Tryptophan biosynthesis and production of other related compounds from indolepyruvic acid by mixed ruminal bacteria, protozoa, and their mixture in vitro

抄録

Tryptophan (Trp) biosynthesis and the production of other related compounds by mixed ruminal bacteria (B), protozoa (P), and their mixture (BP) in an in vitro system were quantitatively investigated by using 1 mM of indole-3-pyruvic acid (IPA) as substrate. Ruminal microorganisms were anaerobically incubated at 39°C for 12 h. Trp and other related compounds in both the supernatants and the microbial hydrolyzates of the incubation were analyzed by HPLC. As a whole, about 334, 440, and 436 μM of Trp were produced from IPA in 12 h by B, P, and BP suspensions, respectively. In the B suspension, a greater portion of synthesized Trp (242 μM) from IPA was accumulated as free form in the medium, whereas a large amount of Trp (92 μM) was incorporated into cell protein in a 12-h incubation. On the other hand, in the P suspension, a large amount of Trp (475 μM) from IPA was also found as free form in the supernatant in a 12-h incubation. Protozoa did not incorporate Trp into cell protein, but they liberated endogenous Trp (34 μM) into the medium. The net productions of Trp from IPA were 344.3 and 447.7 μmol/g of microbial nitrogen in 12 h by B and P suspensions, respectively. The ability of P to synthesize Trp from IPA was about 30% higher than that of B in 12 h. Trp produced from IPA by B, P, and BP suspensions were simultaneously degraded into its related compounds, and among them, indoleacetic acid (IAA) was a major product found in all microbial suspensions. Productions of IAA were 124, 25, and 99 μM from IPA in 12 h by B, P, and BP suspensions, respectively. The formation of indolelactic acid (ILA) from IPA was observed for the first time in all microbial suspensions, and it was about 84, 24, and 54 μM in 12 h by B, P, and BP, respectively. Higher IAA and ILA productions were observed in B when compared with P. A small amount of skatole (SKT) (26 μM) was produced from IPA in B, whereas a sizable amount of SKT (38 μM) was found in BP after a 12-h incubation. p-Cresol (CRL) was also produced from IPA by both B (43 μM) and BP (65 μM) suspensions in 12 h, and this is also the first discovery to show the formation of CRL from IPA by B and BP suspensions. BP suspension was more active to produce both SKT and CRL from IPA, though P suspension has no ability to produce either SKT or CRL from IPA during a 12-h incubation.