EFFECT OF BLOOD AND HIGH-DENSITY-LIPOPROTEIN PREPARATIONS UPON LIPASE DISTRIBUTION AND SPONTANEOUS LIPOLYSIS IN BOVINE-MILK

The mechanism by which high density lipoprotein promotes spontaneous lipolysis in milk was investigated. Spontaneous milk samples were characterized by an elevated cream-associated lipoprotein lipase activity compared with nonspontaneoue milk samples. This cream lipase activity was correlated (.70) with cold storage acid degree value. Blood preparations (serum, plasma) added to warm milk, upon milk cooling, caused correlated (.89) increases in cream lipase activity and acid degree values. Total milk lipase activity was not correlated with level of cold storage lipolysis; however, skim milk lipase activity was negatively correlated (-.38). Addition of high density lipoprotein to recombined milk caused transfer of active lipoprotein lipase from unheated skim milk onto fat globules, which had been recovered from milk heated to inactivate cream-associated lipoprotein lipase. Transfer of lipoprotein lipase to fat globules by high density lipoprotein occurred in a dose-response manner with a correlation of .68 between high density lipoprotein added and cream lipase activity. These observations supported the concept that biological variation in high density lipoprotein content of milk can contribute to the variability in spontaneous lipolysis observed among milk samples. This contribution by high density lipoprotein is mediated primarily through its ability to redistribute lipoprotein lipase toward fat globules rather than activation of the lipoprotein lipase that was initially associated with fat globules.