Relative activation of milk lipoprotein lipase by serum of cows fed varying amounts of fat

A routine laboratory assay to evaluate relative concentrations of lipoprotein lipase activator (apo C-II) in cow serum was developed. The assay was linear for at least 120 min after an initial, unexplained, lag ime of 13 to 15 min. Half-maximal activation was in the range of 1 to 2% serum in the assay. Inhibition of activation was indicated at high amounts (10%) of serum. Activation from plasma was half that from serum, presumably caused by an increase in substrate Km in the presence of heparin. Use of glyceryl tri[9,10-3H] oleate yielded excessively high blanks; [2-3H] glyceryl triolein is suggested for routine assay. Relative amounts of activator were not different between dry and lactating cows fed "conventional" diets. Activator concentration increased linearly with increasing dietary fat and was related to concentration of total lipid in plasma. The assay may provide a useful adjunct in studies on lipoprotein metabolism.