Detoxification of aldehydes by histidine-containing dipeptides: from chemistry to clinical implications

Abstract

Aldehydes are generated by oxidized lipids and carbohydrates at increased levels under conditions of metabolic imbalance and oxidative stress during atherosclerosis, myocardial and cerebral ischemia, diabetes, neurodegenerative diseases and trauma. In most tissues, aldehydes are detoxified by oxidoreductases that catalyze the oxidation or the reduction of aldehydes or enzymatic and nonenzymatic conjugation with low molecular weight thiols and amines, such as glutathione and histidine dipeptides. Histidine dipeptides are present in micromolar to millimolar range in the tissues of vertebrates, where they are involved in a variety of physiological functions such as pH buffering, metal chelation, oxidant and aldehyde scavenging. Histidine dipeptides such as carnosine form Michael adducts with lipid-derived unsaturated aldehydes, and react with carbohydrate-derived oxo- and hydroxy-aldehydes forming products of unknown structure. Although these peptides react with electrophilic molecules at lower rate than glutathione, they can protect glutathione from modification by oxidant and they may be important for aldehyde quenching in glutathione-depleted cells or extracellular space where glutathione is scarce. Consistent with in vitro findings, treatment with carnosine has been shown to diminish ischemic injury, improve glucose control, ameliorate the development of complications in animal models of diabetes and obesity, promote wound healing and decrease atherosclerosis. The protective effects of carnosine have been linked to its anti-oxidant properties, its ability to promote glycolysis, detoxify reactive aldehydes and enhance histamine levels. Thus, treatment with carnosine and related histidine dipeptides may be a promising strategy for the prevention and treatment of diseases associated with high carbonyl load.

Fig. 1

Major pathways of aldehyde detoxification.

Fig. 2. Structures of carnosine - aldehydes reaction products

CAR, carnosine; HNE, 4-hydroxy-trans-2-nonenal; ACR, acrolein; GA, glycolaldehyde; GX, glyoxal; MG, methylglyoxal; MDA, malondialdehyde; ONE, 4-oxo-trans-2-nonenal, CAR-ACR₁, conjugates and dehydration products with 1:1 carnosine-acrolein ratio; CAR-ACR₂, conjugates and dehydration products with 1:2 carnosine-acrolein ratio; CAR-ACR₃, conjugates and dehydration products with 1:3 carnosine-acrolein ratio.

Fig. 3. Carnosine protects phospholipid from oxidation by molecular oxygen

Oxygen was bubbled at a rate of 30 ml/min through a solution containing 1-palmitoyl-2-arachidonyl phosphatidylcholine (PAPC; 25μg/ml) in 3 ml of phosphate-buffered saline (KH₂PO₄ 1.06 mM, NaCl 155.17 mM, Na₂HPO₄ 2.97 mM, pH 7.4) for 1h at 37°C in the presence and absence of 10 mM carnosine. Formation of PAPC oxidation product 1-palmitoyl-2-(5′-oxo-valeroyl)-phosphatidylcholine (POVPC) was determined by ESI mass-spectrometry and POVPC concentration is expressed as the percentage of the parent PAPC ion. *, P<0.05.

Fig. 4. Reaction of carnosine with methylglyoxal

A, UV spectra of a reaction mixture containing methylglyoxal (1 mM) and carnosine (5 and 40 mM) after 12h of incubation. B, Time course of the reaction between methylglyoxal and carnosine monitored at 280 nm. Carnosine at several concentrations was incubated with 1mM methylglyoxal in 150 mM potassium phosphate buffer, pH 7.4, at 37°C. Change in absorbance was acquired every 60 min.

Fig. 5. Carnosine protects glutathione from methylglyoxal-mediated depletion

Carnosine at indicated concentrations was incubated with 0.2 mM GSH and 0.2 mM methylglyoxal in 150 mM potassium phosphate buffer, pH 7.4, at 37 °C. Time-dependent consumption of free GSH was monitored spectrophotometrically after the addition of DTNB.