycles was diminished. In contrast, at lowered Pi concentrations, there was no accumulation of nucleoside monophosphates, but the production of purine bases and nucleosides was increased as a result of enhanced nucleotide entry into the nucleoside cycles. The dephosphorylation of nucleotides thus seems to be regulated by intracellular Pi concentrations. ATP catabolism proceeded mainly via adenylate deamination at both elevated and lowered Pi concentrations. Guanylate formed during GTP catabolism was mainly dephosphorylated, but significant amounts of GMP were also deaminated to yield IMP.

Guanosine triphosphate catabolism in purine nucleoside phosphorylase deficient human B lymphoblastoid cells

GTP catabolism induced by sodium azide or deoxyglucose was studied in purine nucleoside phosphorylase (PNP) deficient human B lymphoblastoid cells. In PNP deficient cells, as in control cells, guanylate was both dephosphorylated and deaminated but dephosphorylation was the major pathway. Only nucleosides were excreted during GTP catabolism by PNP deficient cells and the main product was guanosine. The level of nucleoside excretion was largely affected by intracellular orthophosphate (Pi) level. In contrast, normal cells excreted nucleosides only at low Pi level while at high Pi levels, purine bases (guanine and hypoxanthine) were exclusively excreted. PNP deficiency had no effect on the extent of GMP deamination.

Substance rates of different steps of purine metabolism in normal and preserved red blood cells (RBC) studied in experiments simulating in vivo conditions

A dialysis incubation system simulating the conditions in the circulation can be used: To draw conclusions about the RBC in transporting purines between organs. To improve solutions for "rejuvenating" stored RBC. To design better systems for quality control of stored RBC.