Ketohexokinase (EC 2.7.1.3) was purified to homogeneity from human liver, and fructose-bisphosphate aldolase (EC 4.1.2.13) was partially purified from the same source. Ketohexokinase was shown, by column chromatography and polyacrylamide-gel electrophoresis, to be a dimer of Mr 75000. Inhibition studies with p-chloromercuribenzoate and N-ethylmaleimide indicate that ketohexokinase contains thiol groups, which are required for full activity. With D-xylulose as substrate, ketohexokinase and aldolase can catalyse a reaction sequence which forms glycolaldehyde, a known precursor of oxalate. The distribution of both enzymes in human tissues indicates that this reaction sequence occurs mainly in the liver, to a lesser extent in the kidney, and very little in heart, brain and muscle. The kinetic properties of ketohexokinase show that this enzyme can phosphorylate D-xylulose as readily as D-fructose, except that higher concentrations of D-xylulose are required. The kinetic properties of aldolase show that the enzyme has a higher affinity for D-xylulose 1-phosphate than for D-fructose 1-phosphate. These findings support a role for ketohexokinase and aldolase in the formation of glycolaldehyde. The effect of various metabolites on the activity of the two enzymes was tested to determine the conditions that favour the formation of glycolaldehyde from xylitol. The results indicate that few of these metabolites affect the activity of ketohexokinase, but that aldolase can be inhibited by several phosphorylated compounds. This work suggests that, although the formation of oxalate from xylitol is normally a minor pathway, under certain conditions of increased xylitol metabolism oxalate production can become significant and may result in oxalosis.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• The contribution of (S)-warfarin to the clinical effect of rac-warfarin is well understood. The extent to which (R)-warfarin contributes to the clinical effect of rac-warfarin is unclear. WHAT THIS STUDY ADDS• Using unequivocally pure (R)-and (S)-warfarin we have demonstrated that (R)-warfarin contributes to the hypoprothrombinaemic effect of single large doses of warfarin.• The extent of the interaction is dependent on VKORC1 genotype. AIMS1) To determine the pharmacokinetics and pharmacodynamics of (R)-and (S)-warfarin given alone and in combination and 2) to determine whether the relative potency of (R)-and (S)-warfarin is dependent on VKORC1 genotype. METHODSA three way crossover study was conducted in which 17 healthy male subjects stratified by VKORC1 1173 C>T genotype and all CYP2C9 1*/1* received (R)-warfarin 80 mg, (S)-warfarin 12.5 mg and rac-warfarin sodium 25 mg. Plasma (R)-and (S)-warfarin unbound and total concentrations and prothrombin time were determined at multiple time points to 168 h. RESULTSPharmacokinetic parameters for (R)-and (S)-warfarin were similar to the literature. (R)-warfarin 80 mg alone resulted in a mean AUCPT (0,168 h) of 3550 s h (95% CI 3220, 3880). Rac-warfarin sodium 25 mg containing (S)-warfarin 11.7 mg produced a greater effect on AUCPT (0,168 h) than (S)-warfarin 12.5 mg (mean difference 250 s.h, 95% CI 110, 380, P < 0.002) given alone. In a mixed effects model the ratio of response between (R)-and (S)-warfarin (AUCPT((R)-warfarin) : AUCPT((S)-warfarin)) was 1.21 fold higher (95% CI 1.05, 1.41, P < 0.02) in subjects of VKORC1 TT genotype compared with the CC genotype. CONCLUSIONS(R)-warfarin has a clear PD effect and contributes to the hypoprothrombinaemic effect of rac-warfarin. VKORC1 genotype is a covariate of the relative R/S potency relationship. Prediction of drug interactions with warfarin needs to consider effects on (R)-warfarin PK and VKORC1 genotype.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• Although not well recognized, methadone inhibits CYP2D6 in vivo and in vitro and UGT2B7 and 2B4 in vitro.• We aimed to investigate the effect of methadone on the pathways of codeine metabolism, namely O-demethylation (CYP2D6), 6-glucuronidation (UGT2B4/7) and N-demethylation (CYP3A4/2C8), in subjects maintained on methadone or buprenorphine as a control. WHAT THIS STUDY ADDS• Compared with subjects on buprenorphine, methadone reduced the clearance of codeine to morphine and to codeine-6-glucuronide but had no effect on norcodeine formation.• Plasma morphine concentrations remained unchanged, as although its formation was reduced, its metabolism to M3G and M6G was also reduced.• Metabolic drug interactions with methadone cannot assume substrate-dependent inhibition. AIMSTo compare the O-demethylation (CYP2D6-mediated), N-demethylation (CYP3A4-mediated) and 6-glucuronidation (UGT2B4/7-mediated) metabolism of codeine between methadone-and buprenorphine-maintained CYP2D6 extensive metabolizer subjects. METHODSTen methadone-and eight buprenorphine-maintained subjects received a single 60 mg dose of codeine phosphate. Blood was collected at 3 h and urine over 6 h and assayed for codeine, norcodeine, morphine, morphine-3-and -6-glucuronides and codeine-6-glucuronide. RESULTSThe urinary metabolic ratio for O-demethylation was significantly higher (P = 0.0044) in the subjects taking methadone (mean Ϯ SD, 2.8 Ϯ 3.1) compared with those taking buprenorphine (0.60 Ϯ 0.43), likewise for 6-glucuronide formation (0.31 Ϯ 0.24 vs. 0.053 Ϯ 0.027; P < 0.0002), but there was no significant difference (P = 0.36) in N-demethylation. Similar changes in plasma metabolic ratios were also found. In plasma, compared with those maintained on buprenorphine, the methadone-maintained subjects had increased codeine and norcodeine concentrations (P < 0.004), similar morphine (P = 0.72) and lower morphine-3-and -6-and codeine-6-glucuronide concentrations (P < 0.008). CONCLUSIONMethadone is associated with inhibition of CYP2D6 and UGTs 2B4 and 2B7 reactions in vivo, even though it is not a substrate for these enzymes. Plasma morphine was not altered, owing to the opposing effects of inhibition of both formation and elimination; however, morphine-6-glucuronide (analgesically active) concentrations were substantially reduced. Drug interactions with methadone are likely to include drugs metabolized by various UGTs and CYP2D6.

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Perhexiline's pharmacokinetics exhibit significant enantioselectivity in CYP2D6 extensive/intermediate and poor metabolizers, with both enantiomers displaying polymorphic and saturable metabolism via CYP2D6. Clinical use of rac-perhexiline may be improved by developing specific enantiomer target plasma concentration ranges.