aliphatic amines. MAO is, in itself, an important target in drug design and is discussed
separately in chapter 8 since it represents a non-messenger target in drug design. The
intermediate aldehyde is then oxidized to the corresponding carboxylic acid or, occasion-
ally, is reduced to the alcohol. Monoamine oxidase is found mainly in mitochondrial
membranes, and occurs in multiple isozyme forms. It is a flavoenzyme in that it contains
a riboflavin coenzyme. It seems to act only on certain forms of neurotransmitter. It does
not, for example, affect the bound transmitter stored in vesicles, nor, curiously, the trans-
mitter just released. That MAO inhibitors do not increase the intensity of nerve stimula-
tion implies that there is no enzymatic destruction of freshly released transmitter.
NEUROTRANSMITTERS AND THEIR RECEPTORS 221Figure 4.7Degradative metabolism of norepinephrine.