Summary: PA was grown in glucose limited conditions in continuous culture at pH 7·0 in a chemically defined medium containing either free amino acids or casein as the organic nitrogen source. Apart from aspartate and threonine, which were poorly utilized at the higher dilution rates, all amino acids in the free-amino-acid medium were utilized to various extents. At the higher dilution rates, aspartate actually increased in concentration, probably due to deamidation of asparagine. The amino acid most utilized at all dilution rates was arginine, with up to 99% of the amino acid being consumed. Both casein and its α-casein fraction supported growth at a level only slightly lower than that obtained with the free-amino-acid medium, provided that either cysteine or thioglycollate was present. With the exception of tyrosine, nearly all of the amino acyl residues of α -casein were utilized to some degree. In general, the higher the concentration of each amino acid in the medium (whether free or as part of α-casein) the higher the extent of utilization by PA. Only 50% of the arginyl residues (0·16 mM) of α-casein were utilized compared with 99% of free arginine (1·5 mM) under similar conditions, suggesting that only 50% of the α-casein arginine was accessible to the organism. From a comparison of the amino acid composition of α-casein with that of the high fraction of the culture supernatant it was concluded that leucine, phenylalanine, lysine, histidine, arginine and valine were specifically released from α-casein by the endo- and exopeptidase activity of PA.


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