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Inhibition of Lipase and Lipolysis in Milk with N-Ethylmaleimide¹

Department of Dairy Industry, University of California, Davis

ABSTRACT

Milk lipases have been shown to be sensitive to heat, light, hydrogen peroxide, and copper (4, 5, 10, 11). Their storage stability has been markedly increased by reducing agents and glutathione (5). It has been shown (1, 9) that the activity of a large number of enzymes demands unobstructed sulfhydryl groups on the proteins. Prankel and Tarassuk postulated that these sulfhydryl groups are essential for milk lipase activity (5). Experiences in this laboratory and observations of others (8) indicate that p-chloromercuribenzoate reacts slowly, if at all; N-ethylmaleimide (NEM)² has been shown (6, 7) to react quickly and specifically with sulfhydryl groups under mild conditions. NEM was used as an inhibitor in the present investigation of whether the sulfhydryl groups are actually involved in lipolysis and lipase activity in milk. Freshly drawn, uncooled milk, mixed or from single cows, was obtained in a stainless-steel can and processed immediately after milking. The NEM was added directly to whole milk and the milk was incubated at 37° C. for a certain period before being subjected to activation treatments.

¹ This study was supported in part by funds from the California Dairy Industry Advisory Board.