Purification and characterization of human testis aldose and aldehyde reductase

[Enzymological Studies on the Mechanisms of Pathogenesis of Diabetic Complications]

　Aldose reductase (AR) is involved in the pathogenesis of complications in diabetes. In this study, the enzymatic properties of AR isolated from various sources and a recombinant human AR (rh-AR) were analyzed in detail. The sensitivity of different forms of AR to several AR inhibitors (ARIs) was compared. Our findings enabled us to propose that human AR should be used as the target enzyme in the development of ARIs. An enzyme-linked immunosorbent assay (ELISA) for human AR which employed monoclonal antibodies against rh-AR was created, and this method was used to demonstrate the distribution of AR in human tissues. AR was widely distributed in various organs and blood cell components. The levels of erythrocyte AR (e-AR) were 10.1±1.9 ng/mg Hb and 10.5±3.0 ng/mg Hb in healthy volunteers and diabetic patients, respectively, and thus there was no significant difference between them. The e-AR levels of diabetic patients were assayed using the ELISA developed to investigate the potential correlation between AR levels and the onset of diabetic complications. There were significant correlations between the incidence of diabetic neuropathy and e-AR levels in patients with disease duration of less than 10 years, and between the incidence of diabetic retinopathy and e-AR levels in patients with disease duration of 10-20 years. Our results suggest that measurement of e-AR levels in patients could help optimize drug therapy with ARIs and be a useful method to predict the onset of complications due to the upregulation of the polyol pathway.

Pharmacological Properties of Fidarestat, a Potent Aldose Reductase Inhibitor, Clarified by Using Sorbitol in Human and Rat Erythrocytes

We examined the effect of fidarestat on the increase in sorbitol content in erythrocytes from healthy volunteers in vitro. Fidarestat inhibited the increase with an IC50 value of 18 nmol/l. A subsequent experiment showed that fidarestat had a similar inhibitory effect on the increase in sorbitol content in erythrocytes from diabetic patients. On the other hand, epalrestat, the only aldose reductase inhibitor used clinically, inhibited increase in sorbitol content at a concentration over 500-fold higher than fidarestat. Although the IC50 value of fidarestat was not affected by fasting plasma glucose, HbA1C, age, aldose reductase content or gender, there was a significant positive relationship between the IC50 value of epalrestat and fasting plasma glucose. In addition, in fidarestat (0.25-2 mg/kg)-treated diabetic rats, the inhibitory rate for erythrocyte sorbitol accumulation was well correlated with that for nerve sorbitol accumulation, which indicates that erythrocyte sorbitol is available for assessing the state of sorbitol pathway flux in target tissue after fidarestat administration. These results suggest that fidarestat potently inhibits the increase in sorbitol pathway flux in diabetic patients independent of various factors and that erythrocyte sorbitol is useful for its estimation.

Presence of a closely related subgroup in the aldo-ketoreductase family of the mouse

Aldose reductase (alditol:NAD(P)+ 1-oxidoreductase), an enzyme implicated in the pathogenesis of various diabetic complications, catalyzes the reduction of a variety of aldehydes. From a mouse kidney library, we isolated aldose reductase cDNA that encodes a 316-amino-acid protein with approximately 97% identity to rat lens aldose reductase, approximately 69% identity to the mouse vas deferens protein and also approximately 69% identity to mouse fibroblast growth-factor-1-regulated protein. RNA-blot analysis demonstrated abundant expression of the enzyme transcript in the testis, skeletal muscle and kidney. However, a very low level of the transcript was detected in the sciatic nerve and lens, where abundant expression and involvement of the enzyme in diabetic complications were documented in other animals species. The isolated cDNA was expressed in Escherichia coli and the recombinant protein was purified to homogeneity by affinity chromatography and chromatofocusing. The expressed enzyme demonstrated reductase activity for various aldo sugars but not for the steroids. The enzyme reaction with DL-glyceraldehyde was, however, competitively inhibited by progesterone or 17 alpha-hydroxyprogesterone. The results not only indicate a unique tissue distribution and enzyme attribute of mouse aldose reductase, but also the presence of a closely related subgroup within the aldo-oxidoreductase superfamily in mouse tissues.

NADPH-dependent enzyme-catalyzed reduction of aldophosphamide, the pivotal metabolite of cyclophosphamide

One of the metabolites found in the urine of mammals given the prodrug cyclophosphamide is alcophosphamide, an alcohol. It is most probably generated from cyclophosphamide via aldophosphamide, an aldehyde which otherwise can directly give rise to phosphoramide mustard; the latter effects the cytotoxic action of cyclophosphamide and other oxazaphosphorines. It has already been demonstrated that horse liver alcohol dehydrogenase catalyzes the reduction of aldophosphamide to alcophosphamide. Herein, we report that aldose reductase and aldehyde reductase purified from human placenta also catalyze this reaction. The Km values for aldose reductase- and aldehyde reductase-catalyzed reduction of aldophosphamide to alcophosphamide were 0.15 and 1.6 mM, respectively. Aldose reductase and aldehyde reductase accounted for 94 and 6%, respectively, of total placental pyridine nucleotide-dependent enzyme-catalyzed aldophosphamide (160 microM) reduction. Aldose reductase-catalyzed reduction of aldophosphamide appeared to be noncompetitively inhibited by sorbinil; the Ki value was 0.4 microM. The in vivo significance of these observations is uncertain but could be of some magnitude since alcophosphamide is known to be only weakly cytotoxic.

Quantitative determination of human aldose reductase by enzyme-linked immunosorbent assay. Immunoassay of human aldose reductase

An antibody-sandwich enzyme-linked immunosorbent assay (ELISA) for evaluating tissue levels of aldose reductase was developed using a polyclonal antibody prepared against the recombinant enzyme expressed in a baculovirus system. The specificity of this antibody to aldose reductase was verified by immunoprecipitation, immunoblotting and ELISA. The polyclonal antibody did not crossreact with human aldehyde reductase, an enzyme in the same aldo-keto reductase family structurally and functionally related to aldose reductase. The sensitivity and specificity of this assay method enabled direct determination of aldose reductase level in various human tissues including the erythrocyte. The highest level of aldose reductase was detected in the kidney medulla among tissues investigated. More than a 2-fold variability in the erythrocyte aldose reductase was demonstrated among healthy individuals, indicating the heterogeneity of this enzyme expression in a human population. This assay system may be useful for direct measurement of the level of tissue aldose reductase in conjunction with the evaluation of the efficacy of aldose reductase inhibitors prescribed for the treatment of diabetic complications.

Aldose reductase inhibitors as pathobiochemical probes

In tissues susceptible to damage from chronic diabetes, excess glucose is metabolized by aldose reductase (AR) to sorbitol. Originally, AR-catalyzed sorbitol formation (and accumulation) was found in the diabetic lens; the cataractogenicity of this process was proven by preventing cataract formation with an AR inhibitor (ARI). These findings were extended to the hypothesis that, in diabetic tissues, excessive intracellular sorbitol formation initiates a cascade of metabolic abnormalities which gradually progress to loss of functional and structural integrity. The pivotal role of AR as a trigger for such abnormalities was established by preventing their occurrence in diabetic animals treated with an ARI. By inference, this led to the concept that inhibition of AR should prevent, arrest, and, possibly, reverse the development of late diabetic sequelae. In addition to motivating drug-oriented research, the ARI concept provided a rationale for the use of ARIs as experimental tools to probe the pathogenesis of diabetic complications. By helping to elucidate the metabolic, functional, and structural ramifications of the AR-catalyzed disposal of excess glucose in diabetic schemes, and in addition, by helping to define the applicability of animal models for the study of early functional pathogenic alterations occurring in diabetic subjects, ARIs may enable the discrimination in diabetic tissue of arrestible and reversible from the irreversible abnormalities.