Thomas Spector

Refinement of the Coomassie blue method of protein quantitation: A simple and linear spectrophotometric assay for ≤0.5 to 50 μg of protein

The Coomassie brilliant blue G assay for proteins described by Bradford (1976) (Anal. Biochem.72, 248) was reexamined. It was found that the extinction coefficient of the dye-protein complex solution remained constant over the protein concentration range of 0.8 to 10 μg/ml of solution. This unchanging extinction coefficient, A595 = 0.60 ± 0.0110 μg of protein/ml of solution, enhances both the sensitivity and versatility of the assay. Selection of a volume of dye-reagent (0.5 to 5.0 ml) which dilutes the protein sample to a final concentration of 0.8 to 10 μg/ml permits the application of Beers Law for accurate determinations of ≤0.5 to 50 μg of protein. A combination of Bradfords study and the present one indicates that most common laboratory reagents and chemicals exert little or no influence on the A595 of the dye-reagent.

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.

Oxypurinol as an inhibitor of xanthine oxidase-catalyzed production of superoxide radical.

A recent study of the mechanism by which oxypurinol inhibits uric acid generation [T. Spector, W. W. Hall and T. A. Krenitsky, Biochem. Pharmac. 35, 3109(1986)] showed that xanthine is ineffective in impeding the binding of oxypurinol to reduced xanthine oxidase. This study prompted the present hypothesis that, at elevated concentrations of substrates, oxypurinol would be superior to allopurinol as an inhibitor of the xanthine oxidase-catalyzed production of superoxide radical. It was found that the potency of allopurinol was attenuated by elevated concentrations of xanthine and hypoxanthine, whereas the potency of oxypurinol was relatively unaffected. Oxypurinol produced immediate inhibition of superoxide radical production as well as progressive inhibition with time. In contrast, allopurinol, which is also a substrate for xanthine oxidase, produced very little immediate inhibition and caused progressive inhibition only after conversion to oxypurinol. The theoretical advantages of treating ischemic tissues with oxypurinol are discussed.

Conversion of 2,6-diamino-9-(2-hydroxyethoxymethyl)purine to acyclovir as catalyzed by adenosine deaminase

Adenosine deaminase (ADA) was partially purified from several sources using affinity chromatography. These enzymes have the capacity to catalyze the deamination of 2,6-diamino-9-(2-hydroxyethoxymethyl)purine (A134U) to form the antiviral agent acyclovir [9-(2-hydroxyethoxymethyl)guanine]. Their relative substrate efficiencies (Vmax/Km) with A134U (standardized to adenosine = 100) were: dog ADA, 0.092; human ADA, 0.015-0.029; rat ADA, 0.025; calf ADA, 0.016; and Escherichia coli ADA, 0.0003. In addition to having the lowest efficiency with A134U, the bacterial ADA was also distinguished by its lack of binding of the mammalian ADA inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine and by its weak binding to the 9-(p-aminobenzyl)adenine-agarose affinity column. Four minor metabolites of A134U and acyclovir have been reported to be produced in the rat. These compounds are oxidized on either the C-8 position of the ring or the terminal carbon of the side chain. Neither acyclovir nor any of these metabolites produced significant inhibition of calf intestine ADA. The oxidized metabolites containing an N-6 amino group were extremely slow substrates of this enzyme.

Mammalian adenylosuccinate synthetase: Nucleotide monophosphate substrates and inhibitors

Guanylate cyclase (GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2) activity was examined in preparations from normal rat liver and a series of Morris hepatomas...

5-Ethynyluracil (776C85): Protection from 5-fluorouracil-induced neurotoxicity in dogs

5-Ethynyluracil (776C85) is a potent mechanism-based inactivator of dihydropyrimidine dehydrogenase (DPD), the enzyme that catalyzes the rapid catabolism of 5-fluorouracil (5-FU). Because catabolism is the major route for 5-FU clearance, we studied the effect of 5-ethynyluracil on the pharmacokinetics and toxicity of continuous i.v. 5-FU infusion in the dog. 5-FU at 40 mg/kg/24 hr resulted in a steady-state plasma 5-FU concentration of 1.3 microM and was fatal with dogs dying from apparent neurotoxicity. 5-Ethynyluracil lowered the total clearance of 5-FU from 9.9 to 0.2 L/hr/kg and enabled 1.6 mg/kg/24 hr 5-FU to achieve a steady-state plasma 5-FU concentration of 2.4 microM with no apparent toxicity. 5-FU at 4 mg/kg/24 hr achieved a steady-state plasma 5-FU concentration of 5.3 microM and produced only mild gastrointestinal disturbances in 5-ethynyluracil-treated dogs. Thus, a catabolite of 5-FU appears to be responsible for the 5-FU-induced neurotoxicity in dogs.

Herpes and human ribonucleotide reductases: Inhibition by 2-acetylpyridine 5-[(2-chloroanilino)-thiocarbonyl]-thiocarbonohydrazone (348U87)

The mode of inactivation of herpes simplex virus type 1 and human ribonucleotide reductases by 2-acetylpyridine 5-[(2-chloroanilino)-thiocarbonyl]-thiocarbonohydrazone++ + (348U87) was determined and compared to that described previously [Porter et al. Biochem Pharmacol 39: 639-646, 1990] for 2-acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (A1110U). 348U87 inactivated herpes ribonucleotide reductase faster than did A1110U. Moreover, iron-complexed 348U87 was a considerably more potent inactivator than iron-complexed A1110U. It appeared to efficiently form an initial complex with the viral enzyme prior to rapid enzyme inactivation. The combination of 348U87 and iron-complexed 348U87 inactivated with a rate constant that was slightly greater than the sum of their individual rate constants of inactivation. The corresponding combination of A1110U species inactivated with a rate constant that was much greater than the sum of the individual rate constants of inactivation. Herpes ribonucleotide reductase that had been inactivated by either species of 348U87 was reactivated by diluting the enzyme and inactivators into assay medium containing excess iron. 348U87 was also an effective inactivator of herpes simplex virus type 2 and varicella zoster virus ribonucleotide reductases. The iron-complexed forms of 348U87 and A1110U exhibited very different modes of inactivation of human ribonucleotide reductase. Iron-complexed 348U87 was a tight-binding inactivator, whereas iron-complexed A1110U was only a weak, non-inactivating, inhibitor. Furthermore, the inactivation by iron-complexed 348U87 was not stimulated by either 348U87 or A1110U, whereas the weak inhibition by iron-complexed A1110U was converted to rapid inactivation by A1110U. Excess iron prevented the inactivation by iron-complexed 348U87. Uncomplexed 348U87 was similar to uncomplexed A1110U in that it was not an inhibitor of the human enzyme.

Statistical methods to distinguish competitive, noncompetitive, and uncompetitive enzyme inhibitors

Abstract Statistical methods for distinguishing the common types of enzyme inhibitors are presented. Steady-state kinetic data in the doulbe-reciprocal form are analyzed. The test for competitive and uncompetitive inhibition simply reveals whether there is a significant difference between the sum of the residual variances for each data set (i.e., each line of the double-reciprocal plot) fitted to a straight line and the residual variance generated by fitting the data points of all the data sets to one of these models (in double-reciprocal form). A standard F test is performed to quantitate the significance of the additional error created by fitting the data to the model. F values are converted to probability values which express the degree by which the data conform to the model. The F test is not directly suitable for verifying noncompetitive inhibitors because they produce both slope and intercept effects. Therefore, the data are first transformed so that the point of convergence of the data sets is moved to the origin of the double-reciprocal graph. Equations are presented to fit each transformed data set to a straight line and also to fit the transformed data sets to a family of straight lines that intersect at the origin. The sums of the residual variances of the first fitting and the total residual variance of the second fitting are then amenable to comparison by the F test because the intercept effects have been abolished. Thus, the degree of conformity to a model describing a family of lines with a common intersection can be assessed. Additional verification of noncompetitive inhibition requires the establishment that the point of convergence resides to the left of the 1 v axis, and the statistical rejection of alternate inhibition models.

Dihydropyrimidine dehydrogenase inactivation and 5-fluorouracil pharmacokinetics: allometric scaling of animal data, pharmacokinetics and toxicodynamics of 5-fluorouracil in humans

Abstract The pharmacokinetics of 5-fluorouracil (5-FU) in different animal species treated with the dihy-dropyrimidine dehydrogenase (DPD) inactivator, 5-ethynyluracil (776C85) were related through allometric scaling. Estimates of 5-FU dose in combination with 776C85 were determined from pharmacokinetic and toxicodynamic analysis. Method: The pharmacokinetics of 5-FU in the DPD-deficient state were obtained from mice, rats and dogs treated with 776C85 followed by 5-FU. The pharmacokinetics of 5-FU in humans were then estimated using interspecies allometric scaling. Data related to the clinical toxicity for 5-FU were obtained from the literature. The predicted pharmacokinetics of 5-FU and the clinical toxicity data were then used to estimate the appropriate dose of 5-FU in combination with 776C85 in clinical trials. Results: The allometric equation relating total body clearance (CL) of 5-FU to the body weight (B) (CL=0.47B0.74) indicates that clearance increased disproportionately with body weight. In contrast, the apparent volume of distribution (Vc) increased proportionately with body weight (Vc=0.58 B0.99). Based on allometric analysis, the estimated clearance of 5-FU (10.9 l/h) in humans with DPD deficiency was comparable to the observed values in humans lacking DPD activity due to genetic predisposition (10.1 l/h), or treatment with 776C85 (7.0 l/h) or (E)-5-(2-bromovinyl)-2′-deoxyuridine (BVdUrd, 6.6 l/h). The maximum tolerated dose (MTD) of 5-FU in combination with 776C85 was predicted from literature data relating toxicity and plasma 5-FU area under the concentration-time curve (AUC). Based on allometric analysis, the estimated values for the MTD in humans treated with 776C85 and receiving 5-FU as a single i.v. bolus dose, and 5-day and 12-day continuous infusions were about 110, 50 and 30 mg/m2 of 5-FU, respectively. Discussion: The pharmacokinetics of 5-FU in the DPD-deficient state in humans can be predicted from animal data. A much smaller dose of 5-FU is needed in patients treated with 776C85.

α-Fluoro-β-alanine: Effects on the antitumor activity and toxicity of 5-fluorouracil

We have shown previously that (R)-5-fluoro-5,6-dihydrouracil (FUraH(2)) attenuates the antitumor activity of 5-fluorouracil (FUra) in rats bearing advanced colorectal carcinoma. Presently, we found that alpha-fluoro-beta-alanine (FBAL), the predominant catabolite of FUra that is formed rapidly via FUraH(2), also decreased the antitumor activity and potentiated the toxicity of FUra. In rats treated with Eniluracil (5-ethynyluracil, GW776), excess FBAL, in a 9:1 ratio to FUra, produced similar effects when administered 1 hr before, simultaneously with, or 2 hr after FUra. FBAL also decreased the antitumor activity of FUra in Eniluracil-treated mice bearing MOPC-315 myeloma at a 9:1 ratio with FUra, but not at a 2:1 ratio. FBAL did not affect the antitumor activity of FUra in mice bearing Colon 38 tumors. We also evaluated the effect of thymidylate synthase (TS) and thymidine kinase (TK) from tumor extracts after FUra +/- Eniluracil +/- FBAL treatment. The activity of TK was similar among the three groups at both 18 and 120 hr. There was also no difference in TS inhibition ( approximately 35%) at 18 hr. However, significantly more TS inhibition was observed in the Eniluracil/FUra group than in the FUra-alone group at 120 hr. FBAL did not alter the effect of Eniluracil/FUra in TS inhibition. Neither FUraH(2) nor FBAL affected the IC(50) of FUra in culture. Thus, the effect of FBAL did not result from direct competition with FUra uptake or immediate anabolism. Either another downstream catabolite that is not formed in cell culture is the active agent, or the effect requires the complexity of a living organism or an established tumor.