Elsevier

Biochemical Pharmacology

Volume 58, Issue 3, 1 August 1999, Pages 517-524
Biochemical Pharmacology

Neuroscience
Aldose reductase, a key enzyme in the oxidative deamination of norepinephrine in rats

https://doi.org/10.1016/S0006-2952(99)00121-5Get rights and content

Abstract

The sympathoneural neurotransmitter norepinephrine (NE) is deaminated to 3,4-dihydroxymandelaldehyde (DHMAL) and subsequently converted to either 3,4-dihydroxymandelic acid (DHMA) or 3,4-dihydroxyphenylglycol (DHPG). In this study, we investigated the relative importance of aldose reductase versus aldehyde reductase in the formation of DHPG from DHMAL. The in vitro incubation of NE with aldose reductase in the presence of monoamine oxidase (MAO) resulted in the formation of DHPG, which was confirmed by mass spectrometry. Although aldehyde reductase also generated DHPG, its activity was much lower than that of aldose reductase. With northern blotting, the expression of both aldose reductase and aldehyde reductase was detected in rat superior cervical ganglia. However, with western blotting, only aldose reductase was immunologically detectable. Treatment of rats with aldose reductase inhibitors for 3 days increased the plasma level of DHMA. There was no correlation between the selectivity of inhibitors and effects on NE metabolite levels. A significant decrease in DHPG, however, was obtained only with an extremely high dose (9 mg/kg/day) of the nonselective inhibitor AL 1576. The present study confirmed that aldose reductase generates DHPG from NE in the presence of MAO. In rat sympathetic neurons, aldose reductase appears to be more important than aldehyde reductase for the formation of DHPG. However, when aldose reductase is inhibited, it appears that aldehyde reductase can compensate for the conversion of DHMAL to DHPG, indicating redundancy in the reduction pathway.

Section snippets

Chemicals

NE bitartrate, DHPG, DHMA, DHBA, and pargyline were all obtained from the Sigma Chemical Co. Matrex Gel Orange A was a product of the Amicon Corp. Prestained molecular weight protein standards for SDS–PAGE and peroxidase substrate (4-chloro-1-naphthol and hydrogen peroxide) were purchased from Bethesda Research Laboratories Life Technologies Inc. and Kirkegaard & Perry Laboratories, Inc., respectively. 3-Deoxy-3-fluoro-d-glucose was obtained from Omicron Biochemicals, Inc. The aldose reductase

DHPG formation by aldose reductase

The formation of DHPG from NE was investigated with an in vitro incubation system containing NE, MAO, and aldose reductase (Fig. 1 ). Incubation without MAO and aldose reductase resulted in peaks corresponding to unmetabolized NE and DHBA, an internal standard added to the reaction mixture after the incubation. When MAO was added to the mixture, the NE peak disappeared, indicating that MAO almost completely metabolized the amine. However, DHPG was not detected. The mixture containing NE and

Discussion

The major first step of intraneural metabolism of NE is oxidative deamination by MAO. This step terminates the physiological role of NE as a neurotransmitter and generates the potentially cytotoxic aldehyde intermediate DHMAL [24]. Since the major metabolite formed by this step is the reduction product, DHPG, rather than the oxidation product, DHMA, either aldose reductase or aldehyde reductase must play a major role in the removal of the potentially cytotoxic aldehyde intermediate. The present

Acknowledgements

The authors thank Dr. Simone Koenig and Dr. Henry M. Fales, Laboratory of Biophysical Chemistry, National Heart, Lung, and Blood Institute, National Institutes of Health, for ES-MS analysis

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