7S globulin from soybean is metabolized in human cell cultures by a specific uptake and degradation system

We examined the biological fate of 7S globulin from soybean in a hepatoma cell line (Hep G2) and in human skin fibroblasts (HSF) to gain new insights into the 7S globulin cell process, the final effect of which is an enhanced expression of the LDL-receptor. The ability of 7S globulin to bind and to be internalized and degraded by both cell types was investigated under different experimental conditions. In all cases, specific uptake (binding + internalization) and degradation of 125I-7S globulin were curvilinear functions of substrate concentration at 37 degrees C. The two processes were saturated at around 80 mg/L, a concentration at which an up-regulation of LDL-receptor was previously reported. The specific uptake of 125I-7S globulin at 37 degrees C was a curvilinear function of time, and achieved equilibrium after 6 and 12 h in HSF and Hep G2 cells, respectively. Binding experiments, conducted at 4 degrees C in Hep G2 cells, showed a specific and saturable association of 7S globulin to the cell membrane. Linear Scatchard analysis demonstrated a single population of binding sites. The amount of 7S globulin bound at saturation (Bmax) was about 2.73 mg/L, with an apparent Kd of 21 micromol/L, assuming 175 kDa as the 7S globulin molecular weight. SDS-PAGE of Hep G2 membrane proteins incubated with 125I-7S globulin revealed a specific interaction of 7S globulin with a cell protein component with molecular weight between 14 and 21 kDa. Further studies are needed to ascertain whether this interaction is directly or indirectly related to the observed stimulation of the LDL-receptor.