The Milk Lipase System. I. Effect of Time, pH and Concentration of Substrate on Activity

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Abstract
Information on the lipase system of cows milk was obtained using freeze-dried raw skim milk as enzyme source and pasteurized, homogenized cream as substrate source. Time-activity curves were established over a 5-hour incubation period and over a pH range of 5.2 through 9.8. Activity was noted as low as pH 5.75 but not at 5.2. Activity-pH curves for incubation periods from 1 to 5 hours revealed an optimum to be present always at pH 8.5 and a 2d small optimum present at pH 7.0 in some cases. The effect of varying concentration of milk fat on velocity of lipolysis was investigated over a wide pH range. A plot of the velocity against the quotient of the velocity divided by the substrate concentration from a linear variation of the Michaelis-Menten equation resulted in nonlinear curves at pH 6.6, 7.5, 7.9, 8.35, and pH 8.8. A straight line was obtained at pH 6.2. Non-linear curves suggested the presence of more than one active lipase at these pH levels. Variations in substrate concentration over a wide pH range resulted in attainment of pH optima 6.5-7.0, 8.5 and 7.9 and a broad optimum between pH 7 and 8, the particular optimum being dependent upon concentration of substrate used.