ascorbic-acid and erythrose

Previous model studies have suggested ascorbic acid as one of the major sources of furan, a possibly hazardous compound found in thermally processed foods (e.g. canned products, jars). The study showed that about 2 mmol mol(-1) furan was obtained when dry-heating ascorbic acid, while much lower amounts were formed upon pressure cooking, i.e. 58 micromol mol(-1) at pH 4 and 3.7 micromol mol(-1) at pH 7. Model reactions also generated 2-methylfuran (MF). However, the MF levels were generally very low with the exception of the binary mixture ascorbic acid/phenylalanine (1 mmol mol(-1)). Studies with 13C-labelled ascorbic acid indicated that furan comprises an intact C4 unit, mainly C-3 to C-6, generated by splitting off two C1 units, i.e. CO2 and formic acid. Possible intermediates are 2-deoxyaldoteroses, 2-furoic acid and 2-furaldehyde, which are known as ascorbic acid degradation products. The mechanism of furan formation from ascorbic acid was validated based on the labelling pattern of furan and the identification of 13CO2 and H13COOH. Furan formation is significantly slowed down in binary mixtures, e.g. the presence of erythrose led to 80% less furan under roasting conditions. This is most likely due to competing reactions in complex systems, thus disfavouring furan formation. The mitigation effect is because furan, contrary to MF, is formed without recombination of ascorbic acid fragments. Therefore, furan levels are definitely much lower in foods than expected from trials with pure ascorbic acid. Consequently, conclusions should be drawn with much caution from model reactions, avoiding extrapolation from oversimplified model systems to food products.

Topics: Amino Acids; Ascorbic Acid; Carcinogens, Environmental; Cooking; Dehydroascorbic Acid; Food Analysis; Food Contamination; Fruit; Furans; Gas Chromatography-Mass Spectrometry; Glucose; Hot Temperature; Hydrogen-Ion Concentration; Models, Biological; Solid Phase Microextraction; Tetroses; Vegetables

L-threose is a product of ascorbate oxidation and degradation. By virtue of its free aldehyde group it can form Schiff-bases with tissue proteins, altering their normal function. In this study, we have examined the possibility of its detoxification to L-threitol by aldose reductase in the lens. The rat lens enzyme present in fresh homogenate as well as after 100 fold purification was found to utilize L-threose with a km of 7.1 x 10(-4) M. The specificity of the reaction was affirmed by its inhibition with sorbinil and quercetin, the well known aldose reductase inhibitors. Further studies on the role of this enzyme in preventing toxicity due to degradation products of ascorbate are in progress.

Topics: Aldehyde Reductase; Animals; Ascorbic Acid; Crystallins; Enzyme Inhibitors; Glycosylation; Imidazoles; Imidazolidines; Kinetics; Lens, Crystalline; NADP; Oxidation-Reduction; Quercetin; Rats; Substrate Specificity; Tetroses

L-Threose is a significant degradation product of ascorbic acid at pH 7.0 in the presence of oxygen. When compared to several other ascorbate-derived degradation products, it had the greatest ability to glycate and crosslink lens proteins in vitro. To determine whether L-threose was formed in the lens, the sugars in a TCA-soluble extract from human lenses were reduced to polyols with NaBH4, acetylated and analysed by gas-liquid chromatography. The threitol levels measured were 3.4 +/- 0.8 micrograms per lens (n = 4). GC-MS measurements made after reduction with NaBD4 indicated that threitol, but little or no threose, was originally present in the human lens. Rat lenses were incubated with [1-13C]D-threose for 24 hr, and considerable D-threitol formation was seen by NMR spectroscopy. Analysis of the lenses after medium removal showed that only [1-13C]threitol was present within the lenses indicating a rapid reduction of threose within the lens, presumably by aldose reductase. Assays with human recombinant aldose reductase and with human lens cortical and nuclear extracts all exhibited sorbinil-inhibitable aldose reductase activity with L-threose as substrate. This was confirmed by incubating a preparation of [1-14C]L-tetrose (a mixture of 40% L-threose and 45% L-erythrose) with both the pure aldose reductase and crude lens extracts followed by the subsequent identification of the [1-14C]L-threitol formed by thin layer chromatography. L-Threose degrades very slowly in 0.1 M phosphate buffer at pH 7.0, but the addition of a four-fold excess of N alpha-acetyl-L-lysine accelerated the rate of disappearance of threose 30-fold, indicating a rapid glycation reaction. When [1-14C]L-tetrose was incubated with a complete bovine lens homogenate, a linear incorporation into protein was observed over a 24 hr period. Increasing levels of lens extract exhibited increasing incorporation into protein. These data confirm a rapid reactivity of L-threose with lens protein and argue that glycation would occur in vivo in spite of the presence of aldose reductase.

Topics: Aldehyde Reductase; Animals; Ascorbic Acid; Cattle; Chromatography, Gas; Crystallins; Culture Techniques; Glycosylation; Lens, Crystalline; Lysine; Sugar Alcohols; Tetroses