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 de toxified by ruminal microorganisms. The rates of the detoxification re actions are critical to acquisition of tolerance to the plants by rumi nant animals, but further information is needed about factors which in fluence reaction rates and about the nature of the detoxification reac tions. We found that rates of disappearance of NPA and NPOH varied som ewhat 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 hi gher. We present evidence that ruminal microbes from both cattle and s heep reduce these nitro groups in situ, so that NPA is converted to be ta-alanine and NPOH is converted to 3-amino-1-propanol. These products were identified by thin-layer chromatography and, as their dabsyl der ivatives, separated by high-performance liquid chromatography. The pro duct beta-alanine was itself metabolized by these mixed suspensions of rumen microbes, so its recovery was always less than what would be es timated from NPA loss, but as much as 87% of the NPOH lost from incuba tion mixtures was recovered as 3-amino-1-propanol. Addition of sulfide and ferrous ions to suspensions of ruminal microbes increased the rat e of NPOH reduction about threefold, but rates of NPA reduction were n ot similarly increased. When incubations were under hydrogen gas inste ad of carbon dioxide, the addition of sulfide and ferrous ions led to even greater (five- to eightfold) increases in the rates of NPOH metab olism. The effect of hydrogen gas was not replaced by the effects of a ny of a number of other potential hydrogen donors that were tested.