Veni, Vidi, Vici: Immobilized Peptide-Based Conjugates as Tools for Capture, Analysis, and Transformation

Analysis of peptide biomarkers of pathological states of the organism is often a serious challenge, due to a very complex composition of the cell and insufficient sensitivity of the current analytical methods (including mass spectrometry). One of the possible ways to overcome this problem is sample enrichment by capturing the selected components using a specific solid support. Another option is increasing the detectability of the desired compound by its selective tagging. Appropriately modified and immobilized peptides can be used for these purposes. In addition, they find application in studying the specificity and activity of proteolytic enzymes. Immobilized heterocyclic peptide conjugates may serve as metal ligands, to form complexes used as catalysts or analytical markers. In this review, we describe various applications of immobilized peptides, including selective capturing of cysteine-containing peptides, tagging of the carbonyl compounds to increase the sensitivity of their detection, enrichment of biological samples in deoxyfructosylated peptides, and fishing out of tyrosine–containing peptides by the formation of azo bond. Moreover, the use of the one-bead-one-compound peptide library for the analysis of substrate specificity and activity of caspases is described. Furthermore, the evolution of immobilization from the solid support used in peptide synthesis to nanocarriers is presented. Taken together, the examples presented here demonstrate immobilized peptides as a multifunctional tool, which can be successfully used to solve multiple analytical problems.

Analysis of urinary nucleosides as helper tumor markers in hepatocellular carcinoma diagnosis

Hepatocellular carcinoma (HCC) is a common neoplasm in Taiwan, for which early diagnosis is difficult and the prognosis is usually poor. HCC is usually diagnosed by abdominal sonography and serum alpha-fetoprotein (AFP) detection. Modified nucleosides, regarded as indicators for the whole-body turnover of RNAs, are excreted in abnormal amounts in the urine of patients with malignancies and can serve as tumor markers. We analyzed the excretion patterns of urinary nucleosides from 25 HCC patients and 20 healthy volunteers by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS) under optimized conditions. The HPLC/ESI-MS/MS approach with selective reaction monitoring (SRM) allowed for the sensitive determination of nucleosides in human urine samples. The mean levels of the urinary nucleosides adenosine, cytidine, and inosine were significantly higher in HCC patients than healthy volunteers (average of 1.78-, 2.26-, and 1.47-fold, respectively). However, the mean levels of urinary 1-methyladenosine, 3-methylcytidine, uridine, and 2'-deoxyguanosine were not significantly different. Combined with the determination of serum AFP levels, the higher levels of urinary adenosine, cytidine, and inosine may be additional diagnosis markers for HCC in Taiwanese patients.

Analysis of urinary nucleosides as potential tumor markers in human colorectal cancer by high performance liquid chromatography/electrospray ionization tandem mass spectrometry

BACKGROUND

Increased levels of modified nucleosides have been observed in urine from patients suffering from several cancers. In this study, we evaluated whether urinary nucleosides can serve as potential tumor markers for colorectal cancer by high performance liquid chromatography-electrospray/tandem mass spectrometry (HPLC/ESI-MS/MS).

METHODS

A simple and specific method based on HPLC/ESI-MS/MS was developed to determine the urinary nucleosides from patients with colorectal cancer. We studied the excretion patterns of nucleosides in urine from 26 patients with colorectal cancer and 18 healthy controls.

RESULTS

The LC/MS/MS approach with selective reaction monitoring (SRM) allowed for the sensitive determination of nucleosides in human urine samples with colorectal cancer. The mean levels of 5 urinary nucleosides (adenosine, cytidine, N(2),N(2)-dimethylguanine, 8-hydroxy-2'-deoxyguanosine and uridine) were significantly higher in the patients with colorectal cancer than in the healthy adults.

CONCLUSIONS

The results indicate that urinary nucleosides determined by LC/MS/MS may be useful as biological markers for colorectal cancer. Our findings suggest that LC/MS/MS is a highly specific and sensitive method for rapidly screening a large number of nucleoside that may be useful as markers for cancer in humans.

Role of Nitrogen Lewis Basicity in Boronate Affinity Chromatography of Nucleosides

Urinary modified nucleosides have a potential role as cancer biomarkers, and most of the methods used in their study have utilized low-pressure phenylboronate affinity chromatography materials for the purification of the cis-diol-containing nucleosides. In this study, a boronate HPLC column was surprisingly shown not to trap the nucleosides as would be expected from experience with the classic Affigel 601 resin but showed only partial selectivity toward cis-diol groups while other groups exhibited better retention. In aprotic conditions, trapping of nucleosides was possible; however, the selectivity toward cis-diol-containing compounds was lost with the Lewis basicity of available nitrogens being the main determinant of retention. The experimental findings are compared to and confirmed by DFT calculations.

MALDI-TOF MS analysis of urinary nucleosides

As RNA turnover seems to be impaired in cancer patients, modified nucleosides have been evaluated as potential tumor markers. Modified nucleosides are mainly formed post-transcriptionally in tRNA, set free during RNA metabolism, and excreted in urine. Especially methylated nucleosides play an important role, as their levels are higher in urine from cancer patients. For structural elucidation of known and unknown nucleosides from urine samples of cancer patients, MALDI-TOF MS and MALDI-PSD were used for the first time. This technique generally ensures high sensitivity, mass resolution, and accuracy. In our analytical approach we prepurified nucleosides from urine by affinity chromatography and subsequently separated them by semipreparative high performance liquid chromatography. The different fractions were collected separately and analyzed by MALDI-TOF MS and PSD-MALDI using a mixture of six low molecular weight calibrants for internal or external calibration. The molecular totals formulas based on a mass accuracy of 10 ppm and below were calculated and a systematic data base search was performed. The inherent problem of the MALDI-technique, the reduced sensitivity for low molecular weight substances caused by matrix suppression effects, was reduced by our technique. We identified several nucleosides in urine, which were previously identified via retention times and UV spectra of standards after HPLC analysis. Eight further nucleosides were observed. This work demonstrates for the first time the potential of MALDI-TOF and PSD-MALDI in combination with semipreparative HPLC for assignment of nucleosides in urine. The particularly high mass accuracy of this mass spectrometric method provides opportunities for identifying unknown compounds.

Application of capillary electrophoresis in clinical chemistry: the clinical value of urinary modified nucleosides

Urinary modified nucleosides were determined by capillary electrophoresis using a 300 mM SDS-25 mM sodium tetraborate-50 mM sodium dihydrogenphosphate buffer. The nucleosides were extracted from urine by phenylboronate affinity gel chromatography. In cancer patients the levels of the modified nucleosides are generally elevated. By an artificial neural network method breast cancer patients were differentiated from normal individuals, which indicates that the modified nucleosides could be of clinical value as tumor markers.

Simultaneous determination of adenosine, S-adenosylhomocysteine and S-adenosylmethionine in biological samples using solid-phase extraction and high-performance liquid chromatography

A sensitive and rapid method for measuring simultaneously adenosine, S-adenosylhomocysteine and S-adenosylmethionine in renal tissue, and for the analysis of adenosine and S-adenosylhomocysteine concentrations in the urine is presented. Separation and quantification of the nucleosides are performed following solid-phase extraction by reversed-phase ion-pair high-performance liquid chromatography with a binary gradient system. N6-Methyladenosine is used as the internal standard. This method is characterized by an absolute recovery of over 90% of the nucleosides plus the following limits of quantification: 0.25-1.0 nmol/g wet weight for renal tissue and 0.25-0.5 microM for urine. The relative recovery (corrected for internal standard) of the three nucleosides ranges between 98.1 +/- 2.6% and 102.5 +/- 4.0% for renal tissue and urine, respectively (mean +/- S.D., n = 3). Since the adenosine content in kidney tissue increases instantly after the onset of ischemia, a stop freezing technique is mandatory to observe the tissue levels of the nucleosides under normoxic conditions. The resulting tissue contents of adenosine, S-adenosylhomocysteine and S-adenosylmethionine in normoxic rat kidney are 5.64 +/- 2.2, 0.67 +/- 0.18 and 46.2 +/- 1.9 nmol/g wet weight, respectively (mean +/- S.D., n = 6). Urine concentrations of adenosine and S-adenosylhomocysteine of man and rat are in the low microM range and are negatively correlated with urine flow-rate.

Chromatographic and electrophoretic methods for modified hemoglobins

The discovery of the clinically important glycohemoglobin adducts and their relation to diabetes mellitus have greatly stimulated the study of other minor post-translational modifications of hemoglobin. Chromatographic and electrophoretic procedures have played an important role in these studies. Today several hemoglobin adducts are known and the formation of adducts with glucose, phosphorylated carbohydrates, urea/cyanate, aspirin, vitamins, acetaldehyde, penicillin and acetyl CoA have been described. Furthermore, new adducts, such as those observed using hemoglobin as a biochemical marker monitoring environmental, occupational and lifestyle exposures to reactive toxic chemicals are constantly being reported. This review deals with chromatographic and electrophoretic separation methods available for the study of non-enzymatic post-translational modifications of hemoglobin. Suitability, perspectives and biomedical applications are discussed.

Protein-boronic acid conjugates and their binding to low-molecular-mass cis-diols and glycated hemoglobin

Different methods for covalent linkage of phenylboronic acid (PBA) to structural proteins and enzymes are presented. Protein-PBA conjugates, free in solution or immobilised on magnetizable polymer particles, were tested for their binding of D-sorbitol, D-mannose and glycohemoglobin (GHb). Similarly, alkaline phosphatase-PBA conjugates were used in an attempted enzyme-linked sorbent assay for the detection of GHb. Affinity chromatography on immobilised D-mannose and gel chromatographic studies of protein-PBA complexes with [14C]sorbitol, clearly illustrated a low affinity of the interaction studied. Glycated hemoglobin could not be detected using the enzyme-linked sorbent assay approach. However, GHb was found to be specifically retained on columns filled with protein-PBA-coated particles as affinity matrix, enabling the glycation level of blood samples to be determined.

Regulation of extracellular adenosine levels in the striatum of aging rats

Extracellular adenosine concentrations, evaluated by microdialysis in the striatum of young and aged rats, were 66.8 +/- 0.7 and 71.6 +/- 1.0 nM, respectively. The adenosine deaminase inhibitor EHNA (100 microM) increased the extracellular adenosine levels in young rats only. The adenosine kinase inhibitor iodotubercidin (10 microM) brought about the same increase in young and aged rats. In aged rats the resting adenosine outflow was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist D-(-)-2-amino-7-phophonoheptanoic acid (D-AP7) (1 mM). It is concluded that extracellular levels of adenosine in the striatum are not affected by age, irrespective of the differences in adenosine deaminase activity and that the release of excitatory amino acids is responsible for much of resting adenosine outflow in aged but not in young rats.

Coupling of m-aminophenylboronic acid to s-triazine-activated Sephacryl: use in the affinity chromatography of glycated hemoglobins

An improved process is described for covalent coupling of m-aminobenzeneboric acid to s-triazine-activated Sephacryl matrices. The derivatized Sephacryl gel contained up to 150-200 mumol boronate per ml. It has been applied to the separation of glycated and non-glycated hemoglobins (Hbs) present in red-cell hemolysate. The new bioaffinity support was evaluated by the analysis of 67 diabetic patients and 20 normal adults. The mean value for glycated Hb was 6.6 +/- 0.8% for non-diabetics and 11.2 +/- 2.9% for diabetics. The method effects group-specific separation between glycated and non-glycated Hbs even in presence of foetal Hb and abnormal Hb variants. There is an excellent correlation between the glycated Hb levels obtained by the new method and two established procedures, namely high-performance liquid chromatography (r = 0.933) and affinity Merckotest (r = 0.991). The inter-assay and intra-assay coefficient of variations of less than 3.0% suggest that the method is reproducible. The results indicate that the method may serve as an alternative procedure for the study of glycated proteins. The s-triazine-activated Sephacryl could also be used for immobilizing enzymes and for preparing biospecific absorbents.

The precise radioimmunoassay of adenosine: minimization of sample collection artifacts and immunocrossreactivity

Anti-adenosine antibodies were produced in rabbits immunized with N6-carboxymethyladenosine conjugated to methyl albumin. 125I-N6-Aminobenzyladenosine was synthesized and used as a high-specific-activity, high-affinity ligand. A radioimmunoassay (RIA) was developed that can detect 6.25 nM (312.5 fmol) of underivatized adenosine and cross-reacts less than 0.02% with adenine nucleotides and guanosine and not at all with 1 mM inosine. The sensitivity of the RIA can be increased to a detection limit of 0.125 nM (6.25 fmol) by derivitizing samples with benzyl bromide to form N6-benzyladenosine. The assay was adapted to an automated RIA procedure. Assay precision was increased by: (i) inhibiting slight adenosine deaminase activity present in anti-sera; (ii) treating buffers and albumin used in the RIA with charcoal to remove contaminating adenosine; and (iii) correcting for a small but variable component of immunoreactivity not attributable to adenosine. A second antibody prepared with a 2',3'-disuccinyladenosine-albumin conjugate was also found to detect some non-adenosine-mediated immunoreactivity in plasma samples. Immunointerference in human plasma was eliminated in samples treated with ZnSO4/Ba(OH)2 or partially purified over C18 Sep Paks to remove nucleotides and assayed after sample benzylation or succinylation. Human blood was mixed with a novel "stop" solution that was optimized to inhibit adenosine formation from AMP by greater than 99% and to inhibit adenosine uptake into red cells and degradation by greater than 94%. Human plasma/stop solution was assayed by RIA and HPLC with equivalent results.

Direct labeling of DNA-adducts formed from carcinogenic diol-epoxides with a fluorescent reporter compound specific for the cis vic-diol group

The fluorescent reporter group, N-(5-FLUORESCEINYL),N'-(3-BORONATOPHENYL)THIOUREA (FABA) has been synthesized. This boronate group makes the reporter specific for cis vic-diols. The reporter group is bound to DNA-adducts formed from the reaction of calf-thymus DNA and benzanthracene trans-10,11-dihydrodiol,8,9-epoxide (anti), benzo(a)anthracene-trans-3,4-dihydrodiol-1,2-epoxide (anti) but is not bound to 3-methylcholanthrene-11,12 dihydro-epoxide. Femtomole quantities of adduct may be detected.

Fluorometric determination of plasma adenosine concentrations using high-performance liquid chromatography

Methods are described for the fluorometric determination of plasma adenosine concentrations, using HPLC. Plasma obtained from blood of dogs treated with erythro-(2-hydroxy-3-nonyl)adenine hydrochloride and dipyridamole was deproteinized with perchloric acid and the neutralized sample was put sequentially onto a SepPak C18 and boronic acid affinity column. Subsequently, adenosine in the final elution was converted to 1,N6-ethenoadenosine and was quantitated by HPLC with a fluorescence detector. The percentage recovery of adenosine added to the deproteinized plasma was nearly 100%. In the adenosine deaminase treated plasma, the increase in adenosine concentration of even 4 nM can be accurately determined. The control renal venous plasma concentrations of adenosine in anesthetized dogs were 19.9 +/- 1.9 nM, a significantly higher value than the corresponding arterial concentrations (12.7 +/- 1.1 nM), thereby suggesting the renal release of adenosine. This release was markedly enhanced following the removal of the renal arterial occlusion. Thus, taken together with the in vivo results, the present method is sensitive, hence most useful for the determination of plasma adenosine concentrations.

Ribonucleosides as minor milk constituents

Ribonucleosides are minor milk constituents and show a typical pattern which is assumed to be species-specific. As well as the unmodified components adenosine, cytidine, guanosine, inosine, and uridine, modified compounds such as Nl-methyladenosine and N6-carbamoylthreonyladenosine--products of the transfer RNA catabolism--have been identified and quantified in individual and bulk herd (race: German black pied) milk samples throughout a whole lactation period. The results of our longitudinal study have shown that--with the exception of the colostral phase--the levels of these minor constituents vary only slightly throughout lactation. These findings imply that ribonucleosides are useful for characterizing milk of different species and technological treatment. Ribonucleosides were determined and balanced, for example, in the course of the churning process, showing that the pattern of these minor milk constituents is useful as a "fingerprint" that allows differentiation between the three butter types defined in the German Federal Butter Ordinance.

Boronic acids for affinity chromatography: spectral methods for determinations of ionization and diol-binding constants

Arylboronic acids attached to solid matrices have proved useful for the diol-specific chromatography of biomolecules and affinity purification of enzymes by exchangeable-ligand chromatography. The latter use has been limited by the intrinsic ionization constant (pKa approximately 9) of the most common commercial products. The synthesis of several arylboronic acids with ionization constants near neutrality are described, and the application of a new general, spectral-difference method for determining acid ionization constants and formation constants with fructose is developed. In particular 4-(N-methyl) carboxamido-benzeneboronic acid was found to have a pKa of 7.86 and a formation constant with D-fructose of 8600. It was stable toward acid or base hydrolysis. We suggest that 4-carboxybenzeneboronic acid might be useful for preparing matrices for enzyme affinity chromatography.

On-line sample processing and analysis of diol compounds in biological fluids

We developed a coupled dual column system with an optional post-column derivatization for on-line sample processing, trace enrichment and analysis of aromatic 1,2-diol and aliphatic cis-diol biomolecules (e.g. catecholamines, ribonucleosides). The fully automated high-performance liquid chromatography analyzer tolerates the direct injection of proteinaceous fluids by use of a unique bonded-phase precolumn material which allows the simultaneous performance of covalent affinity and size-exclusion chromatography.

Chemiluminescent determination of adenosine, inosine, and hypoxanthine/xanthine

A fully automatic method for analysis of adenosine, inosine, and hypoxanthine/xanthine which combines the specificity of enzymatic catalysis and sensitivity of chemiluminescence is presented. The hydrogen peroxide formed by sequential catabolism of purines to uric acid is detected by the oxidation of luminol in the presence of peroxidase. The method takes advantage of the fact that light output in the H2O2/luminol system is transient. By adopting a two-step procedure this feature enables selective determination of adenosine, inosine, and hypoxanthine/xanthine. In step 1 any purines lower in the catabolic sequence than the analyte under study are converted to uric acid. Light emission is allowed to decay to baseline levels. During step 2 the analyte is selectively degraded. The H2O2 formed leads to a new light emission which is proportional to the square of analyte concentration. The method can be performed with commercially available reagents and enzymes and requires minimal processing of biological samples. Excellent agreement has been obtained with HPLC analysis. Sensitivity is in the range of 5-10 nmol/liter in as little as 0.1 ml. More than 200 samples per day can be analyzed by a single operator.

Direct clean-up and analysis of ribonucleosides in physiological fluids

We describe the group-selective separation and quantification of unmodified, modified and hypermodified ribonucleosides in physiological fluids (urine, serum) by on-line multidimensional high-performance affinity chromatography (HPAC)-reversed-phase liquid chromatography (RPLC). The excretion levels and patterns of ribonucleosides such as N1-methyladenosine, N1-methylinosine, N2-methylguanosine, N2-dimethylguanosine, N6-carbamoylthreonyladenosine and 2-pyridone-5-carboxamido-N-ribofuranoside were determined in urines from a control group and from patients with different diseases. The HPAC-RPLC method applied represents a powerful tool, e.g. as a non-invasive screening test, a method to investigate disorders in ribonucleoside and/or RNA metabolism, a method for drug monitoring during nucleoside chemotherapy, and a method to study renal ribonucleoside reutilization.

Batch separation of uracil derivatives from urine

Commercial mixed-bed deionizing resins have been modified by conversion to the sulfonic acid and acetate forms and used to prepare extracts of urine that contains uracil derivatives (pKa greater than 8). A rapid batch extraction process, requiring less than 15 min is described for the first time that can be used either for rapid spectrophotometric screening of altered levels of these uracil derivatives or for accurate measurement of uracil and pseudouridine by high-performance liquid chromatography on reversed-phase columns using 0.002 M ammonium formate (pH 6) in water as the solvent. The spectrophotometric assay is based on the spectral shift of uracil derivatives when neutral aqueous solutions are made alkaline. After the batch extraction, the factor for conversion of absorbancy change to nmol is 0.0046 per nmol/ml for uracil and 0.0035 per nmol/ml for pseudouridine. Pseudouridine levels in two test urines were found to be 24 and 19.3 nmol/mumol of creatinine, which are consistent with published values of 17.6-24.

Separation of purine bases, nucleosides and nucleotides by a column-switching technique combining reversed-phase and anion-exchange high-performance liquid chromatography

An on-line two-stage column chromatographic technique is described which combines reversed-phase and anion-exchange chromatography for the separation of purine nucleic acid components. The elution program applied, consisting of two gradient programmes, provides a separation of bases and nucleosides on the octadecyl silica column and a separation of the nucleotides on the anion-exchange column to which they have been switched at the beginning of the elution. This method is easy to modify for special problems and can be used when establishing a complete profile of purines.

Urinary nucleosides in leukemia: laboratory and clinical applications

Urinary nucleosides offer a number of useful laboratory and clinical applications in the study and analysis of leukemia. There are significant differences in the excretion of modified nucleosides between normal individuals and individuals with various forms of leukemia, as well as between leukemia patients at opposite ends of the clinical spectrum, i.e., those with active disease and those in remission. The nucleoside excretion levels correlate to bone marrow tumor burden in certain forms of leukemia, and limited serial data indicate the potential value of the nucleosides for predicting relapse before the disease deterioration can be recognized clinically. In addition, it may be feasible to assess the effectiveness of chemotherapy used in the treatment of leukemia much more rapidly with the urinary nucleoside markers than with conventional invasive methods.

Measurement of urinary 2,3-dinor-thromboxane B2 and thromboxane B2 using bonded-phase phenylboronic acid columns and capillary gas chromatography--negative-ion chemical ionization mass spectrometry

The use of bonded-phase phenylboronic acid columns to selectively extract 2,3-dinor-thromboxane B2 and thromboxane B2 from urine is reported. The compounds were first derivatized as the methoxime and then applied to the phenylboronic acid columns. Subsequent purification by thin-layer chromatography and derivatization to the pentafluorobenzyl ester, trimethylsilyl ether followed by capillary gas chromatography-negative-ion chemical ionization mass spectrometry, monitoring specific ions, allows quantitation in the low-picogram/milliliter range. In healthy male volunteers, the median excretions of 2,3-dinor-thromboxane B2 and thromboxane B2 were 10.3 ng/h (range, 4.5-24 ng/h) and 2.8 ng/h (range, 0.5-7.3 ng/h), respectively. The method offers a noninvasive, specific approach to the study of thromboxane synthesis and platelet function in man. It is much less labor intensive than currently available methods employing electron-impact chromatography-mass spectrometry.

Assay of diadenosine tetraphosphate hydrolytic enzymes by boronate chromatography

A new procedure was described for assay of diadenosine tetraphosphate (Ap4A) hydrolases based on boronate chromatography. Potential reaction products, AMP, ADP, and ATP, of the hydrolysis of Ap4A were separated from residual substrate by chromatography on a boronate-derivatized cation-exchange resin, Bio-Rex 70. Separation was achieved by changing the concentrations of ethanol and ammonium acetate in the elution buffers. Picomole masses of products were detectable, blank dpm values were less than 0.5% of the total dpm, and auxiliary enzymes were not required. The procedure was specifically described for Ap4A pyrophosphohydrolase from Physarum polycephalum. The assay is generally applicable for dinucleoside polyphosphate hydrolases which hydrolyze other substrates such as Ap3A, Ap5A, Ap6A, and Gp4G. Dinucleotide polyphosphates are readily purified by chromatography on this boronate resin in a volatile buffer. Tes, Tricine, and Tris buffers significantly interfered with the chromatography of ATP.

High-performance liquid chromatographic separations on dihydroxyboryl-agarose

This paper (which is part of a series of articles on high-performance liquid chromatographic separations on agarose columns) describes the chromatographic behaviour of catecholamines, nucleosides, aminoacylated and non-aminoacylated tRNA, and glycosylated and non-glycosylated hemoglobins on agarose matrices derivatized with m-aminophenylboronic acid. These matrices primarily bind molecules with two vicinal hydroxy groups in the cis-configuration. The possible existence of secondary electrostatic and hydrophobic interactions has been studied with the aid of diagrams showing retention time as a function of pH and buffer concentration.

Depletion of blood plasma cytidine due to increased hepatocellular salvage in D-galactosamine-treated rats

Pyrimidine nucleosides in blood plasma of rats were identified by different procedures, including chemical peak shift methods, before their quantification by reversed-phase high-performance liquid chromatography. The concentrations of uridine, cytidine, and deoxycytidine were 1.0 +/- 0.2, 10.6 +/- 1.9, and 33.4 +/- 5.4 mumol/l, respectively. Six hours after the administration of D-galactosamine, the level of circulating cytidine was severely depressed to 25% of control values; uridine decreased to 54% while deoxycytidine remained unchanged. 24 h after the dose of the amino sugar, the levels of cytidine and uridine returned to control values in blood plasma. Total acid-soluble uridine, cytidine, guanosine, and adenosine was determined by reversed-phase HPLC after treatment of the neutralized acid-soluble supernatant of freeze-clamped rat livers with phosphodiesterase and alkaline phosphatase. Six hours after its administration, D-galactosamine induced a 2.2-fold and a 1.6-fold rise in total acid-soluble uridine and cytidine, respectively. Co-administration of N-(phosphonoacetyl)-L-aspartate, an inhibitor of de novo pyrimidine synthesis, suppressed the increase in total acid-soluble uridine observed after D-galactosamine alone, but was without effect on the enhancement of total cytidine. Three hours after D-galactosamine and 15 min after [2-14C] cytidine, there was a rapid fall of the labeled nucleoside in blood plasma to 49% of control animals accompanied by a 2.8-fold rise in the total radioactivity of rat liver homogenates. From these results it can be concluded that the hepatocellular rise in total acid-soluble cytidine after D-galactosamine, in contrast to the increase in total acid-soluble uridine, originates from the phosphorylation of blood plasma cytidine via the salvage pathway. The depletion of circulating cytidine in the presence of hepatocellular UTP deficiency points to the importance of the liver and the hepatic UTP level for the clearance of blood plasma cytidine.

Selection and characterization of a murine lymphoid cell line partially deficient in S-adenosylhomocysteine hydrolase

The exact role of S-adenosylhomocysteine hydrolase (EC 3.3.1.1) in mediating the toxic effects of adenosine toward mammalian cells has not been ascertained. The selection and characterization of S-adenosylhomocysteine hydrolase-deficient cell lines offers a biochemical genetic approach to this problem. In the present experiments, a mutant clone (Sahn 12) with 11-13% of wild-type S-adenosylhomocysteine hydrolase activity was selected from the murine T lymphoma cell line R 1.1 after mutagenesis and culture in adenosine, deoxycoformycin, uridine and homocysteine thiolactone-supplemented medium. In the presence of 0.5 mM homocysteine thiolactone and 10-200 microM adenosine, wild-type and mutant cells synthesized S-adenosylhomocysteine intracellularly at markedly different rates, and excreted the compound extracellularly. Thus, at time points up to 10 h, the S-adenosylhomocysteine hydrolase-deficient lymphoblasts required 5-10-fold higher concentrations of adenosine in the medium to achieve the same intracellular S-adenosylhomocysteine levels as wild-type cells. Similarly, the Sahn 12 lymphoblasts were 5-10-fold more resistant than R 1.1 cells to the toxic effects of adenosine plus homocysteine thiolactone. These results establish that (i) 11-13% of wild-type S-adenosylhomocysteine hydrolase activity is compatible with normal growth, (ii) in medium supplemented with both adenosine and homocysteine thiolactone, intracellular S-adenosylhomocysteine is synthesized by S-adenosylhomocysteine hydrolase, (iii) the net intracellular level of S-adenosylhomocysteine is determined by both the rate of S-adenosylhomocysteine synthesis and its rate of excretion, (iv) under such conditions the accumulation of S-adenosylhomocysteine is related to cytotoxicity, (v) in the absence of an exogenous homocysteine source, S-adenosylhomocysteine derives from endogenous sources, and the accumulation of S-adenosylhomocysteine is not the primary cause of adenosine induced cytotoxicity.

RNA turnover in cultured hamster embryo cells: identification of modified nucleoside end products

Fourteen methylated nucleosides (N-2-dimethylGuo, N-2-methylGuo, N-1-methylGuo, N-5-methylUrd, N-3-methylUrd, N-1-methylAdo, N-3-methylCyd, N-5-methylCyd, N-1-methyllno, 2'-Omethyl-Cyd, 2'-O-methylUrd, 2'-0-methylGuo, 2'-0-methyllno, and thymidine) and one methylated base (m7Gua) have been identified as normal excretion products of cultured hamster embryo cells. The methylated nucleosides are excreted in the culture media subsequent to RNA turnover. The excretion pattern of the base-methylated nucleosides was determined by continuous labeling of serum-stimulated quiescent hamster embryo cells with [3H-CH3]methionine and measurement of radioactivity in the excreted nucleosides between 23 and 81 1/2 hours after the label was added. These nucleosides accumulate exponentially until a maximum level is reached after 60 hours. These maximum levels were maintained for at least an additional 20 hours.