The human pyridoxal kinase, a plausible target for ginkgotoxin from Ginkgo biloba.

Ginkgotoxin (4′-O-methylpyridoxine) occurring in the seeds and leaves of Ginkgo biloba, is an antivitamin structurally related to vitamin B6. Ingestion of ginkgotoxin triggers epileptic convulsions and other neuronal symptoms. Here we report on studies on the impact of B6 antivitamins including ginkgotoxin on recombinant homogeneous human pyridoxal kinase (EC 2.7.1.35). It is shown that ginkgotoxin serves as an alternate substrate for this enzyme with a lower Km value than pyridoxal, pyridoxamine or pyridoxine. Thus, the presence of ginkgotoxin leads to temporarily reduced pyridoxal phosphate formation in vitro and possibly also in vivo. Our observations are discussed in light of Ginkgo medications used as nootropics.