Peptide fragments of a porcine benzodiazepine-binding protein were used to isolate the cDNA of a related human protein. The cDNA encodes a polypeptide of 312 amino acid residues that is homologous to a bacterial pyridoxal kinase. Transient expression of the cDNA in human embryonic kidney cells confirmed that it encodes human pyridoxal kinase. The recombinant enzyme displayed a Km value of 3.3 microM for pyridoxal and was inhibited competitively by 4-deoxypyridoxine (Ki = 2.8 microM). Benzodiazepine receptor ligands that bound to the purified porcine protein also exerted a potent inhibitory effect on human pyridoxal kinase activity. Transcripts of the pyridoxal kinase gene were detectable in all human tissues examined, and were particularly abundant in the testes. The gene is localized on chromosome 21q22.3 and represents a candidate gene for at least one genetic disorder that has been mapped to this region (autoimmune polyglandular disease type 1).