Induction of milk lipolysis by lipoprotein components of bovine blood serum.

The major blood lipid component responsible for activation of milk lipolysis was high density lipoprotein with density of 1.063 to 1.21 g . ml-1. Its low molecular weight apolipoprotein fraction, apo C, which activates milk lipoprotein lipase in vitro, was unable to induce milk lipolysis under normal conditions. Mechanical treatment of the milk rendered it highly susceptible to apo C-stimulated lipolysis. Low and very low density lipoprotein fractions, which also contain apo C, showed negligible effect on milk lipolysis. Apo C in combination with serum or heparin induced high lipolysis in normal milk. Also, lysolecithin influenced the degree of serum activated lipolysis. Antiserum raised against bovine apolipoprotein A-I, which does not activate lipoprotein lipase, removed the activating ability of serum. Induction of milk lipolysis is preceded by redistribution of lipoprotein lipase, thus increasing the accessibility of the enzyme to its substrates.