Cross-species drug metabolism and impact of metabolic stability testing under anaerobic condition on predicting pharmacokinetics of keto-enol containing compound in humans

After oral administration of [14C]-S-1360 in rats and dogs, [14C]-S-1360 was absorbed rapidly and the bioavailability was 93.7% in rats and 75.1% in dogs. Based on the results in animals, good systemic exposure would be expected in humans. In contrast to the expectation, the exposure was low in healthy volunteers compared to the exposure expected. In addition, human mass balance study using [14C]-S1360 revealed that a large amount of metabolites existed in human plasma. The major metabolites in human plasma were reduced metabolite (HP1) and S-1360 N-glucuronide, and they respectively accounted for approximately 30% of total AUC. Unchanged S-1360 accounted for only 14% of total AUC. The results showed that a significant difference between humans and animals were observed in metabolism of S-1360. Although S-1360 was stable in human hepatocytes under aerobic condition (approximately 84% remaining at 1 h), S-1360 was labile under anaerobic condition (approximately 55% remaining at 1 h). The present study revealed that the reductive metabolism pathways are the key metabolic pathway of S-1360, especially the metabolic stability test under anaerobic condition is important to predict pharmacokinetics of keto-enol containing compound, such as S-1360.

Improved Predictability of Hepatic Clearance with Optimal pH for Acyl-Glucuronidation in Liver Microsomes

The purpose of this study was to investigate the optimal pH for acyl-glucuronidation formation with carboxylic acid-containing compounds in human and rat liver microsomes to improve the predictability of their hepatic clearance. The optimal pH for acyl-glucuronidation of all 17 compounds was around pH 6.0 in human and rat liver microsomes. Correlation analysis was done with the predicted in vitro intrinsic clearance (CL_{int,in vitro}) and in vivo intrinsic clearance (CL_{int,in vivo}) calculated from available reported data of total clearance (CL_tot) of 11 compounds in humans. For 8 of the 11 compounds, under the pH 6.0 condition, the CL_{int,in vitro} were within 1/3 to 3-fold error of the observed CL_{int,in vivo} whereas, the error was within 1/3 to 3-fold of the observed CL_{int,in vivo} for only 3 of the 11 under the pH 7.4 condition. The intracellular pH in human and rat hepatocytes decreased in the presence of a carboxylic acid-containing compound. These findings suggest that acyl-glucuronidation in liver microsomes at pH 6.0 is closer to physiological conditions in the presence of carboxylic acid compounds, and thus, use of this pH condition is important for physiological interpretation and predictability of intrinsic clearance.

Discovery of Atabecestat (JNJ-54861911): A Thiazine-Based β-Amyloid Precursor Protein Cleaving Enzyme 1 Inhibitor Advanced to the Phase 2b/3 EARLY Clinical Trial

Accumulation of amyloid β peptides (Aβ) is thought to be one of the causal factors of Alzheimer's disease (AD). The aspartyl protease β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is the rate-limiting protease for Aβ production, and therefore, BACE1 inhibition is a promising therapeutic approach for the treatment of AD. Starting with a dihydro-1,3-thiazine-based lead, Compound J, we discovered atabecestat 1 (JNJ-54861911) as a centrally efficacious BACE1 inhibitor that was advanced into the EARLY Phase 2b/3 clinical trial for the treatment of preclinical AD patients. Compound 1 demonstrated robust and dose-dependent Aβ reduction and showed sufficient safety margins in preclinical models. The potential of reactive metabolite formation was evaluated in a covalent binding study to assess its irreversible binding to human hepatocytes. Unfortunately, the EARLY trial was discontinued due to significant elevation of liver enzymes, and subsequent analysis of the clinical outcomes showed dose-related cognitive worsening.

Human mass balance, metabolism, and cytochrome P450 phenotyping of lusutrombopag

The human mass balance of lusutrombopag, an orally bioavailable thrombopoietin (TPO) receptor agonist, was characterised in seven healthy male subjects after a single oral dose of [¹⁴C]-lusutrombopag (2 mg, 100 μCi) in solution. Lusutrombopag was the main component in plasma, accounting for 56% of plasma radioactivity AUC_0-∞. In plasma, the half-life of radioactivity (70.7 h) was longer than that of lusutrombopag (25.7 h), suggesting the presence of long circulating metabolites. The main excretion pathway of lusutorombopag was feces, with a radioactivity recovery of approximately 83% within 336 h post-dose. M6 (lusutrombopag-O-propanol or lusutrombopag-O-acetic acid) and M7 (lusutrombopag-O-ethane-1,2-diol) were also identified as main components in feces, accounting for at most 17.9%, and 16.9% of the dose, respectively, and were β-oxidation related metabolites. Our in vitro metabolism study of lusutrombopag indicated that β-oxidation was a subsequent metabolism of ω-oxidation and CYP4 enzymes, including CYP4A11, were the major isozymes contributing to ω-oxidation. In conclusion, lusutrombopag is primarily eliminated via ω-oxidation and excreted in the feces, where CYP4 enzymes play an important role.

The Novel In Vitro Method to Calculate Tissue-to-Plasma Partition Coefficient in Humans for Predicting Pharmacokinetic Profiles by Physiologically-Based Pharmacokinetic Model With High Predictability

Proper prediction of human pharmacokinetic (PK) profiles can accelerate the compound selection in drug discovery. Recently, we reported a robust bottom-up physiologically-based pharmacokinetic (PBPK) approach (J Pharm Sci. 2019 Aug; 108(8):2718-2727), which uses the in vivo rat distribution volume at the steady state (V_ss) to determine human tissue-to-plasma partition coefficients (Kp_tissue). Here, we report on a bottom-up PBPK approach that can simulate the PK profile with both high-throughput and high-predictive accuracy only using in vitro data. In this study, as an alternative parameter of in vivo rat V_ss which was used for the correction of human Kp_tissue, V_{ss, in vitro} was obtained from protein binding data in rats, and the values of V_{ss, in vitro} for 31 reference compounds showed good correlation with the observed rat V_ss (R² = 0.859). Next, rat and human PK profiles of reference compounds were predicted by the bottom-up PBPK approach using Kp_tissue corrected by rat V_{ss, in vitro}. As a result, the absolute average fold errors for pharmacokinetic parameters were almost less than 2, showing that these PK profiles could be accurately predicted using in vitro data. This method enables the screening of promising compounds with good PK profiles in humans at an early stage of drug discovery.

Metabolite profiling of guanfacine in plasma and urine of healthy Japanese subjects after oral administration of guanfacine extended-release tablets

Guanfacine is used for the treatment of attention‐deficit/hyperactivity disorder (ADHD). Using liquid chromatography–tandem mass spectrometry (LC–MS/MS), metabolite profiling of guanfacine was performed in plasma and urine collected from healthy Japanese adults following repeated oral administration of guanfacine extended‐release formulation. Unchanged guanfacine was the most abundant component in both plasma and urine (from the MS signal intensity). In plasma, the M3 metabolite (a sulfate of hydroxy‐guanfacine) was the prominent metabolite; the M2 metabolite (a glucuronide of a metabolite formed by monooxidation of guanfacine), 3‐hydroxyguanfacine and several types of glucuronide at different positions on guanfacine were also detected. In urine, the M2 metabolite and 3‐hydroxyguanfacine were the principal metabolites. From metabolite analysis, the proposed main metabolic pathway of guanfacine is monooxidation on the dichlorobenzyl moiety, followed by glucuronidation or sulfation. A minor pathway is glucuronidation at different positions on guanfacine. As the prominent metabolites in plasma were glucuronide and sulfate of hydroxyguanfacine, which have no associated toxicity concerns, further toxicity studies of the metabolites, for example in animals, were not deemed necessary.

Application of Intestinal Epithelial Cells Differentiated from Human Induced Pluripotent Stem Cells for Studies of Prodrug Hydrolysis and Drug Absorption in the Small Intestine

Cell models to investigate intestinal absorption functions, such as those of transporters and metabolic enzymes, are essential for oral drug discovery and development. The purpose of this study was to generate intestinal epithelial cells from human induced pluripotent stem cells (hiPSC-IECs) and then clarify whether the functions of hydrolase and transporters in them reflect oral drug absorption in the small intestine. The hiPSC-IECs showed the transport activities of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and peptide transporter 1 (PEPT1), revealed by using their probe substrates ([³H]digoxin, sulfasalazine, and [¹⁴C]glycylsarcosine), and the metabolic activities of CYP3A4, CES2, and CES1, which were clarified using their probe substrates (midazolam, irinotecan, and temocapril). The intrinsic clearance by hydrolysis of six ester prodrugs into the active form in hiPSC-IECs was correlated with the plasma exposure (C_max , AUC, and bioavailability) of the active form after oral administration of these prodrugs to rats. Also, the permeability coefficients of 14 drugs, containing two substrates of P-gp (doxorubicin and [³H]digoxin), one substrate of BCRP (sulfasalazine), and 11 nonsubstrates of transporters (ganciclovir, [¹⁴C]mannitol, famotidine, sulpiride, atenolol, furosemide, ranitidine, hydrochlorothiazide, acetaminophen, propranolol, and antipyrine) in hiPSC-IECs were correlated with their values of the fraction of intestinal absorption (Fa) in human clinical studies. These findings suggest that hiPSC-IECs would be a useful cell model to investigate the hydrolysis of ester prodrugs and to predict drug absorption in the small intestine.

Therapeutic efficacy of peramivir against H5N1 highly pathogenic avian influenza viruses harboring the neuraminidase H275Y mutation

High morbidity and mortality associated with human cases of highly pathogenic avian influenza (HPAI) viruses, including H5N1 influenza virus, have been reported. The purpose of the present study was to evaluate the antiviral effects of peramivir against HPAI viruses. In neuraminidase (NA) inhibition and virus replication inhibition assays, peramivir showed strong inhibitory activity against H5N1, H7N1 and H7N7 HPAI viruses with sub-nanomolar activity in enzyme assays. In H5N1 viruses containing the NA H275Y mutation, the antiviral activity of peramivir against the variant was lower than that against the wild-type. Evaluation of the in vivo antiviral activity showed that a single intravenous treatment of peramivir (10 mg/kg) prevented lethality in mice infected with wild-type H5N1 virus and also following infection with H5N1 virus with the H275Y mutation after a 5 day administration of peramivir (30 mg/kg). Furthermore, mice injected with peramivir showed low viral titers and low levels of proinflammatory cytokines in the lungs. These results suggest that peramivir has therapeutic activity against HPAI viruses even if the virus harbors the NA H275Y mutation.

The relationship between in vivo antiviral activity and pharmacokinetic parameters of peramivir in influenza virus infection model in mice

The purpose of this study was to investigate the relationship between pharmacokinetic (PK) parameters of intravenous (IV) peramivir and in vivo antiviral activity pharmacodynamic (PD) outcomes in a mouse model of influenza virus infection. Peramivir was administrated to mice in three dosing schedules; once, twice and four times after infection of A/WS/33 (H1N1). The survival rate at day 14 after virus infection was employed as the antiviral activity outcome for analysis. The relationship between day 14 survival and PK parameters, including area under the concentration-time curve (AUC), maximum concentration (Cmax) and time that drug concentration exceeds IC95 (T(>IC95)), was estimated using a logistic regression model, and model fitness was evaluated by calculation of the Akaike information criterion (AIC) index. The AIC indices of AUC, Cmax and T(>IC95) were about 114, 151 and 124, respectively. The AIC of AUC and T(>IC95) were smaller than that of Cmax. Therefore, both AUC and T(>IC95) were the PK parameters that correlated best with the antiviral activity of peramivir IV against influenza virus infection in mice.

Substrate specificity for carnitine and glycine conjugation of branched side-chain and cyclic side-chain carboxylic acids in various experimental animals

Substrate specificities for the carnitine and glycine conjugates of branched side-chain and cyclic side-chain carboxylic acids were examined using dog, rabbit, cynomolgus monkey, and squirrel monkey hepatocytes and kidney slices. For all tested samples, the substrate specificity for carnitine or glycine conjugation showed a similar tendency to those for rat experiments reported previously, that is, the best substrate for the carnitine conjugate was cyclopropanecarboxylic acid (CPCA), while that for the glycine conjugate was benzoic acid (BA), followed by cyclohexanecarboxylic acid (CHCA). With respect to carnitine conjugation for CPCA, rat hepatocytes showed the highest ability followed by dog, rabbit, and monkey hepatocytes. Rat kidney slices showed the highest carnitine conjugation ability, followed by rabbit, dog, and monkey kidney slices, in that order. With respect to glycine conjugation for BA, rabbit hepatocytes showed the highest ability, followed by rat and monkey hepatocytes. Dog hepatocytes have no or little glycine conjugate ability for the carboxylic acids studied here. Rabbit kidney slices showed the highest glycine conjugation ability followed by rat, dog, and monkey kidney slices.

Model for the drug–drug interaction responsible for CYP3A enzyme inhibition. II: establishment and evaluation of dexamethasone-pretreated female rats

1. Cytochrome P450 (CYP) 3A catalysis of testosterone 6beta-hydroxylation in female rat liver microsomes was significantly induced, then reached a plateau level after pretreatment with 80 mg kg(-1) day(-1) dexamethasone (DEX) for 3 days. 2. Midazolam was mainly metabolized by CYP3A in DEX-treated female rat liver microsomes from an immuno-inhibition study, and the apparent K(m) was 1.8 microM, similar to that in human microsomes. 3. Ketoconazole and erythromycin, typical CYP3A inhibitors, demonstrated extensive inhibition of midazolam metabolism in DEX-treated female rat liver microsomes, and the apparent K(i) values were 0.088 and 91.2 microM, respectively. The values were similar to those in humans, suggesting that DEX-treated female rat liver microsomes have properties similar to those of humans. 4. After oral administration of midazolam, the plasma midazolam concentration in DEX-treated female rats significantly decreased compared with control female rats. The area under the plasma concentration curve (AUC) and elimination half-life were one-11th and one-20th of those of control female rats, respectively. 5. Using DEX-treated female rats, the effect of CYP3A inhibitors on midazolam pharmacokinetics was evaluated. The AUC and maximum concentration in plasma (C(max)) increased when ketoconazole was co-administered with midazolam. 6. It was shown that the drug-drug interaction that occurs in vitro is also observed in vivo after oral administration of midazolam. In conclusion, the DEX-treated female rat could be a useful model for evaluating drug-drug interactions based on CYP3A enzyme inhibition.

Assessment of the hepatic and intestinal first-pass metabolism of midazolam in a CYP3A drug–drug interaction model rats

In the current study, to understand the characteristics of dexamethasone (DEX)-treated female rats as an animal model for drug-drug interactions, a double-cannulation method was applied and separately assessed for the intestinal and hepatic first-pass metabolism of midazolam. Midazolam was administered intravenously or orally to the animals, and midazolam concentrations in the portal and systemic plasma were simultaneously determined. Next, the rates of elimination from the intestine and liver were estimated using the AUC values. After oral administration of midazolam, the entire drug was absorbed without intestinal first-pass metabolism, and 93% of the administered midazolam was extracted in the liver of the DEX-treated female rats. Seven per cent of the midazolam administered reached the systemic circulation. When ketoconazole was given orally to the animals, in conjunction with midazolam, the extraction ratio in the liver decreased from 93% to 77% in the control rats, and the bioavailability of midazolam increased to 23%. On the other hand, after intravenous administration, the elimination half-life of midazolam was not changed by ketoconazole pretreatment. These results indicated that midazolam is only extracted in the liver of DEX-treated female rats and that ketoconazole inhibits the hepatic first-pass metabolism, but not the systemic metabolism. In conclusion, DEX-treated female rats can be used as a drug-drug interaction model via CYP3A4 enzyme inhibition, especially for the hepatic first-pass metabolism of orally administered drugs.

Glucuronidation Converting Methyl 1-(3,4-Dimethoxyphenyl)-3-(3-ethylvaleryl)-4-hydroxy-6,7,8-trimethoxy-2-naphthoate (S-8921) to a Potent Apical Sodium-Dependent Bile Acid Transporter Inhibitor, Resulting in a Hypocholesterolemic Action

Methyl 1-(3,4-dimethoxyphenyl)-3-(3-ethylvaleryl)-4-hydroxy-6,7,8-trimethoxy-2-naphthoate (S-8921) is a novel inhibitor of the ileal apical sodium-dependent bile acid transporter (ASBT/SLC10A2) developed for the treatment of hypercholesterolemia. The present study investigated the hypocholesterolemic action of S-8921 glucuronide (S-8921G) in rats. The plasma concentration of S-8921G was higher than that of S-8921 after single oral administration of S-8921 in normal rats, and S-8921G was excreted into the bile (13% dose). Oral administration of either S-8921 or S-8921G reduced the serum total cholesterol, particularly nonhigh-density lipoprotein cholesterol, in hypercholesterolemic normal rats. In Gunn rats devoid of UDP glucuronosyltransferase-1A activity, S-8921G was undetectable both in the plasma and bile specimens, and only S-8921G administration significantly reduced the serum nonhigh-density lipoprotein cholesterol. An in vitro inhibition study showed that glucuronidation converts S-8921 to a 6000-fold more potent inhibitor of human ASBT (K(i) = 18 nM versus 109 microM). S-8921G was detected both in the portal plasma and loop when S-8921 was administered into the loop of the rat jejunum, although the cumulative amount of S-8921G recovered in the bile was 5-fold greater than that in the loop. The uptake of S-8921G by freshly prepared rat hepatocytes was saturable, and sodium-dependent and -independent systems were involved. Organic anions, such as bromosulfophthalein, estrone 3-sulfate, and taurocholic acid, inhibited the uptake. These results suggest that UDP glucuronosyltransferase-1 isoforms play a critical role in the hypocholesterolemic action of S-8921 by converting S-8921 to a more potent ASBT inhibitor, and organic anion transporter(s) are also involved in its pharmacological action through the biliary excretion of S-8921G.

Model for the drug-drug interaction responsible for CYP3A enzyme inhibition. I: evaluation of cynomolgus monkeys as surrogates for humans

1. Anti-human cytochrome P450 (CYP) 3A4 antiserum completely inhibited midazolam metabolism in monkey liver microsomes, suggesting that midazolam was mainly metabolized by CYP3A enzyme(s) in monkey liver microsomes. 2. Midazolam metabolism was also inhibited in vitro by typical chemical inhibitors of CYP3A, such as ketoconazole, erythromycin and diltiazem, and the apparent K(i) values for ketoconazole, erythromycin and diltiazem were 0.127, 94.2 and 29.6 microM, respectively. 3. CYP3A inhibitors increased plasma midazolam concentrations when midazolam and CYP3A inhibitors were co-administered orally. However, the pharmacokinetic parameters of midazolam were not changed by treatment with CYP3A inhibitors when midazolam was given intravenously. This suggests that CYP3A inhibitors modified the first-pass metabolism in the liver and/or intestine, but not systemic metabolism. 4. The drug-drug interaction responsible for CYP3A enzyme(s) inhibition was observed when midazolam and inhibitors were co-administrated orally. Therefore, it was concluded that monkeys given midazolam orally could be useful models for predicting drug-drug interactions in man based on CYP3A enzyme inhibition.

Distribution and excretion of boron after intravenous administration of disodium mercaptoundecahydro-closo-dodecaborate to rats

Disodium mercaptoundecahydro-closo-dodecaborate (BSH) is an important compound for boron neutron capture therapy. The pharmacokinetics of boron by BSH were studied in normal rats after rapid intravenous injection at three doses (30, 100, 300 mg/kg) or continuous infusion (100 mg/kg/30 min). The boron concentration in biological samples was measured by inductively coupled plasma atomic emission spectroscopy. The blood half-lives of boron in the elimination phase (t1/2 beta) after rapid injection of BSH at doses of 30, 100 and 300 mg/kg were 1.7, 17 and 19 hr, respectively. AUC (32, 219 and 4030 micrograms.hr/ml) increased with the dose, but there was no proportionality among the values. Total clearance decreased drastically from 233 ml/hr/kg (100 mg/kg) to 38 ml/hr/kg (300 mg/kg). As boron was excreted mainly into urine, these results suggest that renal function failure might occur with dosing of 300 mg/kg. In the case of continuous infusion of 100 mg/kg of BSH for 30 min, the pharmacokinetic parameters were similar to those of rapid injection of 100 mg/kg. The highest boron concentration was observed in the kidney and the lowest in the brain. After multiple dosing of BSH at 100 mg/kg/day x 14 days, the boron concentrations in blood, liver, lung and kidney at 24 hr after the last dosing were higher than those after single dosing and were similar to those of simulated values calculated from the single dosing parameters. These results clearly indicated that boron does not accumulate unexpectedly in any tissue with multiple dosing of 100 mg/kg of BSH for two weeks.

Disposition and metabolism of the new hypocholesterolemic compound S-8921 in rats and dogs

S-8921 (methyl 1-(3,4-dimethoxyphenyl)-3-(3-ethylvaleryl)-4-hydroxy-6,7,8-trimeth oxy-2- naphthoate, CAS 151165-96-7) is a novel hypocholesterolemic agent which was found to inhibit ileal Na+/bile acid cotransporter. In this report, the pharmacokinetic profile of S-8921 was studied in rats and dogs. After dosing of 14C-S-8921 to rats at 1 to 25 mg/kg as 0.5% methylcellulose (MC) suspension, tmax was observed during 5-6 h, and AUCs increased with the dose, but not proportionally. The elimination half-lives were around 38-41 h for the doses examined. The apparent absorption ratio of 5 mg/kg of 14C-S-8921 as MC suspension was about 14%. Most of the radioactivity (98% of dose) was excreted into the feces and only 1-2% into the urine. Biliary excretion of radioactivity after dosing of 1, 5 or 25 mg/kg was 22, 20, 15%, respectively. Saturation of the absorption process was suggested. Even in case of intravenous dosing, about 88% was excreted into the bile. Enterohepatic circulation of biliary metabolites was also observed in rat. Its extent was small (6%), but, it may be contribute to the slow elimination of S-8921 from rat. The highest radioactivity was observed in the liver, with other tissues showing similar radioactivity profiles to that of plasma. The elimination half-lives of radioactivity from tissues were very long, e.g. 68 h for the liver and 58 h for the kidney. After 14 days multiple dosing, most tissues showed about two times higher radioactivity than that after a single dose. The simulation curves of liver and plasma showed a good fit with those of the observed values. These results suggested that there is no serious accumulation of radioactivity in tissues by multiple dosing of 14C-S-8921 in rats. The plasma radioactivity after oral dosing of 5 mg/kg of 14C-S-8921 to dogs as an MC suspension reached maximum concentration (c.a. 33 ng/ml) at 2 h, then decreased very slowly with a half-life of 169 h. The apparent absorption ratio was 4.6% for MC suspension. The excretion of radioactivity into bile, feces and urine after oral dosing of 14C-S-8921 at 5 mg/kg as an MC suspension were 3.0%, 94.6% and 0.3%, respectively. Even in the case of intravenous dosing, urinary excretion was very small (2.2%) and most of the radioactivity was excreted very slowly into the feces. The major metabolite of S-8921 in rat bile was its glucuronide. Other minor metabolites identified were the demethylated forms of 7-methoxy and 4'-methoxy moieties of S-8921. They were also excreted into bile as their glucuronides.

Comparison of in vitro carnitine and glycine conjugation with branched-side chain and cyclic side chain carboxylic acids in rats

The substrate specificity for carnitine conjugation was examined using rat hepatocytes and kidney slices and compared with glycine conjugation which is a competitive pathway through the CoA thioester. For both hepatocytes and kidney slices, the best substrate for the camitine conjugate was cyclopropanecarboxylic acid followed by cyclobuthanecarboxylic acid (CBCA) and cyclohexanecarboxylic acid (CHCA). For the glycine conjugate, the best substrate was benzoic acid, with conjugation also occurring with CHCA and CBCA. These results suggest that carnitine transferase shows substrate specificity for cyclic side chain carboxylic acids of lesser carbon number, while glycine transferase shows inverse specificity. To compare directly the amounts of carnitine and glycine conjugates in the liver and the kidney, we estimated the endogenous amounts of carnitine and glycine and then multiplied the results by the production ratio of each conjugate. With respect to the enzyme activity per unit tissue weight, the kidney tended to show higher activities for both conjugates than the hepatocytes. This is the first report, to our knowledge, of the kidney having high carnitine conjugation activity. Cyclopentanecarboxylic acid (CPECA) was the least effective substrate for glycine and carnitine conjugates in both hepatocytes and kidney slices, CPECA may not readily undergo esterification with CoA. The branched-side chain carboxylic acids, such as pivalic acid (PA) and isobutylic acid, were also poor substrates for carnitine and glycine conjugates in rat hepatocytes and kidney slices.

Aldehyde reductase is a major protein associated with 3-deoxyglucosone reductase activity in rat, pig and human livers

3-Deoxyglucosone reductase activity in the extracts of rat, pig and human livers was potently inhibited by aldehyde reductase inhibitors. The major species of 3-deoxyglucosone reductase purified from human and pig livers were biochemically and immunochemically identical with aldehyde reductase. The two enzymes and rat liver aldehyde reductase exhibited higher catalytic efficiency for 3-deoxyglucosone than for D-glucuronate, a representative substrate of aldehyde reductase.

Inhibition of dimeric dihydrodiol dehydrogenases of rabbit and pig lens by ascorbic acid

The dehydrogenase activity of dimeric dihydrodiol dehydrogenases (DD) purified from pig and rabbit lenses was inhibited by either L-ascorbic acid or its epimer, isoascorbic acid, at pH 7.5. Isoascorbate [IC50 (concn. giving 50% inhibition) = 0.043 mM for the pig enzyme; IC50 = 0.13 mM for the rabbit enzyme] was a more potent inhibitor than ascorbate (IC50 values 0.45 and 0.90 mM respectively), but 1 mM-dehydroascorbate gave less than 30% inhibition. Glucose, glucuronate, gulono-gamma-lactone, glutathione and dithiothreitol did not inhibit the enzyme activity. The inhibition by isoascorbate and ascorbate was instantaneous and reversible, and their inhibitory potency was decreased by addition of ascorbate oxidase. In the reverse reaction, isoascorbate and ascorbate gave low IC50 values of 0.013 and 0.10 mM respectively for the pig enzyme and 0.025 and 0.25 mM for the rabbit enzyme. The inhibition patterns by the two compounds were competitive with respect to dihydrodiols of naphthalene and benzene and uncompetitive with respect to NADP+, but those in the reverse reaction were uncompetitive with respect to both carbonyl substrate and NADPH. The steady-state kinetic measurements in the forward and reverse reactions by the pig enzyme were consistent with an ordered Bi Bi mechanism, in which NADP+ binds to the enzyme first and NADPH leaves last. The results indicate that ascorbate and its epimer directly bind to an enzyme: NADP+ binary complex as dead-end inhibitors. Thus ascorbate may be an important modulator of DD in the lens.

Distribution and characterization of dihydrodiol dehydrogenases in mammalian ocular tissues

The immunological relationship of two forms of dihydrodiol dehydrogenase (DD) in pig lens to pig muscle aldose reductase and kidney aldehyde reductase has been studied. Although the minor enzyme form, a monomer of Mr 35,000, was identical with aldose reductase, the major enzyme form, a dimer of Mr 65,000, was distinct from the two reductases. The two enzyme species, although their amounts were low, were distributed in the cornea, iris-ciliary body, retina and choroid of the pig eye. In other mammals, rabbit lens exhibited much higher DD activity than did lens of mice, rats, cats, hamsters, guinea pigs and monkeys, and contained large amounts of the Mr-65,000 enzyme form as well as the minor enzyme form of Mr 35,000. In contrast, only the Mr-35,000 form of the enzyme was found in the lens of other species, except that a small amount of the high-Mr enzyme was detected in mouse lens. The high-Mr enzyme, purified from rabbit lens, was functionally and immunologically similar to dimeric DD of pig lens. The low-Mr enzyme forms, isolated or partially purified from these animal lenses, showed several features in common with aldose reductases from mammalian tissues. The dimeric enzymes of pig and rabbit lenses were NADP(+)-specific, whereas the low-Mr enzymes exhibited dual cofactor specificity and their activities with NAD+ were more than 3-fold higher than those with NADP+.