Abstract
The failure of the Gunn rat to conjugate bilirubin results from either defective (or absent) bilirubin UDP-glucuronyl transferase or from a defect in the membrane environment of that enzyme. The latter possibility was investigated by constructing Arrhenius plots from 38-8°C of microsomal glucose-6-phosphatase activity of Gunn (jj,n=6), Wistar (JJ,n=9) and heterozygote (Jj,n=8) rats. This enzyme was studied because the Gunn rat is not defective in glucose-6-phosphatase, and this enzyme is tightly bound to the microsome. The plots of jj, Jj and JJ were identical in the following ways: a) the specific activity at 37°C was 2.51 μmol hydrolyzed/mg protein/10 min indicating no jj defect in enzyme activity; b) there was a change in slope at 35°C probably indicating a change in protein structure; c) the energy of activation was 12800 cal/mol. However, a clear difference was observed in the following way: JJ demonstrated a discontinuous plot indicating a lipid phase change at 11°C, but the plot of jj was a continuous straight line to 8°C. Jj exhibited an intermediate discontinuity at 8°C. This shift in the phase change to colder temperatures indicates a difference in membrane fluidity among groups. The proposed explanation for the deficient activity of bilirubin UDP-glucuronyl transferase in the Gunn rat is a difference in membrane fluidity which results in an anomalous environment for the enzyme.
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