Oxalate binding to rat intestinal brush-border membrane in pyridoxine deficiency: a kinetic study

Oxalate bound specifically to the intestinal brush-border membrane (BBM) of pyridoxine-deficient rats, but not to BBM of control rats. The binding of oxalate to intestinal BBM of pyridoxine-deficient rats was rapid, reversible, dependent on concentration of oxalate, temperature sensitive and competitively inhibited by oxalate analogues. Kinetic analysis of the oxalate binding data revealed induction of two distinct classes of receptor site for oxalate. The high-affinity oxalate binding sites, reached saturation at 60-70 nM oxalate, had a Kd of 24.29 nM and the number of binding sites were 30 pmoles (i.e., 1.8.10(13) molecules). The low-affinity oxalate binding sites, could not be saturated under experimental conditions upto 1 microM oxalate. It had a Kd of 487.5 nM and the number of binding sites were 156 pmoles (i.e., 9.4.10(13) molecules). The apparent energy of activation was 19 kcal/mol. The half-saturation concentration of inhibitor (IC50) of oxalate was 0.4.10(-5) M, while all other structural analogues of oxalate had higher IC50 values. Among the competitive inhibitors tested IC50 was in the following order, pyruvate greater than maleate greater than oxaloacetate greater than glyoxylate greater than parabonate greater than oxalate. These kinetic characteristics indicate involvement of a membrane protein in oxalate binding and transport in rat intestinal brush-border membrane in pyridoxine deficiency.