Effects of apoproteins C on lipoprotein lipase activity bound to rat fat cells

During short-term incubation, a suspension of fat cells isolated from female rats hydrolyzed tri[3H]oleyl[14C]glycerol added as substrate and incorporated an average of 42% of the amounts of [3H]oleic acid released. Lipolysis was catalyzed by cell-bound lipoprotein lipase (LPL), whose activity was influenced by apoproteins (apo) C purified from human plasma. ApoC-II enhanced 12-fold the hydrolysis rates with cells from fed rats versus 3,4-fold with cells from fasted rats. In both cases, the stimulation by apoC-II was greater than that by whole serums from led or fasted rats. Maximal LPL activity at the cell surface depended more on the preexisting nutritional state of the cell than on apoC-II or serums added extracellular. ApoC-I, apoC-III1 and apoC-III2 had no or moderate direct effects on LPL activity, whereas apoC-III1 and apoC-III2 reduced markedly the stimulating effect of apoC-II. At all levels of LPL activity, the amounts of [3H]oleic acid liberated by hydrolysis and incorporated as [3H]acylglycerol in cell lipids were highly correlated with LPL activity, suggesting that the uptake process was directly related to enzyme action.