Determination of intracellular levels of 6-mercaptopurine metabolites in erythrocytes utilizing capillary electrophoresis with laser-induced fluorescence detection

Capillary electrophoresis proved to be a useful technique for the analysis of intracellular levels of 6-thioguanosine mono-, di-, and triphosphate with analysis times of 20 min. Conditions required for baseline separation of the thioguanine nucleotides consisted of a 25 mM KH2PO4 (pH 8.0) buffer and a separation voltage of +28 kV. Laser-induced fluorescence detection (lambda ex = 325 nm, lambda em = 410 nm) of the thioguanine nucleotide metabolites of 6-mercaptopurine (6-MP) was possible following oxidation of the thiol functionality. Tedious extraction procedures involving mercury cellulose resins or phenyl mercury adduct formation, which had been required previously for the selective extraction of thiopurines from erythrocytes, were unnecessary due to the overall specificity of the approach. However, the inclusion of 50 mM EDTA in the sample preparation was required to inhibit the anabolic/catabolic enzymatic activity, which was responsible for the degradation of the analytes. The method demonstrated linearity from 5 to 1700 pmol/100 microliters red blood cells for the three analytes (RSDs < or = 8%). The feasibility of the method was demonstrated for the quantitation of 6-thioguanine nucleotides in patients receiving either oral or intravenous 6-MP therapy.

Effect of muscular exercise by bicycle ergometer on erythrocyte purine nucleotides

The effect of muscular exercise by bicycle ergometer on erythrocyte purine nucleotides was investigated in 6 athletes. Muscular exercise increased the concentration of inosine monophosphate from 5.9 +/- 1.1 to 7.3 +/- 1.3 nmol/ml in venous erythrocytes and from 5.7 +/- 1.0 to 6.8 +/- 1.4 nmol/ml in arterial erythrocytes, respectively, while it decreased the concentrations of adenosine diphosphate and adenosine monophosphate from 189.3 +/- 42.7 to 141.2 +/- 26.9 and from 26.0 +/- 7.8 to 15.7 +/- 4.3 nmol/ml in venous erythrocytes and also decreased their concentrations from 195.1 +/- 51.0 to 141 +/- 29.2 and from 26.5 +/- 9.6 to 14.8 +/- 3.0 nmol/ml in arterial erythrocytes, respectively. The muscular exercise also increased the concentration of inorganic phosphate in venous plasma from 1.12 +/- 0.12 to 1.46 +/- 0.22 mmol/l, that of NH3 in blood from 41.90 +/- 6.91 to 150.22 +/- 50.80 mumol/l, that of lactic acid in blood from 7.90 +/- 1.71 to 61.03 +/- 18.43 mg/dl and that of hypoxanthine in venous plasma from 1.32 +/- 0.36 to 18.14 +/- 4.87 mumol/l, respectively. Therefore, in vitro study was performed to investigate whether inorganic phosphate, NH4Cl, lactic acid or hypoxanthine affects nucleotides in erythrocytes. After 2 hour-incubation, 2 mM inorganic phosphate increased the erythrocyte concentration of inosine monophosphate 1.6 fold but decreased the erythrocyte concentrations of adenosine monophosphate and adenosine diphosphate 0.72 and 0.89 fold, respectively, in the suspension (pH 7.35), as compared with 1 mM inorganic phosphate. However NH4Cl, lactic acid or hypoxanthine did not affect erythrocyte purine nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum hepatocyte growth factor in acute hepatic failure in comparison with acute hepatitis

Serum hepatocyte growth factor (HGF) levels in patients with acute hepatic failure has been reported to increase. However, possible mechanisms responsible for HGF elevation in this syndrome remains to be determined. To explore the possible mechanisms, we measured serum HGF in patients with acute hepatic failure and self-limited acute hepatitis, using an immunoradiometric assay. Serum HGF levels in acute hepatic failure were 36-fold higher compared with those in acute hepatitis. Serum HGF values in acute hepatitis were significantly correlated with serum bilirubin and gamma-GTP levels, whereas those in acute hepatic failure were not. These clinical findings suggest that serum HGF levels in acute hepatic failure may be regulated by different mechanisms from those in self-limited acute hepatitis.

Assignment of a gene that determines erythrocytic guanosine-5'-triphosphate concentration (Gtpc) to mouse chromosome 9

Nine inbred mouse strains surveyed for erythrocytic guanosine-5'-triphosphate (GTP) concentration were found to segregate into two discrete groups. Strains having low GTP levels between 1.4 and 3.4 nmol/10(9) cells were C3H/HeJ, C3H/HeHa, A/J, and WB/ReJ. Strains having high GTP levels between 11.0 and 14.8 nmol/10(9) cells were AKR/J, DBA/2J, CBA/J, C57BL/6J, and C57L/J. Erythrocytic ATP levels did not vary significantly among these groups. Crosses between low and high GTP strains gave F1 progeny having intermediate levels of GTP, and the progeny of F1's backcrossed to parental strains segregated in a 1:1 ratio for GTP concentration. We designated the GTP concentration determining trait, Gtpc. Typing the C57BL/6J x C3H/HeJ (B x H) recombinant inbred strains for GTP levels revealed 0/12 strain distribution pattern differences for loci on both chromosomes 5 and 9. Backcross analysis did not provide evidence for linkage of Gtpc to W (dominant white spotting) on chromosome 5 with 15/45 recombinants. A test for linkage of Gtpc to transferrin (Trf) on chromosome 9 gave evidence of linkage with an observed recombination frequency of 14.6 +/- 5.5 and a 99% confidence interval of 3.9-33.9 cM.

Regulation of the human neutrophil NADPH oxidase by the Rac GTP-binding proteins

Recent progress in our understanding of the regulation of the phagocyte NADPH oxidase by the Rac GTP-binding protein(s) has provided the first detailed glimpse into the mechanisms of leukocyte regulation by a small GTP-binding protein. Studies over the past year have indicated that the activity of the NADPH oxidase can be modulated by regulation of the GTP/GDP state of Rac. Proteins exist in leukocytes that are able to modify GTP-binding protein function in this manner, and their activity may be regulated by signals generated upon phagocyte stimulation.

Determination of guanine and its nucleosides and nucleotides in human erythrocytes by high-performance liquid chromatography with postcolumn fluorescence derivatization using phenylglyoxal reagent

A high-performance liquid chromatographic method with on-line postcolumn fluorescence derivatization is described for the simple and sensitive determination of guanine and its nucleosides and nucleotides in human erythrocytes. After deproteinization of the biospecimen, guanine and its nucleosides and nucleotides were separated on a reversed-phase column (TSKgel ODS-120T) by gradient elution with methanol in the aqueous mobile phase consisting of tetra-n-propylammonium phosphate (pH 6.0) and phosphate buffer (pH 6.0). The compounds were then automatically converted into fluorescent derivatives by reaction with phenylglyoxal. This derivatization was selective for guanine-containing compounds. The present method permitted the reliable quantification of GDP and GTP in human erythrocytes. The detection limits (at a signal-to-noise ratio of 3) for guanine and its nucleosides and nucleotides were 3.2-10.0 pmol in a 20-microliters injection volume. The concentrations of GDP and GTP in human erythrocytes were 17.2 +/- 6.2 and 40.2 +/- 5.8 nmol/ml, respectively.

Activation of respiratory burst oxidase is accompanied by desensitization of p47phox in nucleoside-triphosphate binding along with its translocation to cell membrane

A cytosolic component of human neutrophils, p47phox, potentiates respiratory burst oxidase translocating from cytosol to membrane upon cell stimulation. In this study, the nucleotide-binding ability of p47phox was examined using [32P]GTP dialdehyde (oGTP), [32P]oATP, and [32P]oNADPH. p47phox showed affinities for both oGTP and oATP that were 14 times higher than that for oNADPH, suggesting that it is a nucleoside triphosphate (NTP)-binding protein rather than an NADPH-binding protein. Binding analysis of p47phox using either [32P]oGTP or [32P]oATP revealed an apparent binding constant for each individual NTP analogue and the same maximum binding value, which suggests that both NTPs share a common specific binding site. Stimulation of neutrophils with phorbol myristate acetate (PMA) resulted in enhancement of the oxidase activity to generate O2- anion and was accompanied by substantial translocation of p47phox to membrane. However, p47phox derived from the stimulated cell membrane had lost its NTP-binding ability, unlike that from the resting cytosol. These results suggest that the binding of NTP to p47phox may be involved in the process that activates the oxidase and is desensitized in translocated p47phox.

G protein coupling of human platelet V1 vascular vasopressin receptors

We used several approaches to identify the G protein coupled to V1 vascular arginine vasopressin (AVP) receptors of human platelets. In purified platelet membranes, high-affinity specific binding of [3H]AVP but not that of the V1 vascular antagonist [3H]d(CH2)5Tyr(Me)AVP was modulated by guanosine 5'-O-(3-thiotriphosphate) or sodium fluoride both in the presence and absence of MgCl2. AVP failed to modify the [alpha-32P]GTP labeling pattern or the cytosolic translocation of the 24- to 27-kDa GTP-binding proteins. AVP-stimulated GTPase activity of platelet membranes was blocked by antibodies specific for the COOH-terminal of the Gq alpha protein. AVP increased labeling of a 42-kDa platelet membrane protein by the photoreactive GTP analogue [alpha-32P]azidoanilido GTP. Immunoblotting of platelet proteins with various G protein-specific antibodies revealed that the 42-kDa protein labeled with [alpha-32P]azidoanilido GTP was immunoblotted only by antibodies specific for the alpha-subunit of GQ-11. Thus V1 vascular AVP receptors of human platelets are coupled in a divalent cation-dependent manner to a G protein belonging to the Gq-11 family.

Treatment of patients with chronic hepatitis using ursodeoxycholic acid

We treated 82 patients of chronic hepatitis using 300 mg. of ursodeoxycholic acid (UDCA) daily and observed them for a mean of 10 mo before and 16 mo after UDCA administration. Seven liver function tests (AST, ALT, ALP, LAP, GTP, Ch-E and T-cholest) were assessed monthly. The values were compared before and after the administration of UDCA. The AST, ALT, LAP and GTP improved significantly in the UDCA treated patients, whereas ALP, Ch-E and T-cholest. did not show any change throughout the study. Amongst the liver function tests that improved, the serum--GTP level, in particular decreased markedly and rapidly in patients treated with UDCA. Although UDCA 600-mg daily was administered in patients who showed lack of improvement with 300-mg UDCA treatment, no significant improvement was obtained. Repeated liver biopsies were carried out in six of the 42 patients in whom liver biopsy had been performed before the administration of UDCA. We detected no histological changes during the UDCA treatment. There were no side effects related to therapy with UDCA. In conclusion, we confirmed that UDCA is a safe and effective drug for treating patients with chronic hepatitis and may help in prevention of progression of the disease, particularly in patients with a high serum--GTP level.

Erythrocytic nucleoside triphosphates in marine fish

1. Whole blood purine nucleotides were determined in 19 species of selachians and 27 species of marine teleosts. Concomitant ATP and GTP were revealed inside the erythrocytes of almost all species studied. 2. ATP seems to be the main potential modulator of oxygen affinity in rays, sharks and teleosts, and GTP was not detected in only two teleost species. 3. The mean erythrocytic NTP concentration and the ratio between NTP and intraerythrocytic Hb concentrations in rays were the lowest (2.3 mM and 0.6 respectively), increasing in sharks (3.8 mM and 1.0), and further in teleosts (5.9 mM and 2.1). 4. The intraerythrocytic phosphate contents probably reflect different adaptative strategies associated with the fish habits and habitats, and with the Root effect expression.

Absolute requirement for GTP in activation of human neutrophil NADPH oxidase in a cell-free system: role of ATP in regenerating GTP

Guanine and/or adenine nucleotides appear to be involved in the activation of the superoxide-generating NADPH oxidase of phagocytic cells. Their precise roles, however, are unclear, as much of the evidence for their involvement comes from experiments in which nucleotides have been added to complex systems already rich in both endogenous nucleotides and enzymes capable of interconverting them. To circumvent this problem we have examined the role of nucleotides in neutrophil NADPH oxidase activation by using a cell-free system in which adenine and guanine nucleotide concentrations were carefully controlled and monitored by (i) depletion of endogenous nucleotides by extensive dialysis and charcoal treatment; (ii) reconstitution of the depleted system with reagents analyzed for purity; and (iii) measurement of nucleotide levels in cytosol preparations and in oxidase reaction mixtures by HPLC analysis. In contrast to previous reports that have demonstrated only a several-fold enhancement of oxidase activity by GTP or its analogs, we have shown that oxidase activation was absolutely dependent upon GTP in reactions containing dialyzed cytosol in which the total endogenous nucleotide levels were reduced by greater than 99.5%. Kinetic studies revealed that GTP is required at or before the rate-limiting step in oxidase activation. Two nonhydrolyzable analogs of GTP, guanosine 5'-(gamma-thio)triphosphate and guanylyl imidodiphosphate, were even more active than GTP, suggesting the involvement of one or more GTP-binding proteins. In contrast, ATP was neither necessary nor sufficient for oxidase activation. If reaction mixtures were contaminated with GDP and/or GMP, however, ATP (but not its nonhydrolyzable analog adenylyl imidodiphosphate) could indirectly support oxidase activation by means of endogenous enzymes that catalyze the ATP-dependent conversion of GMP and GDP to GTP.

A case of erythropoietic protoporphyria with severe liver dysfunction suggesting a close relationship between erythrocyte protoporphyrin levels and those of gamma-GTP

A case of erythropoietic protoporphyria (EPP) with severe acute abdominal pain and jaundice was reported. Erythrocyte protoporphyrin (PP) levels were constantly high, and liver histology showed a slight fibrosis with inflammatory infiltration. During the investigation period of 18 months, erythrocyte PP levels closely paralleled those of serum gamma-GTP.

Evidence that the signal-initiating membrane protein CD9 is associated with small GTP-binding proteins

F(ab')2 fragments of anti-CD9 mAb aggregate platelets by a thromboxane-dependent pathway implicating CD9 as signal initiating molecule. We demonstrate that mAbs directed against CD9, but not against GPIIb/IIIa specifically immunoprecipitate, from detergent lysates of human platelets, proteins of 25 and 26 kDa which bind [alpha 32P]GTP on nitrocellulose transfers. The binding is specific since it is blocked by GTP, but not by ATP. The GTP-binding proteins do not belong to a Mg(2+)-sensitive subset since they are unaffected by the addition of 2 microM-20 mM Mg2+. The observations demonstrate that CD9 is associated with selected small G-proteins.

Role of erythrocyte organic phosphates in blood oxygen transport in anemic quail

The effects of two different degrees of experimentally induced anemia and the consequent high percentage of circulating immature erythrocytes on oxygen affinity (pH 7.5 and 41 degrees C), erythrocyte organic phosphates, and Hb fractions have been studied in quail. Blood reticulocytes reached percentages of 24 and 69-87% in the first and second experiments, respectively. Variations in the phosphate levels found during the anemic process were related to the amount of circulating reticulocytes. The erythrocyte [ATP] (brackets indicate concentration) and [ATP]/[Hb] molar ratio increased with the release of reticulocytes and returned to control levels as they matured. The erythrocyte [inositol pentakisphosphate (InsP5)] decreased significantly when circulating reticulocytes showed their highest values, whereas there was hardly any effect on the [InsP5]/[Hb] molar ratio, which changed only slightly. Hb-O2 affinity also exhibited no statistical changes associated with acute anemia. These latter findings indicate that InsP5, at physiological concentrations, is the primary modulator of quail Hb function; the observed rise in [ATP] has no additional influence on Hb-O2 affinity. It is suggested that InsP5 tends to maintain the blood oxygen affinity in both mature erythrocytes and reticulocytes. The main compensatory response at blood level is a rapid bulk reticulocyte release from medulla.

Novel pharmacological strategies in the treatment of life-threatening cytomegalovirus infections. Clinical experience with continuous infusion 9-(1,3-dihydroxy-2-propoxymethyl) guanine

Two novel antiviral pharmacologic strategies were used for therapy of life- and sight-threatening cytomegalovirus (CMV) infection; these were continuous drug infusion by portable pump and individualized patient regimen. 9-(1,3-Dihydroxy-2-propoxymethyl)-guanine (DHPG), an active and recently licensed antiviral drug against cytomegalovirus infection, was administered to five immunocompromised patients with chorioretinitis (all patients), colitis (two), and pneumonitis (three). Through dosage escalation, correlations between plasma levels, toxicity (i.e., myelosuppression), and clinical benefit were ascertained for therapy of acute disease (pneumonitis) as well as long-term therapy (chorioretinitis). Resolution of viremia, pneumonitis, colitis, and chorioretinitis was accomplished with steady-state plasma levels of DHPG approximating the mean ID50 of CMV isolates. The most notable clinical benefit was survival from CMV pneumonia and stabilization of vision. Although no adverse toxicity occurred during the DHPG continuous long-term therapy, survival was limited by the underlying disease.

Metabolic control of the K+ channel of human red cells

The effects of cAMP, ATP and GTP on the Ca2(+)-dependent K+ channel of fresh (1-2 days) or cold-stored (28-36 days) human red cells were studied using atomic absorption flame photometry of Ca2(+)-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solution. When high-K+ ghosts were incubated in an isotonic Na+ medium, the rate constant of Ca2(+)-dependent K+ efflux was reduced by a half on increasing the theophylline concentration to 40 mM. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg2+ was added together with ATP, and it was abolished by raising free Ca2+ to the micromolar level. Like theophylline, isobutyl methylxanthine (10 mM) also affected K+ efflux. cAMP (0.2-0.5 mM), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K+ loss when the ghost free-Ca2+ level was below 1 microM, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2 mM) plus either theophylline (10 mM), or isobutyl methylxanthine (0.5 mM), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg2+ and it was not overcome by raising internal Ca2+. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K+ loss. Although this effect was observed in the absence of added Mg2+ (0.5 mM EDTA present), it was potentiated upon adding 2 mM Mg2+. The K+ efflux from ATP-loaded ghosts was not altered by dithio-bis-nitrobenzoic acid (10 mM) or acridine orange (100 microM), while it was increased two- to fourfold by incubating with MgF2 (10 mM), or MgF2 (10 mM) + theophylline (40 mM), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5 mM GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (gamma-32P)ATP under various conditions affecting K+ channel activity, was in direct correspondence to their effect on K+ efflux. The results suggest that the K+ channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins.

Characterization of GTP-gamma-S binding to isolated human platelet plasma membranes and its relationship with the stimulation of a phospholipase C activity

Binding parameters for the interaction of GTP-gamma-[35S] with isolated platelet plasma membranes have been studied. Analysis of the data by a non-linear curve fitting program indicates that the interaction can be satisfactory described by a model with a single, high affinity binding site (Kd = 0.3 +/- 0.07 microM and Bm = 0.4 +/- 0.2 nmoles of GTP-gamma-S/mg of membrane protein). Binding is selectively inhibited by GDP-beta-S and GMP-PNP (1 microM), but not affected by ATP, CTP, ITP, or UTP, even at mM concentration. Optimal conditions for the interaction were 30 degrees C and pH 8.0. Incubation of the isolated membranes with GTP-gamma-S results in a measurable phospholipase C activity (as detected both by a breakdown of phosphoinositides and an increase of inositide phosphates) which under our experimental conditions is only slightly enhanced by addition of cytosolic proteins. Our results indicate that platelet plasma membranes contain all the necessary elements for signal transduction through the diacylglycerol/inositolphosphates pathway.

Nucleotides in lymphocytes of human subjects with zinc deficiency

Cell-mediated immunity in human subjects is affected adversely as a result of zinc deficiency. The mechanism by which a deficiency of zinc may affect lymphocyte proliferation and functions, is not well understood at present. Nucleoside phosphorylase (NPase), a purine catabolic pathway enzyme, is zinc dependent, and a congenital deficiency of this enzyme is known to affect adversely cell-mediated immunity. This effect has been related to an accumulation of toxic nucleotides in lymphocytes as a result of NPase deficiency. Inasmuch as the effect of zinc deficiency on the activity of NPase and the levels of nucleotides in human lymphocytes has not been previously reported, we assayed these parameters in human subjects with zinc deficiency before and after zinc supplementation. A mild deficiency of zinc was diagnosed in those having decreased zinc in two out of three cell lineages (less than 42 micrograms in granulocytes, less than 48 micrograms in lymphocytes, and less than 1.70 microgram in platelets, per 10(10) cells). In comparison with five subjects with sufficient zinc, six subjects with zinc deficiency showed a decrease in the activity of NPase (p = 0.01), an increase in adenosine diphosphate (ADP) level (p = 0.008), a decreased adenosine triphosphate (ATP)-to-ADP ratio (p = 0.0001), and an increase in both guanosine triphosphate (GTP) (p = 0.02) and deoxyadenosine triphosphate (dGTP) (p = 0.04 in the lymphocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

A ras-related protein is phosphorylated and translocated by agonists that increase cAMP levels in human platelets

The antigenicity of platelet proteins was assayed against various monoclonal antibodies (mAbs) that recognize specific epitopes of the ras-encoded p21 protein. mAb M90, which detects the region of p21 protein within amino acids 107-130 and inhibits its GTP-binding activity, strongly reacted with a 22-kDa protein present in the particulate fraction of human platelets. Other mAbs against ras-encoded proteins, including Y13-259, which efficiently detects ras proteins from a variety of organisms, did not recognize the platelet 22-kDa protein. Transfer of the platelet 22-kDa protein to nitrocellulose paper showed that the protein binds [alpha-32P]GTP. Moreover, preincubation of the transferred protein with mAb M90 drastically reduced its GTP-binding activity. Treatment of platelets with iloprost, a prostacyclin analog, caused (i) a time-dependent increase of a 24-kDa protein that is recognized by mAb M90 in particulate and cytosolic fractions and (ii) the gradual decrease of the 22-kDa protein from the particulate fraction. When platelets were labeled with 32P and then treated with iloprost, the 24-kDa protein was found to be phosphorylated. The 32P-labeled 24-kDa protein was specifically immunoprecipitated by mAb M90. These results suggest that appearance of the 24-kDa protein results from phosphorylation of the 22-kDa protein, which shifts its mobility to a higher molecular mass area.

Receptor-regulated formation of GTP[gamma S] with subsequent persistent Gs-protein activation in membranes of human platelets

Preincubation of human platelet membranes with the ATP analog ATP[gamma S] led to persistent adenylate cyclase activation. This stimulation was increased by copreincubation with PGE1 and obliterated by removing endogenous GDP by the NTP-regenerating system, creatine phosphate plus creatine kinase. PGE1 partially reversed the action of the regenerating system. Control formation of GTP[gamma S] from ATP[gamma S] and GDP in platelet membranes was apparently not stimulated by PGE1. In contrast, in the presence of creatine phosphate plus creatine kinase, which prevented formation of GTP[gamma S], PGE1 stimulated formation of this GTP analog, by partially reversing the action of the NTP-regenerating system. The data indicate that GTP[gamma S] can be formed by a membrane-associated nucleoside diphosphokinase from ATP[gamma S] and GDP, resulting in persistent Gs-protein activation, and that this process can be stimulated by an agonist-activated receptor.

"In situ" characterization of guanine nucleotide-binding properties of erythrocyte membranes

Unsealed membranes from human erythrocytes bind GTP and GTP analogs according to first order kinetics, a single rate constant being observed. With [35S]GTP gamma S this is 0.15 +/- 0.2 min-1. Treatment of the membranes with detergents decreases binding considerably. Scatchard plots reveal uncomplicated patterns of ligand association, with Kd values of 10.2 +/- 2.3 nM [35S]GTP gamma S, of 18.2 +/- 4.3 nM [alpha-32P]GTP and of 28.6 +/- 3.5 nM [alpha-32P]GDP, respectively. The stoichiometry with the three ligands is strictly comparable, i.e. 65 +/- 7 picomoles/mg of membrane protein. Binding of each labeled nucleotide is competitively inhibited by the other two unlabeled ligands, the inhibition constants being very close to the corresponding Kd values. Metabolic depletion and subsequent repletion of intact erythrocytes result in membrane preparations still active in guanine nucleotide binding, with unmodified Kd values. However, the stoichiometry falls to 35 picomoles/mg protein with the "depleted" erythrocyte membranes and regains higher values (50 picomoles/mg protein) with the "repleted" cell membranes. Accordingly, the "in situ" characterization of guanine nucleotide-binding properties of erythrocyte membranes seems to represent a new tool for monitoring the metabolic state of intact erythrocytes.