GENERAL HISTOLOGY

Detailed explanation-1: -Dopamine synthesis begins with the amino acid phenylalanine, and proceeds sequentially through tyrosine, DOPA, and then dopamine. Tyrosine hydroxylase is the rate-limiting enzyme in this pathway. Another important enzyme is DOPA decarboxylase, which decarboxylates DOPA to form dopamine.

Detailed explanation-2: -Metabolic Pathways of Levodopa and Dopamine. Metabolism of levodopa and dopamine may proceed by four pathways: decarboxylation, O-methylation, transamination, and oxidation. Levodopa is decarboxylated to dopamine by the enzyme AAAD. This enzyme is ubiquitously distributed in the gut, liver, and kidney.

Detailed explanation-3: -All catecholamines are synthesized from the amino acid l-tyrosine according to the following sequence: tyrosine → dopa (dihydroxyphenylalanine) → dopamine → norepinephrine (noradrenaline) → epinephrine (adrenaline).

Detailed explanation-4: -Phenylalanine and tyrosine constitute the two initial steps in the biosynthesis of dopamine, which, in its turn, is the metabolic precursor of noradrenaline and adrenaline.