INTRASTRIATAL GRAFTING OF COS CELLS STABLY EXPRESSING HUMAN AROMATIC L-AMINO-ACID DECARBOXYLASE - NEUROCHEMICAL EFFECTS

To study the possibility that increasing striatal activity of aromatic L-amino acid decarboxylase (AADC; EC 4.1.1.28) can increase dopamine production in dopamine denervated striatum in response to L-3,4-dihydr oxyphenylalanine (L-DOPA) administration, we grafted Cos cells stably expressing the human AADC gene (Cos-haadc cells) into 6-hydroxydopamin e denervated rat striatum. Before grafting, the catalytic activity of the enzyme was assessed in vitro via the generation of (CO2)-C-14 from L-[C-14]DOPA. The K-m value for L-DOPA in intact and disrupted cells was 0.60 and 0.56 mM, respectively. The cofactor, pyridoxal 5-phosphat e, enhanced enzymatic activity with maximal effect at 0.1 mM. The pH o ptimum for enzyme activity was 6.8. Grafting Cos-haadc cells into dene rvated rat striatum enhanced striatal dopamine levels measured after s ystemic administration of L-DOPA. When measured 2 h after L-DOPA admin istration, the mean dopamine level in the striata of Cos-haadc-grafted animals was 2 mu g/g of tissue, representing 31% of normal striatal d opamine concentration. The mean dopamine concentration in the striata grafted with untransfected Cos cells (Cos-ut cells) was 1 mu g/g. At 6 -8 h after L-DOPA administration, striatal dopamine content in the Cos -haadc-grafted animals was 0.67 mu g/g Of tissue weight, representing 9% of intact striatum dopamine content. By contrast, the average dopam ine content in the Cos-ut-grafted animals was undetectable. These find ings demonstrate that enhancing striatal AADC activity can improve dop amine bioformation in response to systemically administered L-DOPA.