FIGURE 11-7 Biochemical steps in the synthesis of epinephrine by the chromaffin cell of the adrenal medulla.

in Figure 11-9. The major reactions involve oxidation by monoamine oxidase (MAO), a mitochondrial enzyme in liver, kidneys, brain, and adrenergic nerve endings. The second major catabolic reaction is methyl-ation. This is catalyzed by catechol O-methyltransfer-ase (COMT), which is widely distributed, especially in liver and kidneys, but it is absent from nerve endings. Its location may be in the cytosol and possibly in the cell membrane. The aldehydes produced are converted to acids by aldehyde dehydrogenase. The aldehydes can also be reduced to form alcohols. COMT methyl-ates catecholamines at the C-3 hydroxyl of the ring and, to a vastly smaller extent, the p-hydroxyl group. There are variable amounts of MAO and COMT in different tissues, but the catecholamines eventually reach the liver and kidneys, which contain these en zymes. Consequently, the urinary products appear in a fairly consistent pattern (Figure 11-9). The majority of these excretion products are conjugated as glucuron-ides or sulfates, the latter being the more abundant form.

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