The (R)- and (S)-isomers of 5-methylhydantoin (5-MH) and of 5-isopropylhydantoin (5-IPH) were synthesized, and incubations of the individual isomers with a rat liver dihydropyrimidinase preparation (100,000g supernatant fraction) were carried out. Only the (R)-isomer of 5-MH or 5-IPH was ring-opened by the enzyme. Reversibility of the enzymatic ring-opening reaction could be demonstrated with only the (R)-isomer of 2-methylhydantoic acid (2-MHA) or 2-isopropylhydantoic acid (2-IPHA). The results of the present investigation show that the replacement in 5-phenylhydantoin of the phenyl group with an alkyl group does not alter the stereospecificity of the hydantoin substrates in the ring-opening reaction. The results are used to form the concept that (R)-dihydrothymine, the optical isomer previously postulated as the natural substrate to the enzyme, may have a different type of binding at the active site of the enzyme.

Amidohydrolases↗

PubMedDate not supplied

Dihydropyrimidinase. Metabolism of some cyclic imides of different ring size.

The ability of dihydropyrimidinase (EC 3.5.2.2) to hydrolyze cyclic imides of different ring size was investigated. Succinimide, glutarimide, and adipimide are five-, six-, and seven-membered cyclic imides, respectively. The ring-opened compounds that correspond to these cyclic imides are, respectively, succinamic, glutaramic, and adipamic acid. In incubations of cyclic imides (pH 8, 1 hr) with a rat liver dihydropyrimidinase preparation from which omega-amidase had been removed, adipimide was classed as a good substrate and succinimide and glutarimide were classed as very poor but definite substrates. alpha-Phenylsuccinimide, the N-demethylated metabolite of phensuximide, was a much better substrate than succinimide. alpha-Phenylglutarimide was not a substrate. The in vitro studies of the present investigation were in agreement with observations made in previous in vivo studies.

Amidohydrolases↗

PubMedDate not supplied

Stereochemical formulas of beta-D-glucuronides.

We need to recognize that incorrect representations of stereochemistry are appearing with increasing frequency in Drug Metabolism and Disposition and other scientific literature. Many misrepresentations of beta-D-glucuronides now exist in the Journal. To assume correctness of any given beta-D-glucuronide structure, or to "lift" or "copy" stereochemical formulas without making checks for correctness of configuration, is unsafe. Fundamental rules and conventions are available for stereochemical representations; and, although these guidelines are not intended to cover every possible case, they are sufficient for avoiding errors and ambiguities in the future of the type presented here.

Glucuronates↗

PubMedDate not supplied

Partial purification and characterization of dihydropyrimidinases from calf and rat liver.

The partial purification of rat and calf liver dihydropyrimidinase (EC 3.5.2.2) is described. Molecular weights of the native calf and rat liver enzymes were estimated by gel-filtration chromatography to be 252,000 and 266,000 daltons, respectively. Subunit molecular weights of the calf and rat liver enzyme were estimated by SDS-gel electrophoresis to be 59,000 and 62,000 daltons, respectively. The native enzyme in both species is thought to comprise four subunits. The purified enzyme from both species was capable of catalyzing the hydrolytic ring opening of dihydrouracil, 5-phenylhydantoin, hydantoin, and alpha-phenylsuccinimide.

Amidohydrolases↗

PubMedDate not supplied

The role of dihydropyrimidinase in the metabolism of some hydantoin and succinimide drugs.

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Amidohydrolases↗

PubMedDate not supplied

Urinary metabolites of N-methyl-alpha-methyl-alpha-phenysuccinimide (methsuximide) in the dog.

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Acids↗

PubMedDate not supplied

Gas chromatographic-mass spectrometric studies on the metabolic fate of ethotoin in man.

Unchanged ethotoin and 11 metabolic products of ethotoin were detected in the urine of subjects (2 men and 1 woman) receiving ethotoin. Nine of these products were identified by comparison of their retention times and mass spectra with those of authentic synthetic samples. Hydroxylation of the hydantoin ring at the 5-position produced 5-hydroxyethotoin and 5-hydroxy-5-phenylhydantoin. Aryl hydroxylation resulted in the formation of p-hydroxyethotoin, o-hydroxyethotoin, m-hydroxyethotoin, 3-methoxy-4-hydroxyethotoin, and 3,4-dihydroxyethotoin. Most of these were excreted as the glucuronide conjugate. A dihydrodiol of ethotoin and 3-ethyl-5-hydroxy-5-(4-hydroxyphenyl)hydantoin were isolated along with unchanged ethotoin and 5-phenylhydantoin. 2-Phenylhydantoic acid was also isolated and was shown to have the (R)(-)-configuration.

Biotransformation↗

PubMedDate not supplied

Absolute configurations of the dihydrodiol metabolites of 5,5-diphenylhydantoin (phenytoin) from rat, dog, and human urine.

Dihydrodiol metabolites (DHD) of phenytoin (PHT) have been extracted from the urine of rats and dogs, and from that of a patient on chronic PHT therapy. The crystalline rat DHD was dehydrated to give a mixture of (S)-5-(4-hydroxyphenyl)-5-phenylhydantoin and (S)-5-(3-hydroxyphenyl)-5-phenylhydantoin. The absolute configuration of C5 of the hydantoin ring of the DHD can accordingly be assigned as (S). Circular dichroism studies allowed further assignment of absolute configuration to the crystalline rat DHD, the metabolite being identified as (5S)-5-[(3R,4R)-3,4-dihydroxy-1,5-cyclohexadien-1-yl]-5-phenylhydantoin, (5S)-DHD. The human DHD metabolite was identical with the (5S)-DHD. The DHD from dog urine was found to be a mixture of diastereoisomers; the major component was identified as (5R)-5-[(3R,4R)-3,4-dihydroxy-1,5-cyclohexadien-1-yl]-5-phenylhydantoin, (5R)-DHD, and the minor diastereoisomer was identified as (5S)-DHD. The ratio (5R)-DHD/(5S)-DHD was approximately 2. A comparison of the absolute configurations of phenolic metabolites of PHT and of the DHD's offered a basis for speculation as to stereochemical aspects of PHT metabolism in the rat, dog, and man.

Animals↗

PubMedDate not supplied

Stereochemical aspects of the metabolism of 5-ethyl-5-phenylhydantoin (Nirvanol) in the dog.

Enantiomers of 5-ethyl-5-phenylhydantoin (EPH) were administered to dogs, and urinary metabolites were quantitated. After administration of (R)-EPH, the urinary products included unchanged drug, 5-ethyl-5-(4-hydroxyphenyl)hydantoin (p-EHPH), 5-ethyl-5-(3-hydroxyphenyl)hydantoin (m-EHPH), and an N-glucuronide of EPH. Administration of (S)-EPH gave urinary products consisting of unchanged drug, p-EHPH, m-EHPH, an N-glucuronide of EPH, and a dihydrodiol metabolite, which has been isolated and identified as (5 S)-5-[(3R,4R)-3,4-dihydroxy-1,5-cyclohexadien-1-yl]-5-ethylhydantoin. The levorotatory isomers of p- and m-EHPH have been assigned the (R)-configuration. An unidentified metabolite of EPH has been detected through its reactivity under basic conditions to yield 2-ethyl-2-phenylhydantoic acid, which can be cyclized with acid to EPH. Quantitative studies of the disposition of single oral doses of (R)-, (S)-, and (RS)-EPH by these metabolic routes suggest that the metabolism of one enantiomer is unaffected by the presence of the other enantiomer. Stereoselectivities of metabolic pathways are discussed in relation to stereoselectivities observed for phenytoin metabolism in the dog.

Animals↗

PubMedDate not supplied

5-ethyl-5-phenylhydantoin N-glucuronide, the major urinary metabolite of 5-ethyl-5-phenylhydantoin (Nirvanol) in the dog.

A water-soluble metabolite, isolated from the urine of dogs given (S)-5-ethyl-5-phenylhydantoin [(S)-EPH], has been identified as 1-deoxy-1-[(5S)-5-ethyl-5-phenylhydantoin-3-yl] beta-D-glucopyranuronate [(S)-EPH N-glucuronide]. EPH N-glucuronide did not release the aglycone upon acid or beta-glucuronidase treatment, but incubation in alkaline solution (pH 12-13) readily formed 2-ethyl-2-phenylhydantoic acid (EPHA). The EPHA so formed could be quantitatively cyclized to EPH. With the knowledge of the conversion efficiency of EPH N-glucuronide to EPHA, a quantitative GLC assay for the metabolite was developed. EPH N-glucuronide was found to be the major urinary metabolite after administration to dogs of either (R)-, (S)-, or (RS)-EPH.

Animals↗