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Iodine-125-Labeled Lipoprotein Lipase as a Tool to Detect and Study Spontaneous Lipolysis in Bovine Milk

Department of Animal Nutrition, Agricultural University of Norway, P.O. Box 25, N-1432 Ås-NLH, Norway
Department of Physiological Chemistry, University of Umea, S-901 87 Umea, Sweden

ABSTRACT

The distribution of lipoprotein lipase among cream, casein, and milk serum can be evaluated by addition of a trace amount of ¹²⁵I-labeled lipoprotein lipase to milk. Radioactive lipase was distributed in parallel to endogenous lipase under several conditions. In some milk samples, binding of lipase to cream increased when the milk was cooled. Correlation was good between bound labeled lipase and degree of cold-induced lipolysis in corresponding milk samples. Binding of lipase to cream or to casein was not saturable by addition of two- to threefold more lipase than is normally present in milk. In milk with a relatively high fraction of lipase bound to cream, a correspondingly lower fraction was associated with casein, whereas the fraction of lipase in milk serum was similar in all milk samples. Cold-induced binding of lipoprotein lipase to cream was not fully reversed when the milk was warmed again. Heparin released lipase from casein and increased the amount of lipase bound to cream after cooling.