Mechanisms for turnover of lipoprotein lipase in guinea pig adipocytes

Guinea-pig adipocytes released lipoprotein lipase activity to the medium without depletion of cell-associated lipoprotein lipase activity. Heparin caused immediate release of 20-25% of the lipase activity to the medium, and also enhanced the continued release. After addition of cycloheximide, cell-associated lipoprotein lipase activity decreased rapidly. Release of lipase activity to the medium continued unabated for about 30 min, but there was little release thereafter. The release accounted for only about 25% of the initial lipoprotein lipase activity in the absence and about 50% in the presence of heparin. In pulse-chase experiments with [35S]methionine, labeled lipoprotein lipase appeared in the medium within 40 min, and most of the release occurred during the first h of chase. In a 4-h chase the total (cells + medium) amount of labeled lipase decreased to 34%. Thus, degradation was a main fate of the lipase. Heparin markedly increased the amount of labeled lipase that was released to the medium and decreased the amount that was degraded. Heparin did not change the time-course for the release, and the amount of labeled lipase degraded was proportional to the amount not released to the medium, indicating that the effect of heparin was primarily on release, not on degradation as such. This study demonstrates that adipocytes synthesize lipoprotein lipase in excess of what is being released, and that the excess is rapidly degraded.