Role of propiolaldehyde and other metabolites in the pargyline inhibition of rat liver aldehyde dehydrogenase

doi:10.1016/0006-2952(86)90113-9

Biochemical Pharmacology

Volume 35, Issue 9, 1 May 1986, Pages 1481-1489

https://doi.org/10.1016/0006-2952(86)90113-9 Get rights and content

Abstract

The metabolism of pargyline proceeds by way of three separate cytochrome P-450 catalyzed N-dealkylation reactions: N-depropargylation, N-demethylation and N-debenzylation. Propiolaldehyde, a product of N-depropargylation, is a potent inhibitor of aldehyde dehy drogenase (A1DH). The formation of pargyline-derived propiolaldehyde by isolated rat liver microsomes in vitro was confirmed using gas chromatographic/mass spectrometric techniques. The measured rates of propiolaldehyde formation for uninduced and phenobarbital-induced microsomes in vitro were 0.2 ± 0.03 and 0.9 ± 0.2 μ;mole/30 min/ g wet weight liver respectively. However, these rates may have been artificially low due to competition between semicarbazide, the trapping agent, and microsomal proteins for the generated propiolaldehyde. CO significantly inhibited the microsome-catalyzed N-depropargylation reaction in vitro, whereas CoCl₂ pretreatment of rats partially blocked the pargyline-induced rise in blood acetaldehyde after ethanol. Inhibition of the low K_m liver mitochondrial A1DH by propiolaldehyde in vitro exhibited firstorder kinetics, which is consistent with irreversible inhibition. Acetaldehyde did not attenuate the inhibition of A1DH by propiolaldehyde in vitro or by pargyline in vivo. Propargyl alcohol, a substance which is metabolized to propiolaldehyde by alcohol dehydrogenase, also inhibited A1DH in vivo and caused a quantitatively similar rise in blood acetaldehyde after ethanol as pargyline. Other putative metabolites of pargyline, namely benzylamine and propargy lamine, inhibited A1DH in vivo, albeit to a lesser degree than pargyline, but neither of these amines inhibited A1DH directly. Monoamine oxidase was implicated in the conversion of benzylamine to an active inhibitory species, possibly an imine. From these studies, we conclude that propiolaldehyde was the primary metabolite responsible for the pargyline inhibition of A1DH in vivo; however, certain amine metabolites may have contributed to a lesser degree by conversion to yet unknown inhibitory forms.

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Citation Excerpt :
Microsomes isolated from CYP 2B1/2-induced rats were four times more active in generating propiolaldehyde and was inhibited by CO, suggesting that pargyline was N-depropargylated by cytochrome P450. Propargyl alcohol was also slightly more effective than pargyline in vivo at increasing blood acetaldehyde levels after ethanol administration in vivo to fasted rats [3] and propiolaldehyde was shown to inactivate mitochondrial aldehyde dehydrogenase [4]. Other investigators showed that induced microsomes also catalyzed pargyline N-demethylation/N-debenzylation (cytochrome P450 mediated) and pargyline N-oxidation (NADPH-dependent amine oxidase); [5].
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n-Butyraldoxime (n-BO) is known to cause a disulfiram/ethanol-like reaction in humans, a manifestation of the inhibition of hepatic aldehyde dehydrogenase (AlDH). As with a number of other in vivo inhibitors of AlDH, n-BO does not inhibit purified AlDH in vitro, suggesting that a metabolite of n-BO is the actual inhibitor of this enzyme. In re-examination of the effect of n-BO on blood acetaldehyde levels following ethanol in the Sprague-Dawley rat, we found that pretreatment with substrates and/or inhibitors of cytochrome P450 blocked the n-BO-induced rise in blood acetaldehyde in the following order of decreasing potency: 1-benzylimidazole (0.1 mmol/kg) > 3-amino-1,2,4-triazole (1.0 g/kg) > ethanol (3.0 g/kg) > phenobarbital (0.1% in the drinking water, 7 days) > SKF-525A (40 mg/kg). Rat liver microsomes were shown to catalyze the conversion of n-BO to an active metabolite that inhibited yeast AlDH. This reaction was dependent on NADPH and molecular oxygen and was inhibited by CO and 1-benzylimidazole. Hydroxylamine, postulated by others to be a metabolite of n-BO, inhibited AlDH via a catalase-mediated reaction and not through an NADPH-supported microsome-catalyzed reaction. Using GLC-mass spectrometry, 1-nitrobutane (an N-oxidation product) and butyronitrile (a dehydration product) were identified as metabolites from microsomal incubations of n-BO. However, neither of these metabolic products inhibited AlDH directly or in the presence of liver microsomes and NADPH. We conclude that another NADPH-dependent, cytochrome P450-catalyzed metabolic product of n-BO is responsible for the inhibition of AlDH by n-BO.
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Acetonemia is generally associated with the ketogenic states of fasting and diabetes. Disulfiram (DS), an inhibitor of aldehyde dehydrogenase (AlDH) that is used as an alcohol deterrent drug, is also known to elevate blood acetone in humans, but in the absence of a commensurate increase in its metabolic precursor, acetoacetate. We reexamined the effects of DS and other AlDH inhibitors on circulating ketone body levels in male rats of Sprague-Dawley descent and again demonstrated a 6- and 16-fold increase in blood acetone along with normal levels of acetoacetate at 6 and 24 hr after DS. Pargyline, another inhibitor of AlDH, maintained normal blood acetone levels in the presence of reduced acetoacetate levels. A third inhibitor of AlDH, cyanamide, administered to fasted and nonfasted rats, elevated blood acetone levels 10-fold over controls, with, however, a commensurate 5- and 7-fold increase in blood acetoacetate levels. The threshold values for the cyanamide-induced elevation of blood acetone and acetoacetate were equivalent, i.e. approximately 0.25 mmol/kg body weight (i.p.)- The elevation of acetoacetate and the inhibition of hepatic catalase activity by cyanamide are not mechanistically linked, since 3-amino-1,2,4-triazole, another inhibitor of catalase, elevated blood acetone but not acetoacetate levels. These findings suggest that DS-induced acetonemia is due to inhibition of acetone metabolism, whereas enhanced acetone formation through acetoacetate contributes significantly to cyanamide-induced acetonemia.
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Role of propiolaldehyde and other metabolites in the pargyline inhibition of rat liver aldehyde dehydrogenase

Abstract

Biochem. Pharmac.

J. biol. Chem.

Biochem. Pharmac.

Chem. Biol. Interact.

J. biol. Chem.

Life Sci.

Jap. J. Pharmac.

Biochem. Pharmac.

Biochem. Pharmac.

Archs Biochem. Biophys.

Biochem. Pharmac.

J. Pharmac. exp. Ther.

Res. Commun. chem. Path. Pharmac.

Archs Toxic.

Res. Commun. Subst. Abuse

J. med. Chem.

J. med. Chem.

Adv. exp. Med. Biol.