Reversible microsomal binding of hepatic aldolase

Area of Science:

• Biochemistry
• Cell Biology
• Enzymology

Background:

• Fructose-1,6-bisphosphate aldolase (EC 4.1.2.13) is a key glycolytic enzyme.
• Subcellular localization of glycolytic enzymes is debated for physiological relevance.
• Rat liver aldolase exists as a single isozyme found in both microsomal and cytosolic fractions.

Purpose of the Study:

• To investigate the partitioning of fructose-1,6-bisphosphate aldolase between rat liver microsomes and cytosol.
• To determine the factors influencing the association of aldolase with cellular membranes.
• To understand the kinetic properties of membrane-bound versus dissociated aldolase.

Main Methods:

• Differential centrifugation of rat liver homogenate to fractionate cellular components.
• Gel electrophoresis and immunodiffusion to identify and quantify aldolase isozymes.
• Enzyme elution assays using various metabolites and inorganic salts.
• Kinetic analysis (Km determination) of membrane-bound and dissociated aldolase.

Main Results:

• Aldolase was found in both microsomal and cytosolic fractions of rat liver homogenate.
• Enzyme-membrane association was sensitive to metabolites like fructose 1,6-bisphosphate and glucose 1,6-bisphosphate, and salts (Pi, citrate).
• Higher levels of fructose 1,6-bisphosphate in the homogenate correlated with less enzyme in the microsomal fraction.
• The Michaelis constant (Km) for fructose 1,6-bisphosphate was significantly lower for dissociated aldolase (6 x 10(-6) M) compared to membrane-bound aldolase (3 x 10(-4) M).

Conclusions:

• Fructose-1,6-bisphosphate aldolase exhibits dynamic partitioning between liver microsomes and cytosol.
• Metabolite and salt concentrations modulate aldolase's interaction with cellular membranes.
• The altered kinetic properties of membrane-associated aldolase suggest physiological significance for its subcellular localization.