| Abstract: | The regulation of avian lipoprotein lipase by dibutyryl cyclic AMP in cultured adipocytes was studied with quantitative and specific methods for the measurements of enzyme catalytic activity, enzyme protein mass, and immunoadsorption of labeled enzyme. Incubation of adipocytes in 0.5 mM dibutyryl cyclic AMP plus 0.5 mM theophylline results in a time-dependent decrease in cell lipoprotein lipase catalytic activity. The activity is decreased by 70% in 4 h and over 90% by 12 h. The decrease in cellular catalytic activity is due to a decrease in both enzyme content and enzyme catalytic efficiency. 4 h after exposure of adipocytes to cAMP, enzyme protein was decreased from 3.58 +/- 0.5 to 1.92 +/- 0.1 ng/dish and specific activity from 15.1 +/- 2.1 to 8.4 +/- 1.1 nmol/ng. In the presence of 0.5 mM theophylline, the dibutyryl cyclic AMP-mediated decrease in lipoprotein lipase activity was half-maximal at less than 25 microM dibutyryl cyclic AMP. The rate of lipoprotein lipase synthesis was estimated by measuring the incorporation of L-35S]methionine into enzyme protein during 30 min. A method for the quantitative immunoadsorption of lipoprotein lipase from cell lysates was developed. Utilizing this immunoadsorption technique, the rate of incorporation of L-35S]methionine into lipoprotein lipase was 0.0026 +/- 0.002%, when expressed as a percentage of that incorporated into total trichloroacetic acid-precipitable counts. By 2 h after exposure of adipocytes to 0.5 mM dibutyryl cAMP, the relative synthesis rate had already decreased to 64 +/- 4% of the control rate. After 16 h the synthesis rate was 43.2 +/- 13.8% of the control rate. The observed decreased synthesis rate could account for most of the decreased cellular enzyme content and diminished enzyme secretion rate. |
