Theoretical analysis of a translocation-like model with saturable kinetics

Mechanism of hepatocellular uptake of albumin-bound bilirubin

We previously demonstrated that unconjugated bilirubin spontaneously diffuses through phospholipid bilayers at a rate which exceeds albumin dissociation, suggesting that solvation from albumin represents the rate-limiting step in hepatic bilirubin clearance. To further examine this hypothesis, we studied the uptake of bovine serum albumin (BSA)-bound bilirubin by cultured hepatoblastoma (HepG2) cells. Uptake of bilirubin was saturable, with a K(m) and V(max) of 4.2+/-0.5 microM (+/-S.E.M.) and 469+/-41 pmol min(-1) mg(-1) at 25 degrees C. Substantial bilirubin uptake also was observed at 4 degrees C (K(m)=7.0+/-0.8 microM, V(max)=282+/-26 pmol min(-1) mg(-1)), supporting a diffusional transport mechanism. Consistent with reported solvation rates, the cellular uptake of bilirubin bound to human serum albumin was more rapid than for BSA-bound bilirubin, indicative of dissociation-limited uptake. Counterintuitively, an inverse correlation between pH and the rate of bilirubin flip-flop was observed, due to pH effects on the rate of dissociation of bilirubin from albumin and from the membrane bilayer. The identification of an inflection point at pH 8.1 is indicative of a pK(a) value for bilirubin in this range. Taken together, our data suggest that hepatocellular uptake of bilirubin is dissociation-limited and occurs principally by a mechanism involving spontaneous transmembrane diffusion.

Compartmentalized system with membrane-bound glycerol kinase. Activity and product distribution versus asymmetrical substrate supply

An artificial-membrane-bound glycerokinase chosen as a membrane-bound two-substrate-enzyme model has been used to separate two unequal compartments of a specially designed diffusion cell. An interesting feature is the asymmetry of compartments and the existence of a diffusion layer adjacent to only one face of the enzymic membrane. In such a situation the apparent enzyme activity and the product distribution in the system have been studied versus all the possibilities of combination of ATP and glycerol supply. Our approach has lead us to differentiate two different roles played by a diffusion layer adjacent to a permeable enzymic membrane. Depending on the spatial origin of the enzymic substrates (i.e. from which compartment they derive), the diffusion layer can play either the role of a passive additional resistance to that of the membrane or the role of a third compartment in which the reaction product can partially accumulate before splitting on both parts of the membrane. Our results mainly demonstrate that a membrane-bound enzyme activity and the resulting product distribution occurring in a compartmentalized system may be regulated by the cumulative effect due to the asymmetry in volumes of the compartments, the presence of a diffusion layer and the different possibilities of substrate supply. With the topography studied, which is close to that reported for many 'in vivo' situations, the product may be diffused lead to vectorial metabolism processes.

Mimicked translocation of glucose and glucose 6-phosphate with artificial enzyme membranes

An approach to the mechanism which may govern the behaviour of biological compartmentalized systems is presented. Artificial enzyme membranes with immobilized glucose oxidase, invertase or hexokinase were used to separate two compartments of a specially designed diffusion cell. Asymmetry in volume, hydrodynamic conditions and enzyme location was purposely chosen in order to create situations which could not be obtained with an enzyme free in solution, and was then used to tentatively mimic situations existing in vivo. Experiments were conducted and a translocation effect of H2O2, glucose and glucose 6-phosphate was obtained. A theoretical analysis taking into account the different identified parameters of the system was elaborated.