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Bovine Milk Lipase. II. Characterization¹

Department of Food Science and Technology, University of California, Davis

ABSTRACT

The enzyme lipase, isolated from skimmilk in a homogenous form as evidenced by a single band in starch gel electrophoresis, was partially characterized. It kept best frozen at -20 C and was unstable at 37 C. Lyophilization resulted in complete loss of activity. The heat stability of pure lipase was similar to that of the enzyme in skimmilk. The pH of maximum stability was 6.6–7.6. The enzyme was stable to light in the absence of catalyst.

Diethyl-p-nitrophenyl-phosphate and diisopropylfluoro-phosphate were very potent inhibitors, whereas N-ethylmaleimide, p-chloromercuribenzoate, iodine, and sodium arsenite were less so, thus suggesting that serine and histidine may be at the active site of the enzyme. Skimmilk powder and isoelectric casein had no inhibitory effect. Glutathione and 2-mercaptoethanol were inhibitory at higher concentrations and stimulatory at lower ones.

Lipase hydrolyzed simple short-chain fatty acid triglycerides faster than simple long-chain fatty acid triglycerides. It contained 14.8% N, 0.16% P, and 0.6% sialic acid. Amino acid composition was unlike that of any known milk protein, indicating that lipase is a distinct, separate, minor protein moiety.

¹ This investigation was supported in part by Public Health Service Research Grant no. UI-00593-04 from the National Center for Urban and Industrial Health.

² Present address: The Agricultural Institute, Moorepark, Fermoy, County Cork, Ireland.