Human hepatitis B virus (HBV) DNA polymerase activity was inhibited by pyridoxal 5'-phosphate (PLP) specifically and noncompetitively with respect to deoxythymidine triphosphate (DTTP). NaBH4 reduction of PLP-HBV core proteins resulted in the complete inactivation of HBV DNA polymerase, and PLP modification of the enzyme was though to be mediated through Schiff-base formation. HBV DNA polymerase has a Michaelis constant (Km) of 0.31 microM for dTTP and an apparent inhibition constant (Ki) of 0.2 mM for PLP. Its inactivation and modification by PLP may be useful in the study of not only the reaction mechanism of catalysis, but also the physicochemical nature of the enzyme.