Aromatic amino acid catabolism by lactococci

Abstract

While catabolism of amino acids is believed to play an important role in cheese f1avor development, the pathways present in cheese microflora are poorly understood. To determine the pathways of aromatic amino acid catabolism in lactococci and effects of Cheddar cheese ripening conditions on catabolic enzymes and products, eight starter lactococcal strains were screened. Cell-free extracts prepared from these strains were found to contain an oe-ketoglutarate-dependent aminotransferase activity with tryptophan, tyrosine and phenylalanine. Tryptophan, tyrosine and phenylalanine aminotransferase specifie activities (Il mol product formed/mg proteinlmin) ranged from 0.30 to 2.8 10-3 , 0.93 to 7.3 10-3 and 1.5 to 7.2 10-3 , respectively. Metabolites produced from tryptophan by a cellfree extract of Lactococcus lactis S3 were indolepyruvic acid, indoleacetic acid and indole-3-aldehyde. Indoleacetic acid and indole-3-aldehyde can form spontaneously from indolepyruvic acid under the conditions employed. A defined medium was used to determine whether the aminotransferase(s) was expressed and which metabolite(s) accumulate under conditions that simulated those of ripening Cheddar cheese in terms of pH, salt, temperature and carbohydrate starvation. The results indicated that the aminotransferase(s) was expressed and stable under these conditions. The tryptophan metabolites that accumulated were determined to be strain-specific. lactococcus / aminotransferase / aromatic amino acid / catabolism / cheese flavor Rsum -Catabolisme des acides amins aromatiques des lactocoques. Alors que le catabolisme des acides amins est peru comme jouant un rle important dans le dveloppement de la flaveur du fromage; les voies mtaboliques lies la microflore du fromage ne sont que trs partiellement tablies. Pour dterminer les voies du catabolisme des acides amins aromatiques dans les lactocoques, et les effets des conditions d'affinage du Cheddar sur les enzymes cataboliques et sur les produits, huit souches de levains lactocoques on t analyses. Des extraits de cellules prpares partir de ces souches possdaient une activit aminotransfrase oe-ketoglutarate dpendante du tryptophane, de la tyrosine et de la phnylalanine. Les activits aminotransfrases spcifiques du tryptophane, de la