In vitro metabolism of tyrosine by rumen bacteria, protozoa and their mixture

An anaerobic in vitro study of tyrosine (Tyr) metabolism by rumen bacteria (B), protozoa (P) and their mixture (BP) was conducted using an HPLC analytical method to obtain some basic and systematic information on aromatic amino acid metabolism in the rumen. Most of the Tyr (> 96%) disappeared after 12 h incubation in B and BP, but only 58% in P. Tyr was converted mainly to p‐hydroxyphenylacetic acid (HPA) in all microbial suspensions. About 45% of disappeared Tyr in B and P, and about 35% in BP were converted to HPA. An appreciable amount of phenylalanine (Phe), 13 and 3% of disappeared Tyr, and a small amount of tryptophan (Trp), 8 and 1% of disappeared Tyr, were also produced from Tyr by rumen bacteria and protozoa, respectively. Small amounts of p‐hydroxyphenylpyruvic acid (about 4 and 6% of disappeared Tyr) were produced from Tyr in B and P, respectively. A moderately large amount of phenylacetic acid (14% of disappeared Tyr) was produced from Tyr in P which was 1.9 times higher than that in B. Phenylpropionic acid and p‐hydroxybenzoic acid were produced only in B and BP. It was concluded that the Tyr degradation ability of rumen bacteria was about 1.5 times higher than that of rumen protozoa. Degraded Tyr mainly produces HPA and then two other aromatic amino acids, Phe and Trp, which are considered essential amino acids for ruminants. Therefore, it is speculated that the requirement for Phe and Trp in ruminants may be partially fulfilled if Tyr is sufficiently supplied in rations.