Apoprotein-free heparin-binding and non-binding chylomicrons were used as substrates to test the effects on lipoprotein lipase activity of (a) chylomicron protein I; (b) the mixture of proteins I, II and apoprotein E and (c) human beta 2-glycoprotein I. No activation of the enzyme was observed with any of those apoproteins. When rats were injected simultaneously with [3H]cholesterol-labelled heparin-binding chylomicrons (containing proteins I and II) and [14C]cholesterol-labelled non-binding chylomicrons, no differences were detected between the rates of removal from circulation of those two types of particles. Clearance of chylomicrons from circulation was accompanied by the incorporation of 3H and 14C labels into the livers at similar rates. It is concluded that proteins I, II and apoprotein E have no effect on the degradation of chylomicrons by lipoprotein lipase and that the hepatic recognition of remnants does not appear to be affected by proteins I and II.

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