Distribution of pyridoxal-5-phosphate between proteins and low molecular weight components of plasma: effect of acetaldehyde

It is found that approximately 65-70% of pyridoxal-P at physiological concentrations is bound to plasma proteins; 15% of its amount is bound to amino acids and peptides as a result of the Schiff base formation. Over 85% of pyridoxal-P associated with plasma proteins is bound to serum albumin. Inorganic phosphate and NaCl decrease the affinity of pyridoxal-P for albumin or other proteins. Acetaldehyde interacts with the alpha-amino group of the aspartic acid residue of the N-end of the polypeptide chain of the albumin molecule and with two epsilon-amino groups of the lysine residues having anomalously low value of pKa and deprotonated at physiological values of pH of the medium. Acetaldehyde competes with pyridoxal-P for the first (of the highest affinity) binding site of the coenzyme on serum albumin. Acetaldehyde is not bound at the second site of high affinity for pyridoxal-P on serum albumin.