Saturable and nonsaturable process of sugar uptake: Effect of oncogenic transformation in transport and uptake of nutrients

Abstract

Uptake of sugars into cells by a saturable process increased enormously during and after transformation, and uptake by a nonsaturable process increased significantly but less remarkably compared to controls. The drastic change of uptake rates, observed at around 5 × 10⁻³ M sugar during and after transformation, emphasizes the significant observation that transition of the sugar uptake system from a saturable to a nonsaturable process occurs near the physiological concentration of D‐glucose normally seen in animal blood. At concentrations higher than 5 × 10⁻³ M, where a saturable process is barely involved, nonsaturable uptakes of D‐glucose, D‐mannose, D‐galactose, 2‐deoxy‐D‐glucose and 3‐O‐methyl‐D‐glucose proceed tens to hundreds fold faster than the rate of simple diffusion of L‐glucose. These findings suggest that nonsaturable uptake of the sugars known to be substrates for the saturable transport carrier system may not be a physical process or simple diffusion, as observed for L‐glucose uptake. Rather, the nonsaturable uptake might be part of the total physiological process which, along with the saturable process, is controlled by a membrane‐coordination mechanism. A plausible mechanism is discussed in which negative cooperativity of nutrient uptake, such as that found in bacteria, is involved.