 In mammals , norepinephrine is rapidly degraded to various metabolites . The initial step in the breakdown can be catalyzed by either of the enzymes monoamine oxidase ( mainly monoamine oxidase A ) or COMT . From there the breakdown can proceed by a variety of pathways . The principal end products are either Vanillylmandelic acid or a conjugated form of MHPG , both of which are thought to be biologically inactive and are excreted in the urine . 
