Decarboxylation of stereoisomers of 3, 4-dihydroxyphenylserine (DOPS) in vitro

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  • ラット脳およびじんＬ‐芳香族アミノ酸デカルボキシラーゼによる３，４‐ジヒドロキシフェニルセリン（ＤＯＰＳ）からのノルアドレナリンの生成
  • ラット ノウ オヨビ ジン L ホウコウゾク アミノサン ダツ タンサン コウ

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Abstract

The enzymic decarboxylation of stereoisomers of DOPS was examined using rat brain and kidney decarboxylase. Optimal assay conditions for racemic threo-and racemic erythro-DOPS decarboxylation were determined by the experiments concerning 1) time course, linear for 20 min, 2) optimal pH, pH 8.2, 3) optimal temperature, 37°C except racemic L-threo-DOPS decarboxylation by kidney enzyme and 4) protein concentration, 1 to 5 mg in incubation medium. Under the optimal assay condition, Km of brain enzyme for L-threo-DOPS was 1.43 × 10-3M and Vmax, 2.22 nmoles NE/mg/15min, and Km for racemic erythro-DOPS was 10-3M and Vmax, 4.3 nmoles NE/mg/ 15min. Km of kidney enzyme for L-threo-DOPS was 1.37 × 10-3 M and Vmax, 21 nmoles NE/mg/15 min, and Km for racemic erythro-DOPS was 8.7 × 10-4 M and Vmax, 16.7 nmoles NE/ mg/15min. On the other hand, D-threo and D-erythro-DOPS were scarcely decarboxylated. Decarboxylation of L-threo-DOPS in kidney enzyme was markedly inhibited by D-threo-DOPS. Kinetic analysis revealed that the type of inhibition was non-competitive. In helically-cut strips of isolated rabbit aorta, the contractile response to NE (10-8g/ml) formed from L-threo-DOPS was 95% that of 1-NE (10-8g/ml) while the response to NE (10-8g/ml) formed from racemic erythro-DOPS was not detectable. These results suggest that L-threo-DOPS is a more effective precursor of natural 1-NE than racemic threo-DOPS.

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