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Neurochemieal Research, Vol. 21, No. 1, 1996, pp. 47-50 Regulation of Pyridoxal-5'-Phosphate Level by Biogenic Amines in Mouse Brain Tadashi Asakura, 1 Naoto Takahashi, 1 Tae Hirakawa, 1 Kiyoshi Ohkawa, 1 and Nozomu HibF (Accepted October 12, 1995) We investigated the relationship between the concentration of pyridoxal-5'-phosphate (PLP) and biogenic amine in mouse brain. The production of PLP from pyridoxal (PL) by pyridoxal kinase (PLK) was inhibited by the addition of dopamine (DA), norepinephrine (NE) and 5-hydroxytryp- tamine (5-HT), but not by that of epinephrine and N-acetyl-serotonin. DA and NE were combined with PLP by a non-enzymatic reaction, whereas 5-HT was bound only slightly with PLP. The conjugated product of PLP with DA was also detected by HPLC analysis when PLK activity was assayed using PL as a substrate in the presence of DA. In an in vivo investigation, the depletion of DA and 5-HT in mouse brain after an intraperitoneal injection of 5 mg/kg reserpine, led to slight elevation of the PLP level to 120% of the control level. By contrast, the increase in DA in the brain caused by intraperitoneal administration of 150 mg/kg L-DOPA caused the PLP concen- tration to decrease to 70% of the control level. However, no change in PLK activity in the brain was observed when the mice were treated with either reserpine or L-DOPA. These results suggested that the level of PLP in mouse brain was partly regulated by the concentration of biogenic amines, such as DA, NE and 5-HT, without apparent induction of PLK. KEY WORDS: Pyridoxal-5'-phosphate; dopamine; 5-hydroxytryptamine; pyridoxal kinase; reserpine; L- DOPA. INTRODUCTION Pyridoxal kinase (EC 2.7.1.35, PLK) catalyses the formation of the coenzyme, pyridoxal-5'-phosphate (PLP), in the brain and other tissues. PLP is involved at the stage ofsynthesis of dopamine (DA) (1-5), norepi- nephrine (NE) (3), 5-hydroxytryptamine (5-HT) (2, 3, 5, 6), histamine (3, 7) and ",/-aminobutyric acid (GABA) (3, 8), which are important in maintaining the integrity of chemical homeostasis in the tissues (3). It was earlier demonstrated that catecholamine was depleted in rabbit brain after treatment with reserpine (9), that a deficiency of PLP arising from the inhibition of PLK with deoxy- Department of Biochemistry 1, Jikei University School of Medicine, 3-25-8 Nishi-shinbashi, Minato-ku, Tokyo 105, Japan. 2 Biomedical Research Center, SRL Inc., Tokyo 192, Japan. pyridoxine (10) resulted in an increase in PLK activity in rabbit brain, and that an increase in DA level in rabbit brain caused by administration of L-DOPA led to a de- crease in PLK activity of the brain (9). On the other hand, it is presumed that biogenic amine directly regu- lates the concentration of PLP because the primary amine is combined with PLP via the aldehyde-group. We report here on an inverse relationship between the concentration of PLP and DA in mouse brain caused by the conjugation of PLP with DA. 47 EXPERIMENTAL PROCEDURE Materials. Reserpine was purchased from Daiichi Pharmaceuti- cals Inc. (Japan). L-DOPA was obtained from Sankyo Inc. (Japan). Pyridoxal (PL), pyridoxine (PN), pyridoxamine (PM), PLP, and pyri- doxamine-5'-phosphate (PMP) were purchased from Nacalai Tesque 0364-3190/96/0100-0047509.50/0 y 1996 Plenum Publishing Corporation