Effect of pH on in vitro ruminal conversion of L-tryptophan to 3-methylindole and indole

Three in vitro experiments were conducted to determine the effects of pH on ruminal conversion of L-tryptophan (TRP) to 3-methylindole (3-MI) and indole (IND). Experiment 1 involved 2 closed-system incubations, each with triplicate replications of buffered ruminal fluid at initial pH of 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, and 8.0. Highest molar conversions of TRP to 3-MI were 61.5 +/- 2.7% and 64.3 +/- 0.8% at initial pH treatments of 7.5 and 7.0 (final pH 7.3 and 6.9) for the 2 incubations, respectively. Experiment 2 used duplicate continuous cultures at each of 4 pH treatments. Following a common 5-day pretreatment period, effluent pH averaged 5.6, 6.1, 6.5, and 6.9 for the respective treatments. Data for 4 subsequent 5-day time periods showed production of 3-MI was affected by pH treatment (P less than 0.01), time (P greater than 0.01), and treatment X time (P less than 0.01); conversion of TRP to 3-MI reached 78% at pH 6.9 and decreased to less than 1% at pH 5.6. Production of IND was not related to pH treatment (P greater than 0.10). Total moles of volatile fatty acid (VFA) carbon produced showed a pH treatment X time interaction (P less than 0.01) which reflected a trend toward decreasing VFA production at lower pH and increasing VFA production at higher pH. Experiment 3 consisted of 2 pH treatments with additional continuous culture fermenters.(ABSTRACT TRUNCATED AT 250 WORDS)