Characteristics of Lipase-Rich Fractions of Milk Protein

Abstract

Lipase-rich fractions of skimmilk, frozen-thawed skimmilk and a water extract of rennet-casein, were obtained by Sephadex gel filtration. Three similar fractions were obtained from all materials. Protein predominated in the fast-moving fraction and lipase activity concentrated in the two slow-moving components. Slow components showed 10–200 fold greater activity than the starting material, with the slowest moving component being the highest. Slower moving components from frozen-thawed skimmilk and from the water extract of rennet casein were 5–10 times higher in specific activity than comparable skimmilk fractions.Two slow-moving components from the three materials contained sulfur and carbohydrate in all fractions. Molecular weights were less than 10,000. Variations in 200/280 mu absorption UV ratios suggested differences in molecular structure.The second extract from the rennet casein showed a complex composition containing amino sugars, neuraminic acid, glucose or galactose, and an unknown ninhydrin positive component.Lipase activity was unaffected by pressure ultrafiltration but decreased by freeze drying. Polyacrylamide electrophoresis revealed the freeze-dried fractions had a relatively high degree of protein homogeneity, whereas, in the ultrafiltrated fractions the protein had aggregated with a high degree of polydispersity and heterogeneity.