Carbohydrate utilisation by cell suspension cultures of Phaseolus vulgaris

Uptake of sugar by Phaseolus vulgaris cell suspension cultures from a sucrose supplemented medium is predominantly in the hexose form. This is due to a rapid cleavage of the sucrose by an apoplastic acid invertase activity and an apparent very low demand for and uptake of carbon from the medium prior to induction of cell growth and division. Glucose is preferentially taken up, leading to an accumulation of fructose in the medium. However, when the glucose is depleted the cells do take up the fructose at a rate similar to that of glucose. When glucose or fructose is supplied individually to cell cultures, both are utilised very efficiently with growth slightly better on the fructose medium. Hexose uptake is largely an active process with diffusion uptake even at the highest concentrations (> 50 mM) contributing less than 30%. The hexose uptake system of the cells has a greater affinity for glucose (Km= 240 µM) than for fructose (Km= 960 µM) but the maximum uptake (Vmax) is similar. The major difference in the kinetic properties of hexose uptake is that glucose is a strong inhibitor of fructose uptake, while fructose has little effect on glucose uptake. The differences in the kinetic properties of the uptake system for the two hexoses can largely explain the observed pattern of hexose utilisation when both glucose and fructose are present in the medium.