D-dopa is unidirectionally converted to L-dopa by D-amino acid oxidase, followed by dopa transaminase.
1. Many studies have shown that administration of d-3, 4-dihydroxyphenylalanine (D-dopa) produces contralateral rotation in hemi-parkinsonian animals comparable to L-dopa, with less potency and slower onset. It was postulated that D-dopa was converted to L-dopa to produce these effects. 2. To investigate the postulated chiral inversion of D-dopa to L-dopa and the related mechanism, an enantiomeric separation method for D- and L-dopa using HPLC was first established. Then, rat kidney homogenates containing D-dopa or L-dopa were incubated and subjected to HPLC to detect traces of respective enantiomer generation. The mechanism of the chiral inversion of d-dopa was explored by direct measurement of the production of L-dopa in kidney homogenates. D-dopa incubations containing different concentrations of an inhibitor of D-amino acid oxidase (DAAO) and an inhibitor of dopa transaminase were measured for L-dopa generation using HPLC. The role of DAAO in the chiral inversion of D-dopa to L-dopa was further investigated by using purified DAAO and mutant ddY/DAAO- mouse kidney lacking DAAO activity. 3. In rat kidney homogenate, D-dopa was, indeed, converted to L-dopa, whereas L-dopa was not converted to D-dopa. Sodium benzoate, a selective inhibitor of DAAO, blocked L-dopa generation in a concentration-dependant manner. In contrast with kidney homogenates of wild-type ddY/DAAO+ mice, those of mutant ddY/DAAO- mice lacking DAAO activity did not convert D-dopa to L-dopa unless exogenous DAAO protein was added. Conversely, when carbidopa, an inhibitor of dopa transaminase, was added to the homogenates, significant inhibition of L-dopa production was noted. 4. These results prove the proposal that d-dopa undergoes unidirectional chiral inversion and further suggest that D-dopa is first oxidatively deaminated by DAAO to its alpha-keto acid and then transaminated by dopa transaminase to L-dopa.
Wu
Mei
M
Laboratory of Systems Pharmacology, School of Pharmacy, Shanghai Jiao Tong University, Shanghai, China.
Zhou
Xiang-Jun
XJ
Konno
Ryuichi
R
Wang
Yong-Xiang
YX
D000818:Animals; D002230:Carbidopa / Q000378:metabolism; D003605:D-Amino-Acid Oxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004295:Dihydroxyphenylalanine / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007668:Kidney / Q000378:metabolism; D007980:Levodopa / Q000378:metabolism*; D051379:Mice; D051381:Rats; D017207:Rats, Sprague-Dawley; D020160:Sodium Benzoate / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007980:Levodopa; D004295:Dihydroxyphenylalanine; D003605:D-Amino-Acid Oxidase; D002230:Carbidopa; D020160:Sodium Benzoate