RESUMEN
Cataract formation is believed to result from an oxidative insult which decreases the antioxidant defense of the lens, particularly the vitamin C concentration. Upon oxidation, vitamin C contributes with glucose to protein glycation. It also favours tryptophan oxidation, resulting in fluorescent peptide cross-links and protein insolubilisation. The relationship between cataract and lenticular vitamin C was analysed in 48 cataractous lens nuclei classified into four severity grades, considering the sum of the colour and opacity. Ascorbic and dehydroascorbic acids were quantified by HPLC-fluorescence. The Amadori product was measured by means of furosine, advanced glycation end products by their fluorescence and tryptophan concentration by HPLC-UV. The lens vitamin C concentration significantly decreased with cataract severity, but mostly in severe brown cataracts (around 88 mumol/100 g lens in mild cataracts, and 50 mumol/100 g in dark brown lenses). The dehydroascorbic acid concentration was always low and stable (1.9 +/- 0.9 mumol/100 g), as was the furosine concentration (0.4 +/- 0.1 mumol/g). The fluorescence of insoluble advanced glycated end products was significantly higher in severe cataracts than in milder ones. The peptide tryptophan content was stable but the tryptophan to tyrosine ratio decreased and was highly correlated to the ascorbic acid concentration. Vitamin C content appears to be a good indicator of cataract severity, suggesting that oxidation could take part in cataract progression.
Asunto(s)
Ácido Ascórbico/metabolismo , Catarata/metabolismo , Núcleo del Cristalino/metabolismo , Progresión de la Enfermedad , Femenino , Fructosa/metabolismo , Humanos , Lisina/metabolismo , Masculino , Sensibilidad y Especificidad , Triptófano/metabolismoRESUMEN
New liquid UHT milks supplemented with iron (0.9-1.4 mg/100 ml), vitamin C (1-7 mg/100 ml), lactose (2-4 g/100 ml) and linoleic acid (200-400 mg/100 ml), named growth milks, have recently become available to satisfy the specific nutritional needs of children aged 1-3 years. But the iron-vitamin C mixture could activate the lactose-induced Maillard reaction and tryptophan (Trp) oxidation in proteins. We have therefore examined the Amadori product and Trp concentrations of these milks. Forty-two commercial growth milks from five firms were analysed for the Maillard reaction and the soluble protein Trp content and compared with 64 UHT milks. The furosine concentration of total proteins was two to four times higher in 'growth' milks than in standard UHT milks, indicating a proportional loss of available lysine. The Trp fluorescence of undenatured proteins soluble at pH 4.6 was almost three times lower in 'growth' than in standard milks and Trp concentration 36% lower suggesting destruction of this oxidation-sensitive amino-acid. The mechanism of Trp destruction remains to be elucidated, and the roles of iron and Amadori products determined.
Asunto(s)
Alimentos Fortificados/análisis , Hierro/farmacología , Lactosa/farmacología , Reacción de Maillard/efectos de los fármacos , Leche/química , Animales , Cromatografía Líquida de Alta Presión , Manipulación de Alimentos , Lisina/análogos & derivados , Lisina/análisis , Leche/metabolismo , Proteínas de la Leche/análisis , Triptófano/análisisAsunto(s)
Ácido Ascórbico , Enfermedades Cardiovasculares/sangre , Cobre , Diabetes Mellitus/sangre , Triptófano/química , Adulto , Anciano , Anciano de 80 o más Años , Ácido Ascórbico/análisis , Ácido Ascórbico/sangre , Catarata/metabolismo , Cobre/sangre , Dieta , Humanos , Cristalino/química , Persona de Mediana Edad , Oxidación-Reducción , Péptidos/análisis , Valores de Referencia , Análisis de Regresión , Triptófano/sangre , Triptófano/metabolismoRESUMEN
The in vitro oxidation of tryptophan (Trp) by pro-oxidant systems such as iron-ascorbate indicates that Trp is a target for oxygen radicals in vivo. The Trp in albumin and lipoproteins has been reported to be actively oxidized by hydroxyl radical (HO(â¢)) generating systems such as copper-ascorbate or PUFA (polyunsaturated fatty acids) respectively. The super-physiological concentrations of the oxidants used in these studies prompted us to examine the effect of low copper and ascorbate concentrations on Trp oxidation. Trp (10-5000 µmol/L) was incubated with 1.5 µmol/L copper plus ascorbate (0.113 and 1.13 mmol/L) at 37°C and its oxidation followed by fluorescence and high-performance liquid chromatography. The percentage of Trp oxidized by the ascorbate-copper system was inversely related to its concentration and positively related to the ascorbate concentration. High concentrations of Trp (above 50 µmol/L for 0.113 mmol/L and 500 µmol/L for 1.13 mmol/L ascorbate) are not significantly oxidized in the presence of ascorbate. The large drop in the percentage Trp oxidation at higher concentrations may be due to the chelation of copper by Trp. High concentrations of Trp (over 50 µmol/L) strongly prevented ascorbate oxidation by copper, and therefore inhibited the production of HO(â¢) needed for Trp oxidation. Protein Trp is less readily oxidized by the ascorbate-copper system than free Trp. Proteins chelate copper much better than Trp, and so inhibit its oxidative activity, at least against ascorbic acid.