Inhibitory effect of (+)threo-3,4-dihydroxy-phenylserine (DOPS) on decarboxylation of (-)threo-dops.

The decarboxylation of racemic threo and erythro-3,4-dihydroxyphenylserine (DOPS), norepinephrine (NE) precursors, has been reported in mammalian peripheral and central nervous system tissues in vivo (1-5). It has also been demonstrated both in vitro and in vivo that the rate of racemic erythro-DOPS decarboxylation is more rapid than that of racemic threo-DOPS (6). A recent report on the actions of the four stereoisomers of DOPS on the monoamine content of brain and heart stated that among four isomers, (+)erythro-DOPS may be decarboxylated to (,)NE and to a lesser extent, (-)threo-DOPS may also be decarboxylated to (-)NE in vivo (7). In the present work , we examined the enzymic de carboxylation of two optical isomers of threo-DOPS using rat kidney decarboxylase in vitro, and found the inhibition of (-)threo-DOPS decarboxylation by (-)threo-DOPS. Kidneys of male Wistar rats weighing 200-250 g were used as the enzyme source. Homo genates of tissues with 15 volumes of distilled water were centrifuged at 8000 x g for 10 min and the supernatant fraction was used as L-aromatic amino acid decarboxylase. Medium (final volume 2 ml) containing 50 mM phosphate buffer (pH 8.2), 10-4 M pyridoxal phos phate, 2 x 10-4 M Pargyline, 1 to 3 mg of protein of the supernatant fraction and different amounts of substrate, was incubated at 37'C. Reaction was started by addition of the substrate and stopped by the addition of 4 ml of ice cold 0.4 N perchloric acid with 10 mg of sodium metabisulfite and 200 mg of ethylene-diaminetetraacetic acid disodium salt. The amount of reaction product, NE, was determined by the method of Bertler et al. (8), using Dowex 50 x 4 columns (0.7 mm in diameter, wet resin volume 1 ml). Protein con centration of the enzyme fraction was determined by the method of Lowry et al. (9). Other chemicals used were reagent grade. Optical isomers of threo-DOPS, (-)threo and (-)threo-DOPS correspond to L-threo and D-threo-DOPS, respectively (6, 7). These compounds were synthesized in the Laboratory of Kyowa Hakko Kogyo Co. Ltd., Japan. Each purified isomer had a negative rotation of [a]° =-42.6 (purity: 99.5%) and a positive rotation of [a]'O =-43.2 (C=I, N-HCL) (purity : 99.8 %) respectively. Contamination by NE assayed in our laboratory was found to be 0.009% in (-)threo-DOPS, 0.011 in (- ;-)threo-DOPS and 0.039 in racemic threo DOPS. As shown in Fig. 1, formation of NE by decarboxylation of (-)threo-DOPS proceeded linearly for …