In vitro catabolism of histidine by mixed rumen bacteria and protozoa

An in vitro study was conducted to examine the metabolism of histidine (His ) by mixed rumen bacteria (B), mixed rumen protozoa (P), and a combination of the two (BP). Rumen microorganisms were collected from fistulated goats fed with lucerne cubes (Medicago sativa) and a concentrate mixture twice a day. Microbial suspensions were anaerobically incubated with or without 2 m M each of His, or histamine (HTM), or 1 mM urocanic acid (URA) at 39 degree sC for 12 h. His and other related compounds in both supernatant and microb ial hydrolysates were analyzed by HPLC. After 6- and 12-h incubations, the net degradation of His was 26.1% and 51.7% in B, 13.5% and 20.9% in P, and 21.7% and 46.0% in BP, respectively. The rate of the net degradation of His in B (98.0 mu mol/g microbial nitrogen/h) was about 2.6 times higher than that of P during a 12-h incubation period. His was found to be degraded int o urocanic acid (URA), imidazolelactic acid (ImLA), imidazoleacetic acid (I mAA), and histamine (HTM). Of these degraded His was mainly converted into URA in all microbial suspensions. The production of ImLA and ImAA was highe r in B than in P suspensions, whereas the production of HTM was higher in P than in B suspensions. From these results, the existence of diverse catabo lic routes of His in rumen microorganism was indicated.