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Isolated Milk Fat Globules as Substrate for Lipoprotein Lipase: Study of Factors Relevant to Spontaneous Lipolysis in Milk

Department of Animal Nutrition, Agricultural University of Norway, P.O. Box 25, N-1432 Ås-NLH, Norway

Gunilla Bengtsson-Olivecrona

Department of Physiological Chemistry, University of Ume, S-901 97 Ume, Sweden

ABSTRACT

Fat globules isolated from normal and from spontaneous milk samples were compared as substrates for purified lipoprotein lipase. Only slight differences were observed. Fat globules isolated from fresh warm milk were almost resistant to lipolysis. This included globules from milk prone to spontaneous lipolysis. Cooling made the globules accessible to rapid lipolysis even if they were from normal milk. Rewarming the fat globules did not reverse the process. Maximum rate of lipolysis (after rewarming) required fat globules be stored at 10°C or below for 5 to 10 h. Lipolysis at 4°C usually started after a lag time of 3 to 5 h, but with fat globules from spontaneous milk the lag time was shorter. Fat globules isolated from cold milk were a poor substrate at 4°C but were lipolyzed when warmed. When ¹²⁵I-labeled lipase was added to fresh warm milk, some of the lipase bound to the milk fat globules but it caused little lipolysis. Binding increased after cooling, as did lipolysis. Both binding of lipase and lipolysis were impeded by the presence of skim milk. Another way to make fat globules isolated from fresh warm milk susceptible to lipolysis was to treat them with chemicals known to remove proteins.