Serum lipoprotein stimulation of lipolysis and its relevance to free fatty acid development in bovine milk

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Abstract
Summary: The role of blood serum lipoproteins in stimulating milk lipolysis was investigated by adding blood serum and heparin to the milk of normal cows and cows in which elevated somatic cell counts were induced by intramammary infusion ofEscherichia coliendotoxin andStaphylococcus aureus. There was considerable variation between individual milks in the extent to which lipolysis was stimulated. In some milks there was almost no response. For values obtained from 29 cows during the first 6 months of lactation a relationship was observed between free fatty acid (FFA) levels in the untreated and serum-stimulated samples (r= 0·776). FFA values were higher after 18 h at 4 °C in milks from infused quarters than in those from control quarters, but this difference was not entirely due to higher values from the infused quarters at 0 h. Both blood serum and heparin stimulated lipolysis in high cell count milks and control milks. The response to heparin was greater than that to blood serum, but the response to both was highly correlated with FFA levels in the untreated milks. There were no differences in the ability of skim-milks prepared from infused and control quarters to promote lipolysis in recombined milks containing sterilized homogenized milk fat globules as a source of substrate. Incubation of blood serum with 2 μg/ml trypsin for 1 h at 37 °C reduced the ability of the serum to promote lipolysis. The importance of lipase activation by blood serum lipoproteins in relation to milk lipolysis is discussed.