1. The activities of the enzymes involved in catecholamine synthesis, tyrosine hydroxylase, aromatic amino acid decarboxylase and dopamine β-hydroxylase, were markedly enhanced in homogenates of phaeochromocytoma compared with human adrenal medulla homogenates measured under optimum substrate concentrations.
2. It was demonstrated in fresh tumour slices that the rate of formation of dopamine (3,4-dihydroxyphenethylamine) from tyrosine was much slower than the rate of formation of dopamine from dopa (3,4-dihydroxyphenylalanine) (suggesting that tyrosine hydroxylase was the rate-limiting enzyme in noradrenaline synthesis in phaeochromocytoma) and that the tyrosine hydroxylation step was still susceptible to end-product inhibition by catecholamines.
3. It is suggested that catecholamine overproduction in phaeochromocytoma is due to the increased activities of catecholamine synthetic enzymes rather than to an insensitivity of tyrosine hydroxylase to end-product inhibition.
4. The activities of the enzymes involved in catecholamine catabolism, monoamine oxidase and perhaps catecholamine O-methyltransferase, were reduced in phaeochromocytoma. This finding provides a biochemical basis for the previous published observations of nonexocytotic release of catecholamines in these tumours. Thus excess amounts of newly synthesized noradrenaline which cannot be stored in the filled catecholamine storage vesicles may not be degraded owing to the reduced monoamine oxidase activity and could diffuse from the phaeochromocytoma into the circulation.
