D-AMINO ACIDS IN ORGANISMS AND FOOD

Though natural proteins are build up by L-amino acids, small proportions of symmetrical D-molecules may occur in organic matter. Relatively high concentrations of D-amino acids were determined in some marine invertebrates. D-amino acids have also been detected in bacterial cell walls, vertebrates and even in plants. The conversion of L-amino acids into their D-forms is regulated either biochemically by enzymatic activity, or induced by heat and alkaline treatment. As a consequence, D-amino acids are obviously more widespread in food as would have been expected. D-amino acid levels of 1-10 % of the total amino acid content are not rarely to be found. Significant values have been detected particularly in dairy products like yoghurt and cheese. Expressed in absolute figures, consumption of 100g of Emmental cheese could mean an intake of 70 mg of D-amino acids. 100 ml of instant coffee was found to contain 20 mg of D-amino acids. The highest value with nearly 600 mg/100 ml was measured in liquid spice. Whereas roasted coffee and liquid spice may serve as examples for the influence of food processing (roasting, yeast hydrolysis), D-amino acid formation in fermented products is mainly due to microbial activity. The three predominating amino acids D-alanine, D-glutamic acid and D-aspartic acid in these products are integral part of bacterial cell walls. Probably the role of bacteria has been underestimated yet. Systematic studies about the impact of D-amino acid intake by humans are still lacking. Reports of possible toxic effects on liver and kidney were derived from some animal experiments. The results are strongly species-dependend. The assumption, that microwave heating of milk causes formation of specific D-amino acids did not stand experimental verification.