Metabolism of heat-damaged proteins in the rat
J. E. Ford, C. Shorrock
1971
British Journal of Nutrition
I. Freeze-dried cod muscle and casein were subjected to various conditions of heat treatment. Diets containing the different products, or the unheated materials, were given to a group of four adult male rats during successive 48 h periods, and urine was collected during the second 24 h of each 48 h period. A further collection of urine was made from the rats after they had been given protein isolated from heated skim-milk powder. The content and amino acid composition of the 'peptide' and 'free
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... amino acids' in the urines were determined. 2. Heat damage to the cod-fillet protein increased the total urinary excretion of peptidebound amino acids, from 18.6 to 48.8 pmol/rat . d. The composition of the peptide also changed, and in particular there was a marked increase in lysine, from 2.98 to 20.30 pmol yo. Three amino acidslysine, aspartic acid and glutamic acidtogether comprised nearly 70 % of the total amino acid residues. There was a corresponding increase in urinary excretion of free amino acids, from 53'7 to I 14.4 pmol/rat . d. The combined losses of lysine in urinary peptide and free amino acids were 1.5 % of the total lysine ingested, as against 0.3 % for the unheated cod fillet. 3. The effects of similar heat treatment of casein on the composition of the urinary peptide and free amino acids were less marked. There was no increase in total urinary peptide excretion and there was a smaller increase in the lysine content of the peptide. 4. In urine of rats given protein isolated from heated skim-milk powder, the peptide hydrolysate was rich in lysine and in furosine, which together comprised 41 mol % of the total amino acid composition. These compounds were presumably formed, together with a smaller quantity of pyridosine, from lysine-carbohydrate complex in the urine. It is probable that, as compared with free lysine, the lysine-carbohydrate complex was absorbed relatively inefficiently from the rat intestine. 5. The findings are discussed in relation to the wider question of the metabolism of the 'unavailable peptide' that is released in the course of digestion of heat-damaged protein. As is well known, severe overheating of protein foods impairs their nutritional quality, but the chemical mechanism of the heat damage, and the nutritional consequences, are not yet fully understood. A central process is the combination of free amino groups, comprised in the main of the €-amino groups of the lysine, with other reactive groups to form enzyme-resistant inter-and intra-molecular bonds. The solubility and digestibility of the protein are thereby reduced and the time-course of digestion is prolonged. With heated cod fillet, growth tests with rats, and microbiological assays of samples digested enzymically in vitro, indicated considerable differences in the biological availability of lysine, methionine and isoleucine (Ford & Salter, 1966) and it seemed that some of the impairment of nutritive quality could be attributed to large differences in the rates of enzymic release of different amino acids. From an earlier study of chemical and nutritional changes in heated cod muscle, Miller, Carpenter & Milner (1965) had similarly concluded that the damaging effects of heat, demonstrated in growth tests with rats, were by no means explained simply 21
doi:10.1079/bjn19710037
pmid:5571791
fatcat:d56btx6hmnbxrhieqwknwy66ci