Enzyme Response to Thyrotoxicosis and Hypothyroidism in Human Liver and Muscle: Comparative Aspects*

Abstract

Abstract. 1. Activities of phosphoglucomutase, hexokinase, glucose‐6‐phosphate dehydrogenase, triosephosphate dehydrogenase, mitochondrial glycerophosphate dehydrogenase, hexosediphosphatase and phosphoenolpyruvate‐carboxykinase, of enzymes involved in the citric acid cycle and connected pathways, of hydroxyacyl‐CoA dehydrogenase and carnitine acetyltransferase were determined in biopsy specimens of liver and of tibialis anterior muscle from thyrotoxic and hypothyroid patients and from controls. The results are compared with data obtained from liver and red and white muscle of thyrotoxic rats and guinea pigs. 2. Concomitant with diminished glucose tolerance, the glucokinase activity is decreased in thyrotoxic human liver. The decrease of rat liver glucokinase activity as a response to administered thyroid hormones is found to be dose‐dependent. A relationship between the diminished glucose tolerance in thyrotoxicosis and the decrease of glucokinase activity is discussed. The increase of hexokinase activity (isozyme II) is the most interesting finding in thyrotoxic human muscle because of its possible significance with respect to the elevated metabolic rate. The activity of triosephosphate dehydrogenase, (NADP) malate dehydrogenase, phosphoenolpyruvate carboxykinase and carnitine acetyltransferase is markedly enhanced in human thyrotoxic liver, whereas that of phosphoglucomutase is diminished. 3. Mitochondrial glycerophosphate dehydrogenase, which is known to be markedly increased in the liver and red muscle of thyrotoxic rats, is not increased in the liver and tibialis anterior muscle of thyrotoxic patients, nor in the liver and white muscle of thyrotoxic guinea pigs. The enzyme responses to thyrotoxicosis in human liver and muscle are more similar to those of the guinea pig than of the rat.

Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism.

“…There was little or no rise in muscle oxidative capacity in guinea pigs and a decrease in rabbits after excess thyroid hormone administration (33). The very limited information available in man was obtained in small cross-sectional comparisons of hyperthyroid and euthyroid human subjects (36,37). These studies reported conflicting results.…”

Citrate Synthase Activity in Human Skeletal Muscle1

“…This aspect generally seems to have been given little atten tion. Thus, in some papers the mere mention of 'healthy control subjects' is made without further details (7,10,11). The present subjects were -when one considers the purpose of this investigation -practically healthy since they had neither any muscle disease, nor a dimin In spite of the somewhat higher values found here, citrate synthase belongs to those enzymes having a comparatively low activity in human skeletal muscle.…”

“…ished functional activity of the corresponding muscles until a few days before surgery. On the other hand, the comparatively larger group studied certainly offers more information than data from six or three subjects (3,11). In a recent publication, Bass et al (4) described higher values of citrate synthase activity in the m. vastus lateralis quadricipitis of sport-practic ing subjects (9.2 ± 0.83 U/g) and of highperformance athletes (14.8 ± 0.56 U/g wet weight, 25 °C).…”

“…In this investiga tion the preparation of the extracts and the analyses were carried out as rapidly as possible whereas other authors freezed the muscle pieces for variable durations (10,11). Freezing thawing, however, appears to influence nega tively the activity of citrate synthase (1,11).…”

Incorporation Rate of Glucose Carbon, Palmitate Carbon and Leucine Carbon into Metabolites in Relation to Enzyme Activities and RNA Levels in Human Skeletal Muscles

“…Only sparse information is available on enzyme activities in human skeletal muscles [6,18,37,41,42,46,48]. Moreover, the determinations are from different muscles and from biopsies taken with different techniques which may influence the level of activity.…”