Variations in the uptake and metabolism of peptides and amino acids by mixed ruminal bacteria in vitro

Mixed ruminal bacteria, isolated from sheep (Q and W) fed a concentrate and hay diet, were anaerobically incubated with either 14C-peptides or 14C-amino acids. Experiment 1 showed that uptake of both 14C-labeled substrates was rapid, but the rate for amino acids was twofold greater than for peptides (molecular weight, 1,000 to 200) initially but was similar after 10 min. Experiment 2 demonstrated that metabolism was also rapid; at least 90% of either 14C-labeled substrate was metabolized by 3 min. Of the radioactivity remaining in bacteria, approximately 30% was in the form of 14C-amino acids, but only in leucine, tyrosine, and phenylalanine. Supernatant radioactivity was contained only in tyrosine, phenylalanine, and mostly proline for incubations with 14C-amino acids but in up to 10 amino acids when 14C-peptides were the substrates. Short-term incubations (< 5 min; experiment 3) confirmed previous uptake patterns and showed that the experimental system was responsive to substrate competition. Experiment 4 demonstrated that bacteria from sheep Q possessed initial and maximum rates of 14C-amino acid uptake approximately fourfold greater (P < 0.01) than those of 14C-peptides, but with no significant differences (P > 0.1) between four 14C-peptide substrate groups with molecular weights of 2,000 to < 200. By contrast, bacteria from sheep W showed no such distinctions (P > 0.1) between rates for 14C-peptides and 14C-amino acids. Calculations suggested that peptides could supply from 11 to 35% and amino acids could supply from 36 to 68% of the N requirements of mixed ruminal bacteria.(ABSTRACT TRUNCATED AT 250 WORDS)