Interaction between Pyridoxal Kinase and Pyridoxal-5-phosphate-Dependent Enzymes

The interactions of two pyridoxal-5-phosphate (PLP)-dependent enzymes, alanine aminotransferase (ALT) and glutamate decarboxylase (GAD), with pyridoxal kinase (PK) were studied by fluorescence polarization as well as surface plasmon resonance techniques. The results demonstrated that PK can specifically bind to ALT and GAD. Moreover, binding profiles of both enzymes to immobilized PK were altered by excess amount of PLP. The equilibrium affinity constants for ALT in the absence and presence of PLP are 20.4 × 10 ⁴ M^-1 and 6.7 × 10⁴ M^-1, and for GAD are 37 × 10⁴ M^-1 and 20.8 × 10 ⁴ M^-1, respectively. It appears that specific interactions occur between PK and PLP-dependent enzymes, and the binding affinities of PK for PLP-dependent enzymes decrease in the presence of PLP. The results support our hypothesis that PLP transfer from PK to PLP-dependent enzymes requires a specific interaction between PK and the enzyme.