The majority of tyrosine that does not get incorporated into proteins is catabolized for energy production. One other significant fate of tyrosine is conversion to the catecholamines .
Norepinephrine is the principal neurotransmitter of sympathetic postganglionic endings.
Both norepinephrine and the methylated derivative, epinephrine are stored in synaptic knobs of neurons that secrete it, however, epinephrine is not a mediator at postganglionic sympathetic endings. Tyrosine is transported into catecholamine-secreting neurons and adrenal medullary cells where catechaolamine synthesis takes place. The first step in the process requires tyrosine hydroxylase , which like phenylalanine hydroxylase requires tetrahydrobiopterin as cofactor. N-methyltransferase converts norepinephrine to epinephrine. This latter reaction is one of several in the body that uses SAM as a methyl donor generating S-adenosylhomocysteine.