REVIEW When mixed with levodopa, carbidopa inhibits the peripheral conversion of levodopa to dopamine and the decarboxylation of oxitriptan to serotonin by aromatic L-amino acid decarboxylase. This results in increased amount of levodopa and oxitriptan available for transport to the CNS. Carbidopa also inhibits the metabolism of levodopa in the GI tract, thus, increasing the bioavailability of levodopa.
Vickers S, Stuart EK, Bianchine JR, Hucker HB, Jaffe ME, Rhodes RE, Vandenheuvel WJ: Metabolism of carbidopa (1-(-)-alpha-hydrazino-3,4-dihydroxy-alpha-methylhydrocinnamic acid monohydrate), an aromatic amino acid decarboxylase inhibitor, in the rat, rhesus monkey, and man. Drug Metab Dispos. 1974 Jan-Feb;2(1):9-22.
Vickers S, Stuart EK, Hucker HB: Further studies on the metabolism of carbidopa, (minus)-L-alpha-hydrazino-3,4-dihydroxy-alpha-methylbenzenepropanoic acid monohydrate, in the human, Rhesus monkey, dog, and rat. J Med Chem. 1975 Feb;18(2):134-8.