Formation of Schiff Bases of O‑Phosphorylethanolamine and O‑Phospho‑d,l‑serine with Pyridoxal 5′‑Phosphate. Experimental and Theoretical Studies

Pyridoxal 5′-phosphate (PLP) is a B6 vitamer acting as an enzyme cofactor in various reactions of aminoacid metabolism and inhibiting glycation of biomolecules. Nonenzymatic glycation of aminophospholipids alters the stability of lipid bilayers and cell function as a result. Similarly to protein glycation, aminophospholipid glycation initially involves the formation of a Schiff base. In this work, we studied the formation of Schiff bases between PLP and two compounds mimicking the polar head of natural aminophospholipids, namely: O-phosphorylethanolamine and O-phospho-d,l-serine. Based on the results, the pH-dependence of the microscopic constants of the two PLP–aminophosphate systems studied is identical with that for PLP–aminoacid systems. However, the rate and equilibrium formation constants for the Schiff bases of the aminophosphates are low relative to those for the aminoacids. A theoretical study by density functional theory of the formation mechanism for the Schiff bases of PLP with the two aminophospholipid analogues confirmed that the activation energy of formation of the Schiff bases is greater with aminophosphates; on the other hand, that of hydrolysis is essentially similar with aminoacids and aminophosphates.