The biological availability of methionine sulphoxide. By Gabrielle M. Ellinger and R. Palmer, Rowett Research Institute, Bucksburn, Aberdeen

Heat processing of food proteins may impair the biological availability of the methionine they contain, with little effect on the amount of methionine determined by chemical analysis (Ellinger & Boyne, 1965; Miller, Carpenter, Morgan & Boyne, 1965).There is little information regarding the nature of the modification that makes methionine unavailable.One requisite for the product is that it readily converts to methionine (or methionine sulphoxide) during acid hydrolysis; another that it quantitatively oxidizes to methionine sulphone.Methionine sulphoxide is one compound that fits this description.Its labile relationship with methionine during acid hydrolysis is acknowledged; methionine values obtained by ion-exchange chromatography are frequently derived from the sum of methionine and methionine sulphoxide found.Miller & Samuel (1968) found that under conditions when methionine is the limiting amino acid in the net protein utilization (NPU) test, free methionine sulphoxide may partly replace methionine.In Ford's assay, Streptococcus zymogenes responded equally to free methionine and methionine sulphoxide (Miller et al. 1965).In our assays, to avoid the possible reduction of methionine sulphoxide when the medium was autoclaved, we added the sulphoxide to the autoclaved medium through a membrane filter.On an equimolar basis L-methionine sulphoxide then had 90% L-methionine activity for S. zymogenes.The problem of nutritional availability is, however, mainly concerned with amino acids in the peptide-bound state.Accordingly, casein was treated with hydrogen peroxide under conditions (Toennies & Kolb, 1939) that favour the oxidation of methionyl residues to sulphoxide but minimize reactions with other amino acids.Hydrogen peroxide was added to a suspension of casein in 0-5 N-HChmethanol (3:2 v/v) at the rate of 1-2 m-moles/m-mole methionine in the casein.After vigorous stirring for 3 h and settling overnight at room temperature, the casein was washed free from unreacted hydrogen peroxide and dried under reduced pressure at room temperature.A supplement of 1% L-tryptophan was given with oxidized casein in NPU tests.Methionine continued to be the limiting amino acid.Oxidation reduced the NPU of the casein from 71 to 58 (mean for three preparations).The 'available methionine' determined by the S. zymogenes assay fell from 2-9 to 2-3 g/16 g nitrogen.Thus it appears that the oxidation of the peptide-bound methionine to the sulphoxide reduces the availability of methionine in casein to growing rats and S. zymogenes, while free methionine sulphoxide is nearly completely utilized by S. zymogenes.