Catecholamine biosynthetic cascade, showing the six endogenous catechols found normally in human plasma and the associated synthetic and degradative enzymes. TH, tyrosine hydroxylase; DOPA, l-3,4-dihydroxyphenylalanine; DA, dopamine; DBH, dopamine-beta-hydroxylase; NE, norepinephrine; PNMT, phenylethanolamine-N-methyltransferase; EPI, epinephrine; MAO, monoamine oxidase; ALDH, aldehyde dehydrogenase; AR, aldose/alcohol reductase; DOPAC, dihydroxyphenylacetic acid; DHPG, dihydroxyphenylglycol