Further enhancement of 3-nitropropanol detoxification by ruminal bacteria in cattle

Abstract

Majak, W. 1992. Further enhancement of 3-nitropropanol detoxification by ruminal bacteria in catile. Can. J. Anim. . Ruminal fluid obtained from cattle was used to determine the effects of different diets and supplements on the in vitro degradation of 3-nitropropanol, the miserotoxin aglycone of certatn Astragalus species such as timber milkvetch (Leguminosae). Nitroethane, an analog of the aglycone that is much less toxic to mammals, was effective as a supplement when given onllternate days. It enhanced the in vitro rates of nitropropanol degradation in all experiments, bv 70-110% .In asreement, daily doses of timber milkvetch at a level equivalent to 3 mg nitropropanol tg-1 UoOy weight*also enhanced nitropropanol degradation, suggesting an adaptation to the_alip_hatic nitro group by ruminal bacteria. The forage protein content appears to contribute to the microbial efhcacy and this wis verified when diets were supplemented with soybean meal. The protein supplement increased nitropropanol degradation by 37-44% and, as expected, it also promoted cellulose digestion in vitro.The effect of nitroethane supplementation on the absorption of nitropropanol from the rumen was examined when treated and control groups were challenged with timber milkvetch. Plasma levels of 3-nitropropionic acid, the oxidized form of nitropropanol which is detected in the blood, were significantly reduced in response to nitroethane supplementation, and rates of nitropropanol degradation by ruminal microbes were concomitantly enhanced. These combined results further elucidate the factors that control and contribute to nitropropanol detoxification and suggest possible methods ofprevention of timber milkvetch poisoning under rangeland conditions. where it is detected in the oxidized-form as 3-nitropropionic acid (Pass et al. 1984(Pass et al. , 1985 MATERIALS AND METHODS GeneralThe rumen-fistulated cattle in these studies were predominantly Jersey and Hereford steers. The studies were conducted during 1987-1990 were maintained on a test diet for at least 2 wk before strained ruminal fluid was obtained for in vitro incubations with nitropropanol (2 mM). The fluid was always collected at 09:00 h, before the morning feeding. Details on the anaerobic in vitro procedures and spectrophotometric methods of analysis were described previously (Majak et al. 1982(Majak et al. . 1986). Since the incubation times were short (2-4 h), the dilution of rumen fluid with artificial saliva (1: 1) was discontinued in these studies.

Characteristics of a nitropropanol-metabolizing bacterium isolated from the rumen

“…We postpone taxonomic placement of this organism, however, until completion of phylogenic characterization of other strains recently isolated. The amounts of 3-nitropropanol metabolized ( > 20 mM) by strain NPOHl during growth far exceed the amounts metabolized by other known 3-nitropropanol-metabolizing organisms (Majak andCheng 1981, 1983). Thus, bacteria represented by strain NPOH 1 could be of particular importance in the acquisition of tolerance by animals eating poisonous forage such as milk vetch.…”

“…Thus, bacteria represented by strain NPOH 1 could be of particular importance in the acquisition of tolerance by animals eating poisonous forage such as milk vetch. Furthermore, enhanced rates of 3-nitropropanol metabolism observed when cattle were supplemented with nitroethane, a nontoxic analog of 3-nitropropanol (Majak 1992;Majak et al 1986), may have been due to increased numbers of bacteria like strain NPOH 1. Evidence indicates 3-nitropropanol-metabolizing microbes were present at approximately lo4 cells/mL in rumen fluid collected from a cow that had no known exposure to poisonous vetchs (Anderson et al 1996).…”

“…Tolerance to the nitrotoxins can develop in animals consuming Astragalus and is dependent on rates at which the nitrotoxins are metabolized by ruminal microbes (Majak 1992). However, 3-nitropropionate, which is metabolized much more rapidly and absorbed more slowly than 3-nitropropanol, is less toxic than 3-nitropropanol (Majak and Pass 1989).…”

Denitrobacterium detoxificans gen. nov., sp. nov., a ruminal bacterium that respires on nitrocompounds.

“…Rates of ruminal metabolism of both the nitrotoxins are enhanced when animals are fed sublethal amounts of milkvetch forage (Astragalus miser var. serotinus) or a non-toxic analogue (Majak, 1992 ;Majak & Pass, 1989). Thus a selection for, or more appropriately, an in vivo enrichment of bacteria such as those we describe here may explain the host animal's acquisition of tolerance to the toxins.…”

“…Ruminants gradually adapted to forages containing these toxins can acquire a tolerance resulting from enhanced ruminal detoxification and evidence suggests that bacteria like the one isolated by us may play a major role in this detoxification (Anderson et al, 1998 ;Majak, 1992). late, strain NPOH1 T , it became apparent that this anaerobe differed markedly from known bacteria (Anderson et al, 1997).…”