There is some evidence that there are abnormalities in catecholamine metabolism in patients with obesity and diabetes mellitus. In this study we determined the tissue levels of catecholamine-metabolizing enzymes (monoamine oxidase [MAO] and catechol-O-methyltransferase [COMT]) and norepinephrine (NE) in mice with the hereditary obese-hyperglycemic syndrome (ob/ob mice). The ob/ob mice have greater MAO activity in their kidneys (+ 43 per cent), testes (+ 51 per cent), and white adipose tissue (+ 87 per cent) than their normal-weight Httermates. There is no difference in MAO activity of pancreas, liver, brain, brown fat, heart, and islets of Langerhans in ob/ob and normal mice. The ob/ob mice have less COMT activity in their liver (−39 per cent) than normal mice. There is no difference in COMT activity in the kidney, testis, pancreas, and brain between ob/ob and normal mice. The Michaelis constant of kidney MAO for the tryptamine substrate is similar in normal and obese mice, suggesting that the increased MAO activity is due to a greater amount of tissue MAO rather than an altered affinity of MAO for the tryptamine substrate.
The ob/ob mice have a persistent elevation in MAO activity despite eugiycemia (older mice) and a reduction in weight (caloric restriction), suggesting that the elevated MAO activity in ob/ob mice is a primary rather than a secondary disturbance. The total NE content of kidney, white fat, and brown fat is similar in ob/ob and normal mice. The islet NE content is less than 3 pg. per islet in ob/ob and normal mice.