[Synthesis of norepinephrine from 3,4-dihydroxyphenylserine by L-aromatic amino acid decarboxylase of the rat brain and kidneys].
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 degrees 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 X 10(-3)M and Vmax, 2.22 nmoles NE/mg/15min, and Km for racemic erythro-DOPS was 10(-3)M  and Vmas, 4.3 nmoles NE/mg/15min. Km of kidney enzyme for L-threo-DOPS was 1.37 X 10(-3)M and Vmax 21 nmoles NE/mg/15min, and Km for racemic erythro-DOPS was 8.7 X 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(-8)g/ml) formed from L-threo-DOPS was 95% that of 1-NE (10(-8)g/ml) while the response to NE (10(-8)g/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.
D000818:Animals; D001011:Aorta / Q000187:drug effects; D001921:Brain / Q000201:enzymology*; D003653:Decarboxylation; D004296:Dopa Decarboxylase / Q000378:metabolism*; D015103:Droxidopa / Q000378:metabolism*; D005260:Female; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007668:Kidney / Q000201:enzymology*; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009638:Norepinephrine / Q000096:biosynthesis* / Q000494:pharmacology; D011506:Proteins / Q000032:analysis; D011817:Rabbits; D051381:Rats; D012694:Serine / Q000031:analogs & derivatives*; D013237:Stereoisomerism; D013696:Temperature; D013997:Time Factors
D011506:Proteins; D012694:Serine; D004296:Dopa Decarboxylase; D015103:Droxidopa; D009638:Norepinephrine