Activity of purified milk lipase in the presence of milk constituents

Abstract

The effects of varying concentrations of nonfat dry milk solids, purified proteins, lactose, and salts upon the activity of purified milk lipase were investigated. Milk solids inhibited the enzymic activity, and there was a direct but not proportional relationship between the concentration of the solids and the enzyme inhibition. The inhibition of the enzyme by milk solids followed the Freundlich adsorption isotherm, indicating that the inhibition of the enzyme involved adsorption. Purified α-lactalbumin, β-lactoglobulin, acid casein, Hammersten casein, α-casein, α s-casein, β-casein, and γ-casein inhibited the enzyme up to 90%. Usually, the higher the concentration of caseins, the greater the enzyme inhibition. On the other hand, κ-casein, pseudoglobulin, euglobulin, lactalbumin, blood serum albumin, and bovine plasma albumin stimulated the enzyme up to 60%. At higher concentration, however, some of the stimulating proteins inhibited the enzyme. The ultracentrifugal data indicated that the mode of inhibition of the enzyme by β-casein involved the formation of a complex between the protein and the enzyme. Lactose did not inhibit the enzyme significantly, but salts of calcium or magnesium inhibited the enzyme up to 50%. Ca(OH) 2 was the most inhibitory and MgSO 4 was the least inhibitory. These data suggest that in milk the observed lipase activity is the net result of the inhibitory and stimulatory action of the various milk constituents upon the enzyme.