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 T₃ 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 T₃ 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. T₃ replacement during the last phase of the multiple-temperature shift protocol, restored the plasma T₃ 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 T₃ 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.