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

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 x 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 below higher than 5 x 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.