Isolement et détermination de la composition qualitative de peptides issus de la caséine, stimulant la croissance de <em>Streptococcus thermophilus</em>.

In order to ascertain the physiological role of the peptides which enhanced, the growth of Streptococcus thermophilus in milk, fourteen stimulating peptides were isolated and purified and their qualitative amino-acid composition was determined. The purification process was the following: after hydrolysis at pH 7.5 of whole casein by the neutral protease from Micrococcus caseolyticus the stimulating peptides were fractionated by gel chromatography (Bio-Gel P6), by column chromatography on cation-exchange resins AG 50W X2 (0.05 M pyridine-formate buffer at pit 3.10 to 1.5 M pyridine-acetate buffer at pH 5:10). Further purification was made by column chromatography on anion-exchange resins AG1 X2 (0.1 M pyridine-acetate buffer of pH 7.5 to 6 M acetic acid with a concave gradient system) and by paper chromatography (solvent: butanol-acetic acid-water, 4:1:5, v/v/v) and high tension electrophoresis (30 min; 60 V/cm). Gel filtration of enzymic digest of the whole casein led to six peptide fractions. Three fractions were stimulatory and exhibited a mean molecular weight of (B) 2500, (C) 1500, (D) 1000. Fraction C was the most stimulatory. Three other fractions were inhibitory and contained the highest molecular-weight peptides (about 5000) and peptides with high contents of aromatic residues. Ten and twenty peptide fractions were isolated respectively from Fractions B and D by chromatography on ion-exchange resins; but the peptides obtained after further purification were no more stimulatory. Eleven stimulatory and two inhibitory fractions were separated from Fraction C by column chromatography on cation-exchange resins. Purified stimulatory peptides were obtained from two fractions (C1 and C42) only, after anion-exchange resins (three peptides from Fraction C42, eleven peptides from Fraction C1). These fourteen purified stimulatory peptides display a very variable qualitative amino-acid composition. The polar part (Lys, Glx, Asx) was as important as the hydrophobic part (Pro, Leu, Val, Ala, Gly). The number of distinct amino-acid residues from one peptide was also quite variable: fourteen to four residues. Only lysyl residue was present in all peptides and tryptophan was not present. Three peptides that contained cysteine were issued from %-casein [18]; two peptides (C1a2α and C1C3) could be located respectively to Ala26…Gln22… and Ile26… Arg119 regions from bovine αs1B casein. A complete mixture of amino acids exerted growth stimulation. The set of the amino acids present in a stimulatory peptide was always stimulatory. These data substantiate the following conclusions: (a) stimulatory peptides acted as amino acid source; the amino acids have to be supplied in specific proportions in order not to disturb the cellular amino acids pool and the regulation of amino acid synthesis by Str. thermophilus; (b) the requirements in the peptide-length residues and lysyl residue could be explained by a control system of peptide intake, for example, by a peptide-transport system that would differ from that described in Escherichia coli. [ABSTRACT FROM AUTHOR]