Ruminal Microbes of Adult Steers Extensively Degrade L-Glutamine, but Not L-Glutamate or L-Citrulline

The microbial population within the rumen has long been considered to have the capability of extensively degrading all dietary amino acids (AA). Results from our feeding trial showed that this view might not be accurate. In vivo and in vitro studies were conducted to test the hypothesis that certain AA undergo little degradation by ruminal microbes. In the in vivo experiment, at 0.5 h before and 0, 0.5, 1, 2, and 4 h after cannulated adult steers (~500 kg, n = 6) consumed 0.56 kg dried-distillers grain mixed with 70 g L-glutamine plus 70 g L-citrulline, samples of rumen fluid and plasma were collected for AA analysis using HPLC. Results showed that the concentrations of glutamine in the rumen fluid declined rapidly, but that of glutamate (a product of glutamine catabolism) increased gradually between 0.5 and 4 h after consumption of the meal. In contrast, the concentration of citrulline in the rumen fluid was constant through the 4-h period after steers consuming the meal. The concentrations of L-citrulline and L-arginine, but not L-glutamine or L-glutamate, were increased in the steers at 1 and 2 h after the meal, respectively, when compared with the values at 0 h. In the in vitro experiments, whole rumen fluid (3 mL) from the steers was incubated at 37^oC with 5 mM L-glutamine, 5 mM L-glutamate, or 5 mM L-citrulline for 0.5, 1, 2 or 4 h, and 20 µl samples were collected at the predetermined time points for AA analyses. Results showed extensive hydrolysis of L-glutamine into L-glutamate, but little degradation of either L-glutamate or L-citrulline by rumen microbes during the 4-h period of incubation. Collectively, these in vivo and in vitro results indicate that rumen microbes of adult steers extensively degrade L-glutamine, but not L-glutamate or L-citrulline. We suggest that L-citrulline without any encapsulation or protection from rumen microbes can be effectively supplemented to the diets of ruminants to increase its concentrations and of L-arginine in plasma for utilization and metabolism by various organs and tissues. Supported by funds from Texas A&M AgriLife Research.