Activity of Adrenal Catecholamine-Producing Enzymes and Their Regulation after Stress

Abstract

The pathway for biosynthesis of catecholamines from tyrosine was first proposed by Blaschko in 1939 (3), but only recently have separate enzymatic reactions been characterized. Conversion of tyrosine to dopa is catalyzed by the enzyme tyrosine hydroxylase (TH)(23), which is believed to be the rate-limiting step in the biosynthesis of the catecholamines (17). High levels of the relatively nonspecific aromatic L-amino acid decarboxylase (7,18) rapidly decarboxylate dopa to dopamine; decarboxylation therefore appears to play little or no role in regulation of catecholamine synthesis. Dopamine enters the chromaffin granules to be converted by the enzyme dopamine-beta-hydroxylase (DBH) to norepinephrine (16). Conversion of norepinephrine to epinephrine by phenyl-ethanolamine-N-methyltransferase (PNMT) is the final step in the biosynthesis of catecholamines in the adrenal medulla (1,8).