Methionine: oxidation state in processed foods and enzyme-catalyzed reaction with adenosine triphosphate

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Virginia Polytechnic Institute and State University


Two conditions of alkaline hydrolysis of proteins, (1) 2M NaOH, 18 hours, 100°C and (2) 3M NaOH, 16 hours 110°C, prior to ion-exchange chromatography were tested on free amino acids and model protein systems to determine the better set of conditions for measurement of methionine sulfoxide in food proteins. Recoveries of methionine, methionine sulfoxide, and methionine sulfone from base-hydrolyzed amino acid mixtures were, respectively, 89, 100, and 105% with the 2M NaOH conditions and 83, 90, and 98% with the 3M NaOH conditions. The percentages of total methionine, determined by performic acid oxidation, recovered as methionine, methionine sulfoxide, and methionine sulfone after hydrolysis with 2M NaOH were, respectively 101, 0, and 0% in lysozyme, 68, 25, and 0% in oxidized lysozyme, 74, 3, and 0% in casein and 0, 74, and 0% in oxidized casein. The presence of glucose in the hydrolysis mixture with the model proteins caused as much as 8% oxidation of methionine to methionine sulfoxide. The presence of copper (II) and iron (II) ions along with sugars did not increase the amount of methionine generated in casein and a soy isolate. Methionine sulfone was never generated in any of the model systems. These results suggested that determination of methionine sulfoxide after basic hydrolysis with 2M NaOH in foods low in carbohydrates is valid but in foods high in carbohydrates the procedure may slightly overestimate the methionine sulfoxide content.

Acid hydrolysis of free methionine sulfoxide reduced 15% of the methionine sulfoxide to methionine while acid hydrolysis of oxidized lysozyme and oxidized casein led to reduction of all the methionine sulfoxide in these proteins.

Eight food products were analyzed for methionine, methionine sulfoxide, and methionine sulfone. Total methionine was measured by the performic acid oxidation method, methionine sulfone by ion-exchange chromatography after acid hydrolysis, methionine sulfoxide by ion-exchange chromatography after hydrolysis with 2M NaOH for 18 hours at 100°C, and methionine by the difference between total methionine and the sum of methionine sulfoxide and sulfone. Only a trace of methionine sulfone and less than 6% of total methionine was present as methionine sulfoxide in a soy flour and a concentrate. Two soy isolates contained 74 and 8% of total methionine as sulfoxide and 6 and 4%, respectively, as sulfone. Two soy-based infant formulas contained 17 and 12% of total methionine as the sulfoxide and 12 and 8%, respectively, as sulfone. Two milk-based formulas contained 18 and 9% as sulfoxide and 8 and 13%, respectively, as sulfone.

The feasibility of using ATP:L-methionine S-adenosyltransferase to determine the number of unaltered methionine residues in food proteins was also explored. Di- and tripeptides composed of methionine appeared to be able to function as well as L-methionine as substrates. Spectrophotometric studies suggested that the enzyme could act on methionine residues in two soy isolates; however, these results could not be confirmed by amino acid analyses of the isolates after incubation with ATP and the enzyme.