METABOLISM OF THE PLANT TOXINS NITROPROPIONIC ACID AND NITROPROPANOL BY RUMINAL MICROORGANISMS

The nitro toxins 3-nitro-1-propionic acid (NPA) and 3-nitro-1-propanol (NPOH), which are found in many leguminous plants, are known to be detoxified by ruminal microorganisms. The rates of the detoxification reactions are critical to acquisition of tolerance to the plants by ruminant animals, but further information is needed about factors which influence reaction rates and about the nature of the detoxification reactions. We found that rates of disappearance of NPA and NPOH varied somewhat between samples of ruminal fluid but were usually about 0.4 and 0.1 mumol/ml of ruminal fluid per h, respectively, and that rates with threefold-concentrated cells from rumen fluid were correspondingly higher. We present evidence that ruminal microbes from both cattle and sheep reduce these nitro groups in situ, so that NPA is converted to beta-alanine and NPOH is converted to 3-amino-1-propanol. These products were identified by thin-layer chromatography and, as their dabsyl derivatives, separated by high-performance liquid chromatography. The product beta-alanine was itself metabolized by these mixed suspensions of rumen microbes, so its recovery was always less than what would be estimated from NPA loss, but as much as 87% of the NPOH lost from incubation mixtures was recovered as 3-amino-1-propanol. Addition of sulfide and ferrous ions to suspensions of ruminal microbes increased the rate of NPOH reduction about threefold, but rates of NPA reduction were not similarly increased. When incubations were under hydrogen gas instead of carbon dioxide, the addition of sulfide and ferrous ions led to even greater (five- to eightfold) increases in the rates of NPOH metabolism. The effect of hydrogen gas was not replaced by the effects of any of a number of other potential hydrogen donors that were tested.