PYRIMIDINE METABOLISM

Predicting 5-fluorouracil toxicity: DPD genotype and 5,6-dihydrouracil:uracil ratio

AIM

Decreased DPD activity is a major cause of 5-fluorouracil (5-FU) toxicity, but known reduced-function variants in the DPD gene (DPYD) explain only a part of DPD-related 5-FU toxicities. Here, we evaluated the baseline (pretherapeutic) plasma 5,6-dihydrouracil:uracil (UH2:U) ratio as a marker of DPD activity in the context of DPYD genotypes.

MATERIALS & METHODS

DPYD variants were genotyped and plasma U, UH2 and 5-FU concentrations were determined by liquid chromatography-tandem mass spectrometry in 320 healthy blood donors and 28 cancer patients receiving 5-FU-based chemotherapy.

RESULTS

Baseline UH2:U ratios were strongly correlated with generally low and highly variable U concentrations. Reduced-function DPYD variants were only weakly associated with lower baseline UH2:U ratios. However, the interindividual variability in the UH2:U ratio was reduced and a stronger correlation between ratios and 5-FU exposure was observed in cancer patients during 5-FU administration.

CONCLUSION

These results suggest that the baseline UH2:U plasma ratio in most individuals reflects the nonsaturated state of DPD and is not predictive of decreased DPD activity. It may, however, be highly predictive at increased substrate concentrations, as observed during 5-FU administration.

A rapid HPLC-ESI-MS/MS method for determination of dihydrouracil/uracil ratio in plasma: evaluation of toxicity to 5-flurouracil in patients with gastrointestinal cancer

BACKGROUND

A liquid chromatography-tandem mass spectrometry method for the simultaneous quantitation of endogenous uracil (U) and dihydrouracil (UH2) was developed and tested in a Brazilian population of patients with gastrointestinal cancer previously exposed to 5-fluorouracil (5FU).

METHODS

The analytes were extracted by a liquid-liquid method using 5-clorouracil as internal standard. The separation was performed on a reversed-phase XTerra C18 column with a mobile phase composed of methanol and aqueous 0.1% ammonium hydroxide (15:85). Mass spectrometry detection was carried out using negative electrospray ionization and selected reaction monitoring. Bovine serum albumin was employed as an alternative matrix to prepare the calibration standards, aiming to avoid the measurement of physiologic U and UH2. Calibration curves were constructed over the range of 5-200 ng/mL for U and 10-500 ng/mL for UH2.

RESULTS

The mean RSD values in the intrarun precision were 6.5% and 10.0% and in the interrun precision were 7.8% and 9.0% for U and UH2, respectively. The mean accuracy values were within the range of 90%-110% for both analytes. The analytes were stable in plasma under different conditions of temperature and time. The validated method was successfully applied to determine the plasma concentrations of U and UH2 in patients with gastrointestinal cancer (n = 32) previously treated with 5FU and for whom clinical toxicity was well documented. U concentrations varied from 21.8 to 56.6 ng/mL, whereas UH2 concentrations varied from 57.7 to 271.5 ng/mL. UH2/U ratio ranged from 1.56 to 6.18.

CONCLUSIONS

The method has proved to provide a quick, reliable, and reproducible quantitation of the plasma concentrations of U and its metabolite UH2. The UH2/U ratios did not discriminate patients previously exposed to 5FU with and without severe toxicities, possibly due to the small sample. Further studies in a larger population are desirable.

Isomer differentiation via collision-induced dissociation: the case of protonated alpha-, beta2- and beta3-phenylalanines and their derivatives

A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments is used to examine the gas-phase fragmentation reactions of the three isomeric phenylalanine derivatives, alpha-phenylalanine, beta(2)-phenylalanine and beta(3)-phenylalanine. Under collision-induced dissociation (CID) conditions, each of the protonated phenylalanine isomers fragmented differently, allowing for differentiation. For example, protonated beta(3)-phenylalanine fragments almost exclusively via the loss of NH(3), only beta(2)-phenylalanine via the loss of H(2)O, while alpha- and beta(2)-phenylalanine fragment mainly via the combined losses of H(2)O + CO. Density functional theory (DFT) calculations were performed to examine the competition between NH(3) loss and the combined losses of H(2)O and CO for each of the protonated phenylalanine isomers. Three potential NH(3) loss pathways were studied: (i) an aryl-assisted neighbouring group; (ii) 1,2 hydride migration; and (iii) neighbouring group participation by the carboxyl group. Finally, we have shown that isomer differentiation is also possible when CID is performed on the protonated methyl ester and methyl amide derivatives of alpha-, beta(2)- and beta(3)-phenylalanines.

The significance of the expression of dihydropyrimidine dehydrogenase in prostate cancer

OBJECTIVE

To measure dihydropyrimidine dehydrogenase (DPD), an enzyme involved in the metabolism of 5-fluorouracil (5-FU), expression in prostate cancer and determine whether 5-chloro-2,4-dihydroxypyridine (CDHP), a potent inhibitor of DPD, enhances the antitumoral activity of 5-FU against prostate cancer.

PATIENTS, MATERIALS AND METHODS

In all, 44 prostate tissue specimens were obtained from men who had a radical prostatectomy alone for prostate cancer, and 38 specimens from men who had had neoadjuvant hormonal therapy. We analysed the cancerous tissue and normal prostate tissue for DPD expression using immunohistochemistry, and determined its prognostic significance. In cultured human prostate cancer lines (DU145 and LNCaP), we compared the cytotoxicity of 5-FU/CDHP with that of 5-FU alone. Finally, in experiments on immunodeficient mice, we studied the effect of oral administration of tegafur, a pro-drug for 5-FU, with or without CDHP on the growth of tumours introduced by injection of DU145 cells.

RESULTS

The expression of DPD was significantly higher in cancerous than normal prostate tissue; 36 of 44 (82%) specimens of prostate cancer expressed DPD, whereas only 25 of 44 (57%) specimens of normal prostate tissue expressed DPD. For men with prostate cancer who had radical prostatectomy alone, men with negative DPD expression tended to have a longer recurrence-free survival than those with positive expression; there were no recurrences in men with prostate cancer and negative DPD expression in the 5-year follow-up. DPD expression was significantly lower in men with prostate cancer who received neoadjuvant hormonal therapy. In vitro treatment of human prostate cancer cell lines with 5-FU/CDHP showed more cytotoxicity than with 5-FU treatment alone. Finally, DU145 tumours in mice treated with tegafur and CDHP were significantly smaller than in mice given tegafur alone.

CONCLUSION

The present study showed that DPD expression is elevated in prostate cancer, and indicate that DPD inhibitors might enhance the antitumour activity of 5-FU against prostate cancer.

Increased prevalence of dihydropyrimidine dehydrogenase deficiency in African-Americans compared with Caucasians

PURPOSE

African-American patients with colorectal cancer were observed to have increased 5-fluorouracil (5-FU)-associated toxicity (leukopenia and anemia) and decreased overall survival compared with Caucasian patients. One potential source for this disparity may be differences in 5-FU metabolism. Dihydropyrimidine dehydrogenase (DPD), the initial and rate-limiting enzyme of 5-FU catabolism, has previously been shown to have significant interpatient variability in activity. Several studies have linked reduced DPD activity to the development of 5-FU toxicity. Although the distribution of DPD enzyme activity and the frequency of DPD deficiency have been well characterized in the Caucasian population, the distribution of DPD enzyme activity and the frequency of DPD deficiency in the African-American population are unknown.

EXPERIMENTAL DESIGN

Healthy African-American (n=149) and Caucasian (n=109) volunteers were evaluated for DPD deficiency using both the [2-(13)C]uracil breath test and peripheral blood mononuclear cell DPD radioassay.

RESULTS

African-Americans showed significantly reduced peripheral blood mononuclear cell DPD enzyme activity compared with Caucasians (0.26+/-0.07 and 0.29+/-0.07 nmol/min/mg, respectively; P=0.002). The prevalence of DPD deficiency was 3-fold higher in African-Americans compared with Caucasians (8.0% and 2.8%, respectively; P=0.07). African-American women showed the highest prevalence of DPD deficiency compared with African-American men, Caucasian women, and Caucasian men (12.3%, 4.0%, 3.5%, and 1.9%, respectively).

CONCLUSION

These results indicate that African-Americans, particularly African-American women, have significantly reduced DPD enzyme activity compared with Caucasians, which may predispose this population to more 5-FU toxicity.

Hand-Foot Syndrome Variant in a Dihydropyrimidine Dehydrogenase–Deficient Patient Treated with Capecitabine

We present a case with dihydropyrimidine dehydrogenase (DPD) deficiency that manifested a variant of hand-foot syndrome (HFS). A 52-year-old man received capecitabine for adjuvant treatment of rectal cancer. On the ninth day of the first cycle, he presented to the clinic with a rash on the dorsum of both hands accompanied by symptoms of pain, erythema, swelling, and desquamation consistent with grade 3 HFS. The palms of his hands and soles of his feet were only tender with no apparent rash or discoloration. Dihydropyrimidine dehydrogenase activity was evaluated by radio assay using peripheral blood mononuclear cells. Dihydropyrimidine dehydrogenase activity was below normal: 0.12 nmol/minute/mg protein. Capecitabine was not resumed, and the rash resolved in 3 weeks with the use of pyridoxine and Udderly Smooth balm. Interestingly, HFS is rarely seen with 5-fluorouracil regimens containing selective DPD-inhibitors. This patient with DPD deficiency manifested a variant of HFS. The pharmacologic basis for the development of HFS in DPD-deficient patients warrants further investigation. Dihydropyrimidine dehydrogenase deficiency, if undiagnosed, can lead to death. In addition to severe to life-threatening toxicities akin to 5-fluorouracil, capecitabine can lead to unusual variants of common toxicities, including HFS, in DPD-deficient patients.

Physiologically based pharmacokinetic modelling of the three-step metabolism of pyrimidine using C-uracil as an in vivo probe

AIMS

Approximately 80% of uracil is excreted as beta-alanine, ammonia and CO2 via three sequential reactions. The activity of the first enzyme in this scheme, dihydropyrimidine dehydrogenase (DPD), is reported to be the key determinant of the cytotoxicity and side-effects of 5-fluorouracil. The aim of the present study was to re-evaluate the pharmacokinetics of uracil and its metabolites using a sensitive assay and based on a newly developed, physiologically based pharmacokinetic (PBPK) model.

METHODS

[2-(13)C]Uracil was orally administrated to 12 healthy males at escalating doses of 50, 100 and 200 mg, and the concentrations of [2-(13)C]uracil, [2-(13)C]5,6-dihydrouracil and beta-ureidopropionic acid (ureido-(13)C) in plasma and urine and (13)CO2 in breath were measured by liquid chromatography-tandem mass spectrometry and gas chromatograph-isotope ratio mass spectrometry, respectively.

RESULTS

The pharmacokinetics of [2-(13)C]uracil were nonlinear. The elimination half-life of [2-(13)C]5,6-dihydrouracil was 0.9-1.4 h, whereas that of [2-(13)C]uracil was 0.2-0.3 h. The AUC of [2-(13)C]5,6-dihydrouracil was 1.9-3.1 times greater than that of [2-(13)C]uracil, whereas that of ureido-(13)C was 0.13-0.23 times smaller. The pharmacokinetics of (13)CO2 in expired air were linear and the recovery of (13)CO2 was approximately 80% of the dose. The renal clearance of [2-(13)C]uracil was negligible.

CONCLUSION

A PBPK model to describe (13)CO2 exhalation after orally administered [2-(13)C]uracil was successfully developed. Using [2-(13)C]uracil as a probe, this model could be useful in identifying DPD-deficient patients at risk of 5-fluorouracil toxicity.

The uracil breath test in the assessment of dihydropyrimidine dehydrogenase activity: pharmacokinetic relationship between expired 13CO2 and plasma [2-13C]dihydrouracil

PURPOSE

Dihydropyrimidine dehydrogenase (DPD) deficiency is critical in the predisposition to 5-fluorouracil dose-related toxicity. We recently characterized the phenotypic [2-(13)C]uracil breath test (UraBT) with 96% specificity and 100% sensitivity for identification of DPD deficiency. In the present study, we characterize the relationships among UraBT-associated breath (13)CO(2) metabolite formation, plasma [2-(13)C]dihydrouracil formation, [2-(13)C]uracil clearance, and DPD activity.

EXPERIMENTAL DESIGN

An aqueous solution of [2-(13)C]uracil (6 mg/kg) was orally administered to 23 healthy volunteers and 8 cancer patients. Subsequently, breath (13)CO(2) concentrations and plasma [2-(13)C]dihydrouracil and [2-(13)C]uracil concentrations were determined over 180 minutes using IR spectroscopy and liquid chromatography-tandem mass spectrometry, respectively. Pharmacokinetic variables were determined using noncompartmental methods. Peripheral blood mononuclear cell (PBMC) DPD activity was measured using the DPD radioassay.

RESULTS

The UraBT identified 19 subjects with normal activity, 11 subjects with partial DPD deficiency, and 1 subject with profound DPD deficiency with PBMC DPD activity within the corresponding previously established ranges. UraBT breath (13)CO(2) DOB(50) significantly correlated with PBMC DPD activity (r(p) = 0.78), plasma [2-(13)C]uracil area under the curve (r(p) = -0.73), [2-(13)C]dihydrouracil appearance rate (r(p) = 0.76), and proportion of [2-(13)C]uracil metabolized to [2-(13)C]dihydrouracil (r(p) = 0.77; all Ps < 0.05).

CONCLUSIONS

UraBT breath (13)CO(2) pharmacokinetics parallel plasma [2-(13)C]uracil and [2-(13)C]dihydrouracil pharmacokinetics and are an accurate measure of interindividual variation in DPD activity. These pharmacokinetic data further support the future use of the UraBT as a screening test to identify DPD deficiency before 5-fluorouracil-based therapy.

Dihydropyrimidine dehydrogenase deficiency in an Indian population

BACKGROUND

Dihydropyrimidine dehydrogenase (DPD) deficiency is prevalent in 3-5% of the Caucasian population; however, the frequency of this pharmacogenetic syndrome in the Indian population and other racial and ethnic groups remains to be elucidated.

PATIENTS AND METHODS

We describe an Indian patient who presented to clinic for the treatment of gastric adenocarcinoma with 5-flurouracil (5-FU) therapy who subsequently was diagnosed with DPD deficiency by using the peripheral blood mononuclear cell (PBMC) DPD radioassay. This observation prompted us to examine the data generated from healthy (cancer-free) Indian subjects who were enrolled in a large population study to determine the sensitivity and specificity of the uracil breath test (UraBT) in the detection of DPD deficiency. Thirteen Indian subjects performed the UraBT. UraBT results were confirmed by PBMC DPD radioassay.

RESULTS

The Indian cancer patient demonstrated reduced DPD activity (0.11 nmol/min/mg protein) and severe 5-FU toxicities commonly associated with DPD deficiency. Of the 13 Indian subjects [ten men and three women; mean age, 26 years (range: 21-31 years)] enrolled in the UraBT, 12 Indian subjects demonstrated UraBT breath profiles and PBMC DPD activity within the normal range; one Indian subject demonstrated a reduced breath profile and partial DPD deficiency.

CONCLUSIONS

DPD deficiency is a pharmacogenetic syndrome which is also present in the Indian population. If undiagnosed, the DPD deficiency can lead to death. Future epidemiological studies would be helpful to determine the prevalence of DPD deficiency among racial and ethnic groups, allowing for the optimization of 5-FU chemotherapy.

Purification and expression of a processing protease on beta-alanine-oxoglutarate aminotransferase from rat liver mitochondria

GABA[arrow beta]AlaAT convertase is an endopeptidase that processes brain-type 4-aminobutyrate aminotransferase (GABA AT; EC 2.6.1.19) to liver-type beta-alanine-oxoglutarate aminotransferase (beta-AlaAT I) in rats. Its molecular mass was 180 kDa as determined by gel filtration. A subunit molecular mass of 97652 Da was measured using MALDI-TOF MS. The N-terminal sequence of the purified GABA[arrow beta]AlaAT convertase was SRVEVSKVLILGSGGLSIGQAGEFDYSGSQAV- and was identical to residues 418-449 of carbamoyl-phosphate synthetase I (CPS I; EC 1.2.1.27) purified from rat liver. The subunit molecular mass and the N-terminal amino acid sequence suggested that GABA[arrow beta]AlaAT convertase was the 418-1305 peptide of CPS I. An expression vector containing the coding region of the 418-1305 peptide of rat CPS I was transfected into NIH3T3 cells and the extract of the cells showed GABA[arrow beta]AlaAT convertase activity.

?2-amino acids?syntheses, occurrence in natural products, and components of ?-peptides1,2

Although they are less abundant than their alpha-analogues, beta-amino acids occur in nature both in free form and bound to peptides. Oligomers composed exclusively of beta-amino acids (so-called beta-peptides) might be the most thoroughly investigated peptidomimetics. Beside the facts that they are stable to metabolism, exhibit slow microbial degradation, and are inherently stable to proteases and peptidases, they fold into well-ordered secondary structures consisting of helices, turns, and sheets. In this respect, the most intriguing effects have been observed when beta2-amino acids are present in the beta-peptide backbone. This review gives an overview of the occurrence and importance of beta2-amino acids in nature, placing emphasis on the metabolic pathways of beta-aminoisobutyric acid (beta-Aib) and the appearance of beta2-amino acids as secondary metabolites or as components of more complex natural products, such as peptides, depsipeptides, lactones, and alkaloids. In addition, a compilation of the syntheses of both achiral and chiral beta2-amino acids is presented. While there are numerous routes to achiral beta2-amino acids, their EPC synthesis is currently the subject of many investigations. These include the diastereoselective alkylation and Mannich-type reactions of cyclic- or acyclic beta-homoglycine derivatives containing chiral auxiliaries, the Curtius degradation, the employment of transition-metal catalyzed reactions such as enantioselective hydrogenations, reductions, C-H insertions, and Michael-type additions, and the resolution of rac. beta2-amino acids, as well as several miscellaneous methods. In the last part of the review, the importance of beta2-amino acids in the formation of beta-peptide secondary structures is discussed.

Thymidine kinase in epithelial ovarian cancer: relationship with the other pyrimidine pathway enzymes

TK is a pyrimidine metabolic pathway enzyme involved in salvage DNA synthesis. What roles TK may play in epithelial ovarian cancer and the relationships between TK and the other pyrimidine pathway enzymes remain unclear. We examined TK1 gene expression by RT-PCR and related it to gene expression of TS, TP and DPD in 69 samples from epithelial ovarian cancer, 8 low-malignant-potential tumors, 16 benign ovarian tumors and 34 normal ovaries. Additionally, cytosolic and serum TK activities were determined by radioenzymatic assay. TK1 gene expression, the ratio of TK1 to TS gene expression, that of TK1 to TP and that of TK1 to DPD were significantly higher in epithelial ovarian cancer than in normal ovaries. In epithelial ovarian cancer, TK1 gene expression correlated with cytosolic and serum TK activities, TS and TP gene expression and the ratio of TP to DPD gene expression. Patients with high-TK1 gene expression had a significantly poorer survival than those with low TK1 gene expression. Combined analysis demonstrated that the relative risk of cancer death for tumors with high TK1, high TS and high TP gene expression was greater than that for tumors with high TK1 gene expression alone. TK1 gene expression together with TS, TP and DPD gene expression may play important roles in influencing the malignant behavior of epithelial ovarian cancer. Combination therapy including TK inhibitor is a possible therapeutic intervention in patients with epithelial ovarian cancer.

Gene expression for dihydropyrimidine dehydrogenase and thymidine phosphorylase influences outcome in epithelial ovarian cancer

PURPOSE

Dihydropyrimidine dehydrogenase (DPD) is a pyrimidine salvage enzyme responsible for degradation of thymine, which is produced from thymidine by thymidine phosphorylase (TP). Our purpose was to determine whether DPD affects prognosis in patients with epithelial ovarian cancer and how the two enzymes may interact in such effects.

PATIENTS AND METHODS

DPD gene expression was analyzed by reverse transcription-polymerase chain reaction in 27 samples from normal ovaries and the 85 epithelial ovarian cancers previously studied with regard to TP gene expression.

RESULTS

DPD gene expression was significantly lower in epithelial ovarian cancers than in normal ovaries (P: <.0001), whereas TP gene expression and the ratio of TP to DPD gene expression (TP:DPD) were significantly higher in epithelial ovarian cancer (P: <.0001 for both). In patients with epithelial ovarian cancer, DPD gene expression and the TP:DPD ratio did not significantly correlate with any clinicopathologic factors. Patients with a high TP:DPD ratio (higher than the median) had significantly poorer outcomes than those with lower ratios (P: =.0002). The difference in survival between groups with high and low TP:DPD ratios was greater than the difference between groups with high and low TP gene expression. Multivariate analysis showed the TP:DPD ratio to be the independent prognostic factor (P: =.0002). In tumors with high TP gene expression, low DPD gene expression significantly correlated with poor survival (P: =. 04).

CONCLUSION

Downregulation of DPD gene expression may enhance the negative prognostic effect of high TP gene expression in patients with epithelial ovarian cancer. Certain newly available chemotherapeutic choices may take the TP:DPD ratio into consideration.

A radiochemical assay for beta-ureidopropionase using radiolabeled N-carbamyl-beta-alanine obtained via hydrolysis of [2-(14)C]5, 6-dihydrouracil

A radiochemical assay was developed to measure the activity of beta-ureidopropionase in human liver homogenates which is based on the detection of the reaction product (14)CO(2) by liquid scintillation counting. Radiolabeled N-carbamyl-beta-alanine was prepared within 15 min by a simple hydrolysis of [2-(14)C]5, 6-dihydrouracil under alkaline conditions at 37 degrees C. The enzymatic reaction proved to be linear with time up to at least 3.5 h and protein concentrations up to at least 1 mg/ml. Human beta-ureidopropionase obeyed Michaelis-Menten kinetics with an apparent Km for N-carbamyl-beta-alanine of 15.5 +/- 1.9 microM. The assay proved to be very accurate and sensitive with an intraassay coefficient of variation of 2% and a detection limit of 28 pmol for the product CO(2).

The mature size of rat 4-aminobutyrate aminotransferase is different in liver and brain

The amino acid sequence predicted from a rat liver cDNA library indicated that the precursor of beta-AlaAT I (4-aminobutyrate aminotransferase, beta-alanine-oxoglutarate aminotransferase) consists of a mature enzyme of 466 amino acid residues and a 34-amino acid terminal segment, with amino acids attributed to the leader peptide. However, the mass of beta-AlaAT I from rat brain was larger than that from rat liver and kidney, as assessed by Western-blot analysis, mass spectroscopy and N-terminal sequencing. The mature form of beta-AlaAT I from the brain had an ISQAAAK- peptide on the N-terminus of the liver mature beta-AlaAT I. Brain beta-AlaAT I was cleaved to liver beta-AlaAT I when incubated with fresh mitochondrial extract from rat liver. These results imply that mature rat liver beta-AlaAT I is proteolytically cleaved in two steps. The first cleavage of the motif XRX( downward arrow)XS is performed by a mitochondrial processing peptidase, yielding an intermediate-sized protein which is the mature brain beta-AlaAT I. The second cleavage, which generates the mature liver beta-AlaAT I, is also carried out by a mitochondrial endopeptidase. The second peptidase is active in liver but lacking in brain.

Molecular cloning and sequencing of a cDNA encoding dihydropyrimidinase from the rat liver

A cDNA encoding dihydropyrimidinase has been isolated from a rat cDNA library. The N-terminal and an internal amino acid sequences were determined, and PCR primers were designed based on these sequences. Using a cDNA fragment amplified by RT-PCR with these primers, three cDNA clones were isolated from a rat liver library. The clone with the longest insert of 2129 bp contained a 1557 bp open reading frame encoding a polypeptide of 519 residues with a molecular mass of 56,832 Da.

Analysis of cyclic feed intake in rats fed on a zinc-deficient diet and the level of dihydropyrimidinase (EC 3.5.2.2)

The body weight and feed intake of rats fed on a Zn-deficient diet for 28 d were reduced compared with those of control rats. The feed intakes of the Zn-deficient and control groups during the period were 10.2 (SE 0.3) and 15.7 (SE 0.2) g/d respectively. Cyclic variations in feed intake and body-weight changes were found in analysis not only of all the data for five rats but also that in each individual rat. Cosinor analysis revealed that the cyclical period of both the feed intake and body-weight change in the Zn-deficient rats was 3.5 (SE 0.1) d. The mesor and amplitude value of the feed intake in the Zn-deficient rats was 10.1 (SE 0.4) g/d and 3.5 (SE 0.5) g/d respectively, and that of body-weight change was 1.4 (SE 0.1) g/d and 7.9 (SE 1.3) g/d respectively. Among pyrimidine-catabolizing enzymes, dihydropyrimidinase (EC 3.5.2.2) activity showed significant retardation in the Zn-deficient rat liver with decrease of the enzyme protein. The ratio of apo-form to holo-form dihydropyrimidinase in the liver was not affected by the Zn-deficient diet.

Purification, characterization and inhibition of dihydropyrimidinase from rat liver

Dihydropyrimidinase (DHPase) was purified 564-fold over the initial rat liver extract, using heat, ammonium sulfate fractionation, DEAE-Sepharose CL-6B, carboxymethyl-Sepharose CL-6B, hydroxyapatite and Sephacryl S-300 chromatography. The purified enzyme was shown to be homogeneous by gel electrophoresis both in the presence and absence of SDS. Its molecular mass, determined by gel filtration, was 215 kDa and the subunit mass was 54 kDa. DHPase catalyzed the reversible cyclization of 5,6-dihydrouracil (H2Ura) to N-carbamoyl-beta-alanine or 5,6-dihydrothymine (H2Thy) to N-carbamoyl-beta-aminoisobutyric acid. Authentic 5-bromo-5,6-dihydrouracil (BrH2Ura) and commercially available H2Thy were racemic. However, these 5-substituted 5,6-dihydropyrimidines were hydrolyzed by over 96% and 98%, respectively, by DHPase. These results suggest that dihydropyrimidinase has no stereo specificities for 5-substituents of H2Ura. The addition of H2Ura and H2Thy competitively inhibited the enzyme activity against BrH2Ura. However, the addition of N-carbamoyl-beta-alanine or N-carbamoyl-beta-amino-isobutyric acid showed hyperbolic mixed-type inhibition, when BrH2Ura was used as the substrate. The values of the dissociation constants of BrH2Ura, N-carbamoyl-beta-alanine and N-carbamoyl-beta-aminoisobutyric acid were 17 microM, 0.38 mM and 0.38 mM, respectively. DHPase from the rat liver contains 4 mol Zn2+/mol active enzyme, presumably one atom/subunit. Zn2+ also inhibited the hydrolysis of BrH2Ura by the enzyme. The Ki for Zn2+ as an inhibitor of DHPase was 23 microM, and the maximum rate of inactivation was 0.057 min-1 at 37 degrees C. H2Ura and H2Thy protected the enzyme activity from Zn2+ inactivation.

5-Ethynyluracil (776C85): inactivation of dihydropyrimidine dehydrogenase in vivo

5-Ethynyluracil (776C85), a potent, mechanism-based, irreversible inactivator (Porter et al., J Biol Chem 267:5236-5242, 1992) of purified dihydropyrimidine dehydrogenase (DPD, uracil reductase, EC 1.3.1.2), readily inactivated DPD in vivo. DPD was assayed in tissue extracts by measuring the release of 14CO2 from [2-14C]uracil with an improved method. Specific activities from 0.1 to > 1000 U/mg protein were reproducibly measured. After rats were orally dosed with 20 micrograms/kg 5-ethynyluracil, liver, intestinal mucosa, lung, and spleen DPD were inactivated by 83-94%. The dose required to inactivate rat liver, rat brain, and mouse liver DPD by 50% was 1.8, 11, and 8.9 micrograms/kg, respectively. Rat liver DPD was inactivated completely within 25 min after an oral dose of 500 micrograms/kg 5-ethynyluracil. New DPD was synthesized with a half-time of 63 hr. We also developed an assay based on stoichiometric inactivation of DPD by 5-ethynyluracil to measure 5-ethynyluracil in plasma samples. Samples containing 5-ethynyluracil were incubated with rat liver extract for 24 hr at 12 degrees and then assayed for DPD. DPD activity decreased linearly with the concentration of 5-ethynyluracil (between 0 and 20 nM 5-ethynyluracil). The assay could detect 5-ethynyluracil at concentrations as low as 6 nM in human plasma and was not affected by high concentrations of uracil.

Identity of D-3-aminoisobutyrate-pyruvate aminotransferase with alanine-glyoxylate aminotransferase 2

D-3-Aminoisobutyrate-pyruvate aminotransferase (EC 2.6.1.40) and alanine-glyoxylate aminotransferase 2 (EC 2.6.1.44) were co-purified from rat liver as a single protein. The ratio of the two activities remained constant after Sephacryl S-200 chromatography and chromatofocussing. The Km value for beta-alanine as a substrate with 1 mM glyloxylate as amino group acceptor was 1.4 mM. The activity was inhibited by (S)-alanine with Ki = 2.2 mM. The Km for (S)-alanine as substrate with 1 mM glyoxylate as amino group was 6 mM. This activity was inhibited competitively by beta-alanine with Ki = 0.7 mM. (R)-3-aminoisobutyric acid, 5-aminolevulinic acid, NG,NG'-dimethyl-(S)-arginine, and (S)-2-aminobutyric acid were active competitively with respect to beta-alanine with Km of 0.12 mM, 2.1 mM, 6.4 mM and 11.3 mM, respectively. Antiserum to rat liver D-3-aminoisobutyrate-pyruvate aminotransferase inhibited alanine-glyoxylate aminotransferase activity in rat liver in the same way as that of D-3-aminoisobutyrate-pyruvate aminotransferase. Alanine-glyoxylate aminotransferase activity and D-3-aminoisobutyrate-pyruvate aminotransferase activities were inactivated competitively with respect to beta-alanine by 5-fluorouracil and 6-azauracil, which are chemotherapeutic reagents used to cancer. These experiments indicate that D-3-aminoisobutyrate-pyruvate aminotransferase is identical with alanine-glyoxylate aminotransferase 2, aminolevulinate aminotransferase, 2-aminobutyrate aminotransferase and dimetylarginine-pyruvate aminotransferase.

Inhibition of fluoropyrimidine catabolism by benzyloxybenzyluracil. Possible relevance to regional chemotherapy

Regional infusion with fluoropyrimidines is useful in the treatment of hepatic metastases. However, the effectiveness of regional infusion is minimized by rapid degradation of 5-fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) by the liver which limits the availability of drug for anabolism to active metabolites. 5-Benzyloxybenzyluracil (BBU) is a potent inhibitor of dihydropyrimidine dehydrogenase (DPD), the initial enzyme in FUra catabolism (Naguib FMN, el Kouni MH and Cha S, Biochem Pharmacol 38: 1471-1480, 1989). The effect of BBU on fluoropyrimidine catabolism in the liver was evaluated using the isolated perfused rat liver (IPRL). BBU infused at 0.35 microM over the course of 1 hr demonstrated no hepatotoxicity as measured by bile flow, O2 uptake and lactate dehydrogenase leakage. The effect of BBU (0.35 microM) on the catabolism of FUra (10 microM) or FdUrd (10 microM) was quantitated by HPLC at 5- or 10-min intervals over a 1-hr period. BBU maximally inhibited FUra catabolism by approximately 83%. Further studies utilizing short-term (20 min) infusion of BBU prior to administration of FUra suggested that the inhibition of DPD was reversible. While FdUrd phosphorolysis was not affected, subsequent catabolism of FUra decreased by 70%. Studies on isolated hepatocytes indicated that the increased FUra level in perfusate resulted from inhibition of FUra catabolism and not from inhibition of FUra transport. The significant inhibition of FUra catabolism suggests that BBU may be useful in modulating regional chemotherapy by these fluoropyrimidines.

Purification, characterization and inhibition of D-3-aminoisobutyrate aminotransferase from the rat liver

D-3-Aminoisobutyrate-pyruvate aminotransferase was purified 2000-fold from rat liver extract using heat treatment, ammonium sulfate fractionation, carboxylmethyl-Sepharose CL-6B, DEAE-Sepharose CL-6B, hydroxyapatite, Sephacryl S-200 and electrofocusing chromatographies. The purified enzyme was shown to be homogeneous by gel electrophoresis both in the presence and absence of SDS. Its molecular mass, determined by gel filtration, was 220 kDa and the subunit molecular mass was 52 kDa. The enzyme exhibited absorption maxima at 280 nm and 412 nm with a shoulder at 330 nm at neutral pH. The pH optimum for enzyme activity was 9.5 and the Km values for beta-alanine and pyruvic acid were calculated to be 0.81 mM and 0.45 mM, respectively. The purified enzyme catalyzed the transamination of omega-amino acids; beta-alanine and D-3-aminoisobutyric acid served as good amino donors, and pyruvic acid, glyoxylic acid and oxaloacetic acid were favorable amino acceptors. 6-Azauracil and 6-azathymine were found to be potent inhibitors of purified rat liver D-3-aminoisobutyrate-pyruvate aminotransferase. 6-Azauracil acted as a competitive inhibitor with respect to beta-alanine, and was an uncompetitive inhibitor with respect to pyruvic acid with a Ki of approximately 8.9 mM.

Metabolism of pyrimidine analogues and their nucleosides

The pyrimidine antimetabolite drugs consist of base and nucleoside analogues of the naturally occurring pyrimidines uracil, thymine and cytosine. As is typical of antimetabolites, these drugs have a strong structural similarity to endogenous nucleic acid precursors. The structural differences are usually substitutions at one of the carbons in the pyrimidine ring itself or substitutions at on of the hydrogens attached to the ring of the pyrimidine or sugar (ribose or deoxyribose). Despite the differences noted above, these analogues, can still be taken up into cells and then metabolized via anabolic or catabolic pathways used by endogenous pyrimidines. Cytotoxicity results when the antimetabolite either is incorporated in place of the naturally occurring pyrimidine metabolite into a key molecule (such as RNA or DNA) or competes with the naturally occurring pyrimidine metabolite for a critical enzyme. There are four pyrimidine antimetabolites that are currently used extensively in clinical oncology. These include the fluoropyrimidines fluorouracil and fluorodeoxyuridine, and the cytosine analogues, cytosine arabinoside and azacytidine.

Purification and properties of 5,6-dihydropyrimidine amidohydrolase from calf liver

5,6-Dihydropyrimidine amidohydrolase was isolated from an acetone powder of calf liver and purified to homogeneity. Purification made use of heat treatment, ammonium sulfate fractionation and chromatography on Chelating Sepharose and DEAE-Sepharose with 44% recovery of total activity. The native enzyme has a molecular mass of 217 kDa consisting of four subunits with a molecular mass of 54 kDa each. The amidohydrolase is a metalloenzyme containing one zinc atom/subunit. The enzyme can slowly be inactivated by chelating agents. The kinetic parameters for substrates, 5,6-dihydrouracil, 5,6-dihydrothymine and glutarimide were determined. From log Vmax/KM data, a pKa of 7.6 could be calculated suggesting the formation of a zinc-bound hydroxyl ion which carries out the nucleophilic attack on the C-4 of dihydrouracil.

Bisfunction of propionic acid on purified rat liver beta-ureidopropionase

Propionic acid and isobutyric acid, which are structural analogues of N-carbamoyl-beta-alanine and N-carbamoyl-beta-aminoisobutyric acid, respectively, acted as an allosteric activator as well as a competitive inhibitor of purified rat liver beta-ureidopropionase. Propionic acid and isobutyric acid had a Ki value of approx. 0.3 mM at pH 7.0. The Hill coefficient for N-carbamoyl-beta-alanine was 2.0, but the cooperatively decreased to 1.0 in the presence of 1 mM propionic acid. The K1/2 value towards N-carbamoyl-beta-alanine was calculated to be 0.17 mM from Hill plots and the Km value was determined to be 0.06 mM from replots of the apparent Km vs propionic acid.

Purification and properties of beta-ureidopropionase from the rat liver

beta-Ureidopropionase was purified 1000-fold over the initial rat liver extract, using heat treatment, ammonium sulfate fractionation, CM-Sepharose CL-6B, DEAE-Sepharose CL-6B, hydroxyapatite and Sephacryl S-300 chromatographies. The purified enzyme was shown to be homogeneous by gel electrophoresis both in the presence and absence of sodium dodecyl sulfate. Its molecular mass, determined by gel filtration and sucrose density gradient centrifugation, was 327,000 +/- 9000 and 323,000 +/- 13,000 respectively, and the subunit molecular mass was 54,000 +/- 600. The pH optimum for enzyme activity was 7.0 and pI was 6.4. The enzyme catalyzed the amidohydrolysis of N-carbamoyl-beta-alanine and N-carbamoyl-DL-beta-aminoisobutyrate but did not catalyze that of other ureido compounds including N-carbamoyl-DL-aspartate. With N-carbamoyl-beta-alanine and N-carbamoyl-DL-beta-aminoisobutyrate as substrate, the enzyme exhibited positive cooperativity with a Hill coefficient h = 2. The enzyme activity was proportional to the enzyme concentration between 0.2 nM and 0.5 microM. Arrhenius plots of the influence of temperature on the catalytic activity of the enzyme showed a sharp break at 19 degrees C.

Decreased host toxicity in vivo during chronic treatment with 5-flourouracil

Chronic weekly administration of FUra to CD8F1 female mice bearing spontaneous mammary tumors produced body weight loss during the first 2 weeks of treatment, which became less severe during subsequent weeks of therapy. To our knowledge, the development of such a decrease in FUra toxicity in vivo during chronic treatment with the drug has not been described previously, and a study of this phenomenon was therefore undertaken in tumor-free CD8F1 female mice. Weekly administration of FUra at 85 mg/kg resulted in toxicity expressed in body weight loss and in depressed peripheral WBC levels; however, the magnitude of these toxic effects decreased significantly by the 5th week of treatment. Pretreatment of normal mice with FUra for 7 weeks resulted in a dose-related shift in the LD50 of FUra administered as a subsequent challenge. Compared with an LD50 of 240 mg/kg for FUra in normal mice, the LD50 in mice pretreated with FUra at 50 or 85 mg/kg per week was found to be significantly elevated to 370 and 460 mg/kg, respectively. Pretreatment with FUra at 85 mg/kg for 7 weeks did not alter the activity of the enzymes responsible for the activation of FUra, namely uridine kinase or orotate phosphoribosyltransferase, in the intestinal epithelium or bone marrow, but it did decrease the 24-h urinary excretion of intact [3H]FUra by almost 40% (P less than 0.01). In addition, the FUra pretreatment schedule resulted in a 31% (P = 0.14) increase in the activity of dihydrouracil dehydrogenase in the liver. These results suggest that increased degradation of FUra can be induced by chronic treatment with the drug. Finally, knowledge of the development of increased drug catabolism was used to increase the therapeutic effectiveness of FUra by its incorporation into an increasing-dose regimen. Mice bearing 24-h transplants of the murine breast tumor were treated with a constant dose of FUra for 12 weeks or with a dose that was increased, after 7 weeks, to a dose normally causing a high degree of drug-related mortality. The group receiving the incremented FUra dose had a significantly slower tumor growth rate without an increase in drug-related toxicity. These results are discussed in light of their obvious clinical implications.

Pyrimidine catabolism: individual characterization of the three sequential enzymes with a new assay

We have developed a one-dimensional thin-layer chromatography procedure that resolves the initial substrate uracil and its catabolic derivatives dihydrouracil, N-carbamoyl-beta-alanine (NCBA) and beta-alanine. This separation scheme also simplifies the preparation of the radioisotopes of N-carbamoyl-beta-alanine and dihydrouracil. Combined, these methods make it possible to assay easily and unambiguously, jointly or individually, all three enzyme activities of uracil catabolism: dihydropyrimidine dehydrogenase, dihydropyrimidinase, and N-carbamoyl-beta-alanine amidohydrolase. Earlier reports had presented data suggesting that these three enzyme activities were combined in a complex because they appeared to be controlled at a single genetic locus [Dagg, C. P., Coleman, D.L., & Fraser, G.M. (1964) Genetics 49, 979-989] and because they appeared able to channel metabolites [Barrett, H.W., Munavalli, S.N., & Newmark, P. (1964) Biochim. Biophys. Acta 91, 199-204]. Although the three enzymes from rat liver have similar sizes, with apparent molecular weights of 218 000 for dihydropyrimidine dehydrogenase, 226 000 for dihydropyrimidinase, and 234 000 for NC beta A amidohydrolase, they are easily separated from each other. Kinetic studies show no evidence of substrate channeling and therefore do not support a model for an enzyme complex. The earlier reports may be explained by our studies on the amidohydrolase, which suggest that under certain conditions this enzyme may become the rate-limiting step in uracil catabolism.

Capacity of rat liver for pyrimidine synthesis and catabolism during fetal and neonatal development

The capacity of minces of rat liver to synthesize and degrade pyrimidines during fetal and neonatal development was examined. Pyrimidine synthesis was determined by measuring the rate of incorporation of NaH14CO3 into orotic acid. Pyrimidine catabolism was estimated by measuring the generation of 14CO2 from [2-14C]uridine. The incorporation of [2-14C]uridine into RNA was determined simultaneously with measurements of uridine catabolism. The activity of beta-ureidopropionase, the enzyme which catalyzes the terminal reaction in the dihydropyrimidine catabolic pathway, was also monitored in cell-free extracts of liver throughout the perinatal period. Catabolic activity was detected at the earliest stage of gestation examined (16 days) and rose sharply during fetal development to reach adult levels at birth or shortly thereafter. A similar rise in the activity of beta-ureidopropionase was somewhat delayed when compared with the rise in overall catabolic activity; the enzyme activity at birth was about half the adult level. By way of contrast, the incorporation of NaH14CO3 into orotic acid and [2-14C]uridine into RNA were highest in 16-day fetal liver and declined sharply with fetal and neonatal development. These results demonstrate an appreciable capacity for pyrimidine catabolism in fetal liver, and also contribute to growing evidence that fetal tissues are capable of meeting their pyrimidine requirements through de novo synthesis. The contrast observed between the rate of synthesis of orotic acid and the capacity for pyrimidine degradation throughout perinatal development fits the pattern which has emerged from other studies showing the pathways for the anabolism and catabolism of pyrimidines to be regulated inversely to one another.