Decarboxylation of newly formed DOPA by caudate nucleus synaptosomal particles

The present study compares dopamine formation from two different sources of DOPA: preformed and added to the medium and that newly formed by the hydroxylations of phenylalanine or tyrosine. Synaptosomal preparations from rat brain caudate nucleus were incubated with 3H-labeled DOPA mixed as cosubstrate with either [14C]phenylalanine or [14C]tyrosine. Following the incubation, DOPA and dopamine were separated and their isotope ratios (isotope in phenylalanine (tyrosine) X 100/isotope in DOPA cosubstrate) were determined and compared. The results show that the ratio in dopamine was not only different from that in DOPA but was in fact 8.3-15.0 times higher. Although the absolute values of the isotope ratios were affected by any changes of the cosubstrate concentrations, the ratio in dopamine was found to be higher than in DOPA at all the substrate concentrations tested. When the synaptosomes were separated post-incubation from the medium and analyzed, the ratio in dopamine was also found to be higher than that in DOPA. Also, the large difference between the intrasynaptosomal ratios persisted throughout the incubation period ranging from 10 to 45 min. The results suggest that the DOPA which is newly formed by the hydroxylations, (a) may not be freely exchangeable with added DOPA because of compartmentation and (b) may be preferentially decarboxylated to dopamine.