Studies on the Formation of 3-Deoxyglucosone- and Methylglyoxal-Derived Hydroimidazolones of Creatine during Heat Treatment of Meat.

Dicarbonyl compounds such as methylglyoxal (MGO) and 3-deoxyglucosone (3-DG) are formed via caramelization and the Maillard reaction in food during heating or in vivo as byproducts of glycolysis. Recently, it was shown that creatine, an amino compound linked to the energy metabolism in vertebrate muscle, reacts rapidly with methylglyoxal under physiological conditions to form N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine (MG-HCr), a methylglyoxal-derived hydroimidazolone of creatine. Based on the observation that heated meat contains only small amounts of MGO and 3-DG when compared to many other foodstuffs, the aim of this study was to investigate a possible reaction of creatine with 3-DG and MGO in meat. From incubation mixtures consisting of 3-DG and creatine, a new hydroimidazolone of creatine, namely N-(4-butyl-1,2,3-triol-5-oxo-1-imidazolin-2-yl)sarcosine (3-DG-HCr), was isolated and characterized via spectroscopic means. To quantitate 3-DG-HCr and MG-HCr, meat and fish products were analyzed via HPLC-MS/MS using isotopically labeled standard material. Whereas samples of raw fish and meat contained only trace amounts of the hydroimidazolones (below 5 µg/kg), up to 28.3 mg/kg MG-HCr and up to 15.3 mg/kg 3-DG-HCr were found in meat and fish products. The concentrations were dependent on the heat treatment and presumably on the smoking process. In comparison to the lysine and arginine derivatives CEL, pyrraline, and MG-H1, the derivatization rate of creatine as MG-HCr and 3-DG-HCr was higher than of lysine and arginine, which clearly demonstrates the 1,2-dicarbonyl scavenging properties of creatine in meat.

Creatine is a scavenger for methylglyoxal under physiological conditions via formation of N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine (MG-HCr).

Following incubation of methylglyoxal and creatine under physiological conditions, N-(4-methyl-5-oxo-1-imidazolin-2-yl)sarcosine (MG-HCr) was isolated and identified by NMR and mass spectrometry. Due to its rapid formation, MG-HCr represents a specific product following "scavenging" of methylglyoxal by creatine. Using hydrophilic interaction chromatography coupled to mass spectrometry, MG-HCr was analyzed in urine samples of healthy volunteers. Daily MG-HCr excretion of nonvegetarians ranged from 0.35 to 3.84 µmol/24 h urine (median: 0.90 µmol/24 h urine) and of vegetarians from 0.11 to 0.31 µmol/24 h urine (median: 0.19 µmol/24 h urine), indicating that formation of MG-HCr in vivo is influenced by the dietary intake of creatine. The trapping of methylglyoxal by creatine may delay the formation of advanced glycation compounds in vivo and, therefore, could be of special importance in situations in which the body has to deal with pathophysiologically increased amounts of dicarbonyl compounds ("carbonyl stress"), for instance in diabetic patients.

Advanced glycation end products, physico-chemical and sensory characteristics of cooked lamb loins affected by cooking method and addition of flavour precursors.

The influence of the addition of a flavour enhancer solution (FES) (d-glucose, d-ribose, l-cysteine and thiamin) and of sous-vide cooking or roasting on moisture, cooking loss, instrumental colour, sensory characteristics and formation of Maillard reaction (MR) compounds in lamb loins was studied. FES reduced cooking loss and increased water content in sous-vide samples. FES and cooking method showed a marked effect on browning development, both on the meat surface and within. FES led to tougher and chewier texture in sous-vide cooked lamb, and enhanced flavour scores of sous-vide samples more markedly than in roasted ones. FES added meat showed higher contents of furosine; 1,2-dicarbonyl compounds and 5-hydroxymethylfurfural did not reach detectable levels. N-ε-carboxymethyllysine amounts were rather low and not influenced by the studied factors. Cooked meat seems to be a minor dietary source of MR products, regardless the presence of reducing sugars and the cooking method.

Dietary influence on urinary excretion of 3-deoxyglucosone and its metabolite 3-deoxyfructose.

3-Deoxyglucosone (3-DG), a reactive 1,2-dicarbonyl compound derived from d-glucose in food and in vivo, is an important precursor for advanced glycation endproducts (AGEs). At present, virtually no information about the metabolic transit of dietary 3-DG is available. One possible metabolic pathway of 3-DG during digestion is enzymatic transformation to less reactive compounds such as 3-deoxyfructose (3-DF). To study the handling of dietary 1,2-dicarbonyl compounds by the human body, 24 h urinary excretion of 3-DG and its metabolite, 3-deoxyfructose, was investigated. Urinary 3-DG and 3-DF excretion was monitored for nine healthy volunteers following either a diet with no dietary restrictions or a diet avoiding the ingestion of 3-DG and other Maillard reaction products ("raw food" diet). During the "raw food" diet, the urinary 3-DG and 3-DF excretion decreased approximately to 50% compared to the excretions during the diet with no restrictions. When subjects received a single dose of wild honey (50 g) naturally containing a defined amount of 3-DG (505 µmol), median excretion of 3-DG and 3-DF increased significantly from 4.6 and 77 to 7.5 and 147 µmol/day, respectively. The obtained experimental data for the first time demonstrate a dietary influence on urinary 3-DG and 3-DF levels in healthy human subjects.

Metabolic transit of dietary methylglyoxal.

Methylglyoxal (MGO) is responsible for the pronounced antibacterial activity of manuka honey, in which it may reach concentrations up to 800 mg/kg. As MGO formed in vivo is discussed to play a role in diabetic complications, the metabolic transit of dietary MGO was studied within a 3 day dietary recall with four healthy volunteers. Determination of MGO in 24 h urine was performed with GC-MS after derivatization with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine, and D-lactate was quantified enzymatically. Following a diet virtually free from MGO and other glycation compounds, a defined amount of MGO (500 µmol in manuka honey) was administered in the morning of day 2. Renal excretion was between 0.1 and 0.4 µmol/day for MGO and between 50 and 220 µmol/day for D-lactate. No influence on excretion of both compounds was observed following administration of MGO. To investigate the stability of MGO under physiological conditions, a simulated in vitro gastrointestinal digestion was performed with MGO-containing honey. After 8 h of in vitro digestion, only 5-20% of the initial methylglyoxal was recovered. This indicates that dietary MGO is rapidly degraded during the digestion process in the intestine and, therefore, exerts no influence on the MGO level in vivo.

1,2-dicarbonyl compounds in commonly consumed foods.

1,2-Dicarbonyl compounds, formed from carbohydrates during thermal processing in the course of caramelization and Maillard reactions, are intensively discussed as precursors for advanced glycation endproducts in foods and in vivo. To obtain information about the uptake of individual compounds with commonly consumed foods, a comprehensive analysis of the content of 3-deoxyglucosone (3-DG), 3-deoxygalactosone (3-DGal), and methylglyoxal (MGO) together with 5-hydroxymethylfurfural (HMF) in 173 food items like bakery products, pasta, nonalcoholic and alcoholic beverages, sweet spreads, and condiments was performed. Following suitable cleanup procedures, 1,2-dicarbonyl compounds were quantitated after derivatization with o-phenylenediamine via RP-HPLC with UV detection. 3-DG proved to be the predominant 1,2-dicarbonyl compound with concentrations up to 410 mg/L in fruit juices, 2622 mg/L in balsamic vinegars, and 385 mg/kg in cookies, thus exceeding the corresponding concentrations of HMF. 3-DGal was found to be of relevance in many foods even in the absence of galactose. MGO was only of minor quantitative importance in all foods studied, except for manuka honey. Dietary intake was estimated to range between 20 and 160 mg/day for 3-DG and 5 and 20 mg/day for MGO, respectively.

3-deoxygalactosone, a "new" 1,2-dicarbonyl compound in milk products.

1,2-Dicarbonyl compounds are formed in food during Maillard and caramelization reactions. 3-Deoxy-D-threo-hexos-2-ulose (3-deoxygalactosone, 3-DGal) and galactosone, two 1,2-dicarbonyl compounds originating from the degradation of galactose, were synthesized and converted to the respective quinoxalines, which were characterized by NMR spectroscopy. Analytical separation of the quinoxalines from the epimeric glucose-derived quinoxalines of 3-deoxyglucosone (3-DG) and glucosone was achieved by RP-HPLC on an RP-phenyl column. This method was used to study the relevance of galactose-derived 1,2-dicarbonyl compounds in a variety of foods. 3-DG and 3-DGal were quantified besides 3-deoxypentosone, methylglyoxal, and glyoxal after derivatization with o-phenylenediamine in lactose-hydrolyzed UHT milk, ranging from 2.5 to 18 mg/L and from 2.0 to 11 mg/L, respectively. The concentrations of both compounds tended to be higher in other lactose-hydrolyzed food items as well. During storage of lactose-hydrolyzed milk, the concentrations of the 3-deoxyhexosones first increased, but especially the concentration of 3-DGal tended to decrease on prolonged storage, pointing to lower stability of the compound. 3-DGal was also detected in galactose-free food items such as apple juice and beer. The possible formation of 3-DGal from 3-DG by 3,4-dideoxyglucosone-3-ene as an intermediate is discussed. Compared to the relatively high concentrations of 3-DG and 3-DGal, 3-deoxypentosone, methylglyoxal, and glyoxal were of only minor quantitative importance in all foods studied.