Nutrient oxidation by rat intestinal epithelial cells is concentration dependent

The kinetics of oxidative metabolism of glucose, glutamine, acetate and butyrate were determined in cells isolated from the jejunum and colon of young, fed rats. Jejunal and colonic cells were isolated from the same animal and incubated with a single radiolabeled substrate at concentrations ranging from 0.005 to 25 mmol/L, and (14)CO2 production was determined. Carbon dioxide production was concentration dependent and saturable for all substrates. Data points within the range of the apparent half-maximal oxidation rate were transformed by use of a Lineweaver-Burk plot to calculate K(OX), the concentration at which there was half-maximal oxidation, and Vmax, the calculated maximal rate of oxidation. In jejunal cells, the K(OX) for glucose and glutamine were 0.40 and 0.45 mmol/L, respectively. The K(OX) for glucose, glutamine and acetate ranged from 0.80 to 0.88 mmol/L in colonocytes, whereas the K(OX) for butyrate was 0.33 mmol/L. Except for butyrate in colonocytes, the observed maximal rate of oxidation was comparable to the calculated maximal rate. The substrate concentration required to ensure that substrate was not limiting for its oxidation was estimated at 5 mmol/L for glucose and glutamine in enterocytes and colonocytes, 5 mmol/L for acetate in colonocytes, and 0.50 mmol/L for butyrate in colonocytes. This study showed that attention needs to be given to the concentration of substrate used for in vitro studies.