The strategy for therapy for dopamine deficiency in the substantia nigra of individuals with Parkinson's disease is indicated by which of the following?
Feedback inhibition of dopamine oxidation
Competitive inhibition of biosynthesis from histidine
Provision of metabolites in the tyrosine pathway
Stimulation of monoamine oxidase
Dopamine is produced from L-dopa, which in turn is made from tyrosine. Therapy with the L-dopa precursor increases dopamine concentrations and improves the rigidity and immobility that occur in Parkinsons disease. Dopamine is degraded in the synaptic cleft by monoamine oxidases A and B (MAO-A and MAO-B), producing 3,4- dihydroxyphenylacetaldehyde (DOPAC). DOPAC is in turn broken down to homovanilhic acid, which can be measured in spinal fluid to assess dopamine metabolism. Inhibitors of MAO-A and MAO-B have some use in treating Parkinson's disease. The metabolism of histidine or alanine is not related to that of dopamine. but phenylalanine is a precursor of tyrosine and L-dopa.