A concerted response of the enzymes of urea biosynthesis during thyroxine-induced metamorphosis of Rana catesbeiana.

Abstract

Abstract During natural metamorphosis, concomitant, linear increases were found in tadpole liver for the activities of the five enzymes in the ornithine-urea cycle: carbamyl phosphate synthetase I, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase. Upon immersion in 2.6 x 10-8 m l-thyroxine for two weeks, the biochemical changes which occur during natural metamorphosis were simulated with an elevation of the same enzymes by factors of 8.0, 3.3, 2.4, 2.3, and 3.0, respectively. The lag period prior to the initial rise of each enzyme activity was about 4 days. On the basis of the rate of change of the leg to tail ratio, the thyroxine-exposed tadpoles had an earlier, steeper rise in the levels of each of the five enzymes than that observed during natural metamorphosis. Argininosuccinate synthetase had the lowest specific activity of the ornithine-urea cycle enzymes, even though the assay was improved by the inclusion of urease in the incubation mixture, yielding higher activities than earlier reported. Plotting one enzyme against each of the other ornithine-urea cycle enzymes gave straight lines with high correlation coefficients and similar slopes for both natural and thyroxine-induced metamorphosis. These results demonstrate the simultaneous, concerted response of both mitochondrial and extramitochondrial enzymes in the ornithine-urea cycle enzymes during thyroxine-induced metamorphosis.