Effects of Different Skim Milk Fractions on Activity of Cow Milk Purified Lipoprotein Lipase

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Abstract
Hydrolysis of a long-chain triglyceride substrate by purified cow milk lipoprotein lipase, in the absence of blood serum, was changed by adding heated skim milks. Preincubation of the substrate with small amounts of several skim milks increased (+ 50 to 300%) lipoprotein lipase activity. By contrast, greater amounts of skim milk increased further or decreased lipoprotein lipase activity (+ 300 to -90%). The inhibitory effect was due to substrate surface protection. Results obtained after removal of casein (by isoelectric precipitation or by ultracentriguation) and of whey proteins (by heat treatment) showed that the effect of skim milk was due to the proteose-peptone fraction. Fractionation of the proteose-peptone fraction into component 5-enriched fraction and component 3-enriched fractions suggested that the activating effect was due partially to proteose-peptone component 5, and the inhibitory effect resulted from component 3. These effects were overcome by blood serum.