CARBOHYDRATE METABOLISM IN THE THYROID GLAND*†

Abstract

The presence of the 7 principal tricarboxylic acid cycle enzymes[long dash]aconitase, fumarase, condensing enzyme, and isocitric, a-ketoglutaric, succinic, and malic dehydrogenase [long dash]was demonstrated in sheep and human thyroid tissue. Quantitative assays were adapted for the measurement of aconitase, fumarase, isocitric, succinic, and malic dehydrogenase in homogenates of this tissue. It was found that methimazole, potassium perchlorate and thiocyanate did not inhibit significantly any of these 5 enzymatic activities in sheep thyroid homogenates. These 5 enzymes were not found exclusively in mitochondria. The sheep thyroid content of aconitase, fumarase, and isocitric, malic, and succinic dehydrogenase is much lower than the liver content of these enzymes. This finding in a metabolically highly active tissue suggests the existence in this tissue of an alternate pathway of glucose catabolism. The content of 5 of the aforementioned enzymes in thyroids from treated thyrotoxic patients was correlated with the degree of hyperplasia of these glands.