Red blood cell catechol O-methyltransferase activity in thyroid dysfunction.

Using a modification of the method of Mannl et al. (Mannl, H.F.K., Hempel, K., and Kubler, W. 1972. Catechol O-methyltransferase in human erythrocytes. Arch. Pharmacol. 272, 265-276), we have measured the activity of catechol O-methyltransferase (COMT) (EC 2.1.1.6) in red blood cells (RBC) of patients with hyperthyroidism and hypothyroidism to establish whether thyroid dysfunction is associated with alterations in catecholamine catabolism. The activity of COMT averaged 4.4+/-0.54 nmol/ml RBC per hour of incubation (mean+/-SEM) in euthyroid subjects compared to 4.76+/-0.64 nmol/ml RBC per hour of incubation in hyperthyroidism and 4.42+/-0.81 nmol/ml RBC per hour of incubation in hypothyroidism; these values are not significantly different. There were no significant differences observed in urinary excretion of vanillylmandelic acid, epinephrine, and norepinephrine among the three groups. These data are compatible with the possibility that thyroid status has little influence on the degradation of circulating catecholamines and suggest that hypothyroidism, with its attendant elevations in serum norepinephrine concentration, may be related to a compensatory noradrenergic response.