Thymidine catabolism and the reutilization of its degradative products in Tetrahymena pyriformis Metabolism of [2,6-14C2]thymidine and [2-14C]methylmalonic acid

Niemann, Marilyn A.; Berech, John

doi:10.1016/0005-2787(81)90124-6

Cited by 9 publications

(7 citation statements)

References 21 publications

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“…Consistent with that hypothesis, thymine and BAIBA were detected when D4T was incubated in either rat or human liver homogenates (36 (26). Therefore, it would appear that in addition to salvage pathways which lead to reincorporation of purine and pyrimidine bases into nucleic acids, the breakdown products of these bases may be reutilized for the synthesis of proteins, carbohydrates, and lipids.…”

Section: Discussionmentioning

confidence: 68%

“…Therefore, it would appear that in addition to salvage pathways which lead to reincorporation of purine and pyrimidine bases into nucleic acids, the breakdown products of these bases may be reutilized for the synthesis of proteins, carbohydrates, and lipids. Indeed, labeled macromolecules such as glycogen, glucose, ribose, alanine, aspartate, glutamate, and lipids have been isolated following exposure of cells to radiolabeled thymidine (26). Since D4T is cleaved in vivo to thymine and subsequently metabolized to BAIBA, degradation and/or conversion to macromolecules and products which are shuttled into salvage pathways may account for the unrecovered portion of the D4T.…”

Section: Discussionmentioning

confidence: 99%

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In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

Cretton

Zhu

Kidd

et al. 1993

Antimicrob Agents Chemother

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The disposition and metabolic fate of 3'-deoxy-2',3'-didehydrothymidine (D4T) were 3'-Deoxy-2',3'-didehydrothymidine (D4T) is a thymidine analog with potent anti-human immunodeficiency virus activity in vitro comparable to that of 3'-azido-3'-deoxythymidine (AZT) (23,24). Previous studies on D4T metabolism, by several groups including our own, have demonstrated that D4T is sequentially phosphorylated by cellular enzymes to D4T-TP, the active form which competitively inhibits TTr incorporation by human immunodeficiency virus reverse transcriptase (3,19,34,35). D4T has shown efficacy in patients with AIDS without substantial hematological toxicity (1, 14). The reduced bone marrow toxicity is probably due to the lower steady-state levels of D4T in nuclear DNA of human bone marrow cells compared with those of AZT under similar conditions (34). Peripheral neuropathy, also reported after treatment with 2',3'-dideoxycytidine or 2',3'-dideoxyinosine, was the dose-limiting toxicity and was reversible (6). Inhibition of mitochondrial DNA synthesis has been suggested as a potential mechanism of 2',3'-dideoxynucleoside-induced peripheral neuropathy (8, 21), but other mechanisms may also be involved (31).Understanding of the catabolic fate of D4T is still limited. Pharmacokinetic studies of D4T in mice, rats, dogs, and monkeys have suggested that this agent is not substantially * Corresponding author. degraded. Approximately 50 to 80% of the administered dose of D4T, in animals, is eliminated in urine unchanged (28,29). In a recent pharmacokinetic study of D4T in humans (13), about half of the administered dose was recovered unchanged. The metabolic fate of the remainder is unknown. The in vivo formation of a glucuronide conjugate is still controversial. Schinazi et al. (29) have suggested the presence of a D4T glucuronide in urine of monkeys, while others have not detected such a conjugate (28). Until recently no D4T metabolites, other than its 5'-phosphorylated derivatives, were reported either in vitro or in vivo (18). D4T was recently demonstrated by this laboratory to be a substrate for Escherichia coli thymidine phosphorylase in vitro, leading to the formation of thymine (30). Interestingly, we also detected the in vitro degradation of D4T to thymine, with subsequent formation of thymidine, in human bone marrow cells (34). This suggests that D4T is cleaved to thymine and that subsequent degradation and/or utilization of thymine in vivo accounts for the unrecovered D4T in clinical trials. Understanding the metabolic fate of D4T may be important for clinical trials design, as well as provide information on the mechanism(s) of action and the toxicity of this agent. The present study examines both the in vitro and the in vivo metabolic disposition of D4T in isolated hepatocytes and in nonhuman primates. Isolated rat hepatocytes were chosen, since we had previously demonstrated the usefulness of this in vitro system in examining the metabolic disposition of AZT and 3'-azido-2',3'-dideoxyuridine, which are structur-

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Section: Discussionmentioning

confidence: 68%

Section: Discussionmentioning

confidence: 99%

In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

Cretton

Zhu

Kidd

et al. 1993

Antimicrob Agents Chemother

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“…The previously reported (23, 24) recovery of thymidine radioactivity in the chromatographically identified end product of this reductive catabolic pathway, P-aminoisobutyric acid (PAIB), also supported the assumption that T. pyriformis was able to reductively catabolize this pyrimidine. The preliminary demonstration of the first enzyme in the proposed thymidine reductive catabolic pathway, dihydrothymine dehydrogenase (29), further supported this assumption. Presuming that this organism was indeed capable of catalyzing the required intermediate reductive catabolic interconversions (23, 24, 29, 3 I), we therefore decided to investigate how the reductive thymidine degradative end product PAIB was further metabolized to account for the recovery of thymidine radioactivity in macromolecules other than DNA.…”

mentioning

confidence: 70%

“…These studies thus initially indicated a number of qualitative similarities between pAIB oxidase and D-amino acid oxidase. In addition, both of these oxidases failed to show any significant increase in activity on repeated freeze-thawing and behaved similarly on differential centrifugation (29) and initial (NH,),SO, precipitation (Table V). The high K, of this oxidase in crude homogenates also indicated that perhaps pAlB was not the preferred substrate of this enzyme.…”

Section: Discussionmentioning

confidence: 94%

Demonstration and Preliminary Characterization of an Enzyme Capable of the Further Metabolism of the Thymidine Catabolite, β‐Aminoisobutyric Acid, in Tetrahymena pyriformis Strain GL¹

Niemanw

Berech

1981

The Journal of Protozoology

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ABSTRACT. The enzyme that catalyzes the critical conversion of the proposed reductive thymidine catabolic end product, P‐aminoisobutyric acid, to the initial anabolic reutilization substrate, probably methylmalonic semialdehyde, was investigated to un‐ ambiguously substantiate the operation of a reductive pyrimidine catabolic and reutilization pathway in Tetrahymena pyriformis strain GL. Although most of the studies on the further metabolism of P‐aminoisobutyric acid in other organisms suggested transamination of p‐aminoisobutyric acid to methylmalonic semialdehyde followed by its further oxidation to methylmalonic acid, such a transaminating system could not be demonstrated. By means of a sensitive fluorometric assay system, however, a low, but significant amount of P‐aminoisobutyric acid oxidase activity was detected. This enzymatic activity exhibited the following characteristics in homogenates: good activity in alkaline 0.2 M Tris‐HCI buffer with a rather broad pH optimum ranging from 7.8 to 9.0; optimum activity at a temperature of 37°C; stimulation on the addition of exogenous FAD; inhibition on the addition of divalent cations, EDTA, or PCMB; little stimulation on the addition of detergents; and no increase in activity on repeated freezing and thawing. In addition, crude preparations of this oxidase were found to be relatively stable when stored up to one week either refrigerated or frozen, to have a specific activity of 2.8 nmoles/min/mg of protein, and to have a K, of 3.6 × 10− M for d,l‐β‐aminoisobutyric acid. Whether this high Km, is physiologically significant or not, however, will have to await further investigation. Preliminary (NH4)2SO4 fractionation and affinity chromatography studies also indicated that this enzyme appears to be a unique and specific oxidase whose activity is separable from other marker enzymes, including other oxidases.

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“…Hydrolysis of the latter gives rise to ␤-ureidoisobutyric acid, which is further hydrolyzed to ␤-aminoisobutyric acid, CO 2 , and NH 3 . The ␤-aminoisobutyric acid can be partly excreted intact in the urine, can be further metabolized to other compounds such as methylmalonate, an intermediate in the metabolism of propionic acid, or can be subjected to further oxidation and ultimately to CO 2 and H 2 O (Niemann and Berech, 1981;Griffith, 1986). The proposed biotransformation pathways for [1Ј- 14 C]stavudine are shown in Scheme 1.…”

Section: Disposition Of Stavudine In Humansmentioning

confidence: 99%

Disposition of [1′-14C]Stavudine after Oral Administration to Humans

Zhou

Kaul²,

Liu-Kreyche³

et al. 2010

Drug Metabolism and Disposition

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Thymidine catabolism and the reutilization of its degradative products in Tetrahymena pyriformis Metabolism of [2,6-14C2]thymidine and [2-14C]methylmalonic acid

Cited by 9 publications

References 21 publications

In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

Demonstration and Preliminary Characterization of an Enzyme Capable of the Further Metabolism of the Thymidine Catabolite, β‐Aminoisobutyric Acid, in Tetrahymena pyriformis Strain GL¹

Disposition of [1′-14C]Stavudine after Oral Administration to Humans

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Thymidine catabolism and the reutilization of its degradative products in Tetrahymena pyriformis Metabolism of [2,6-14C2]thymidine and [2-14C]methylmalonic acid

Cited by 9 publications

References 21 publications

In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

In vitro and in vivo disposition and metabolism of 3'-deoxy-2',3'-didehydrothymidine

Demonstration and Preliminary Characterization of an Enzyme Capable of the Further Metabolism of the Thymidine Catabolite, β‐Aminoisobutyric Acid, in Tetrahymena pyriformis Strain GL1

Disposition of [1′-14C]Stavudine after Oral Administration to Humans

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Demonstration and Preliminary Characterization of an Enzyme Capable of the Further Metabolism of the Thymidine Catabolite, β‐Aminoisobutyric Acid, in Tetrahymena pyriformis Strain GL¹