Characterization of γ‐glutamyltranspeptidase in the liver of the frog: 2. Response to season, temperature and thyroid hormone in Rana pipiens

The impact of season and temperature on frog liver γ‐glutamyltranspeptidase was assessed by measuring the activity of this enzyme in plasma membranes isolated from the livers of Rana pipiens obtained as summer and winter frogs; subjected to short‐term (3 weeks) temperature acclimation; and subjected to multiple‐temperature shifts. Plasma levels of T 3 were determined. γ‐Glutamyltranspeptidase was found to be 2·2‐fold higher in the summer frog relative to the winter frog; decreased by 44 percent in the summer frog by cold acclimation and increased by 1·7‐fold in the winter frog by warm acclimation; and increased by 1·9‐fold in the summer frog and 2·8‐fold in the winter frog subjected to multiple‐temperature shifts. Plasma T 3 levels were found to be 42‐fold higher in the summer frog relative to the winter frog; decreased by 42 percent by cold acclimation and increased by 2·9‐fold by warm acclimation; and decreased by 39 percent and 38 percent in the summer and winter frogs subjected to multiple temperature shifts. T 3 replacement during the last phase of the multiple‐temperature shift protocol, restored the plasma T 3 levels to 75 percent of the control levels and prevented the increase evoked by the multiple‐temperature shifts in γ‐glutamyl‐transpeptidase activity. Indeed, enzyme activity in the T 3 replaced state was 19 percent lower than in the control state. The involvement of thyroid hormone as a negative regulator of enzyme activity is discussed.