Effects of ionophores, vitamin B 6 and distiller's grains on in vitro tryptophan biosynthesis from indolepyruvic acid, and production of other related compounds by ruminal bacteria and protozoa

Abstract

Tryptophan (Trp) is an essential amino acid (AA) and it may be limiting to ruminants when feed rations are based on corn (maize). Distiller's grains (DGs) from barley are the dried residue remaining after the starch fraction of corn is fermented using selected yeasts and enzymes to produce alcohol. DG is a rich source of nutrients, because it contains polyphenol, peptides, yeast extracts (nucleic acids and amino acids), citric acid, fermented barley extract and fiber. The ionophore-type feed additives (monensin and salinomycin) modify the movement of monovalent (sodium and potassium) and divalent (calcium) ions across biologic membranes, modify the rumen microflora pattern, decrease acetate and methane production, increase propionate, may improve nitrogen utilization and can increase dry matter digestibility in ruminants. Vitamin B 6 (B 6) acts as a coenzyme in many enzymatic reactions that are involved in the metabolism of AA. An in vitro study was conducted to examine the effects of salinomycin (SL), monensin (MN), B 6 and DG on the production of tryptophan (Trp) and other related compounds from indolepyruvic acid (IPA) by mixed ruminal bacteria (B), protozoa (P) and their mixture (BP). Ruminal microbial suspensions were anaerobically incubated at 39 °C for 12 h, and analyses of Trp and its related metabolites were carried out by HPLC. When 1 mM of IPA was used as a substrate, it was mainly converted to Trp and indoleacetic acid (IAA), resulting in concentrations of 300 and 131, 410 and 28, and 435 and 112 μM in B, P and BP suspensions, respectively. A considerable amount of skatole (SKT) (in B and BP) and indolelactic acid (ILA) (in B, P and BP), and small amount of p-cresol (CRL) (in B and BP) were also produced from IPA. The production of Trp from IPA with SL, MN, B 6 and DG were increased ( P < 0.05) by 18.9–55.3, 9.0–50.4 and 5.1–16.1% in B, P and BP, respectively. The increased Trp production with these additives occurred by the simultaneous inhibition of IAA from IPA. On the other hand, addition of B 6 into B, P and BP enhanced ( P < 0.05) IAA from IPA by 18.9, 26.5 and 5.1%, respectively. Trp produced from IPA with SL was 13.8, 8.8 and 5.0% higher than that with MN in B, P and BP, respectively. Salinomycin and MN completely inhibited the SKT and CRL (in B and BP), and partially inhibited ILA (in B, P and BP) from IPA. On the other hand, B 6 stimulated the production of SKT and CRL in B and BP, and ILA in B, P and BP from IPA. No effects were observed on the production of SKT, CRL and ILA by DG. We conclude that addition of these additives to feeds may increase duodenal Trp supply in ruminants. In addition, SL and MN may also inhibit the production of clinically important Trp metabolites (SKT, CRL and ILA).