Alkaline phosphatase of milk. 1. Association of the enzyme with a particulate lipoprotein complex

Abstract

The Alkaline phosphatase activity of cow''s milk is very low (Qp20) compared to kidney and intestine. About 30-40% of the enzyme is adsorbed to the butterfat globules and may be concentrated by separation of cream. The enzyme is released into the aqueous phase on churning, or by elution with distilled water. The phosphatase in fat-free separated milk is not in true soln., but is associated with an insoluble particulate lipoprotein complex. This particulate material may be sedimented together with a portion of the casein particles by high-speed centrifuging. Precipitation of casein from separated milk with acids, salts or organic solvents results in coprecipitation of the lipoprotein particles together with the associated phosphatase. The more specific precipitation of casein with rennin (or pepsin) permits some separation of the casein and lipoprotein particles. Precipitation of casein with acid and certain salts produces partial denaturation, resulting in occlusion of the phosphatase in the protein precipitate and apparent loss of enzymic activity. Inorganic phosphate partially protects casein from denaturation during salt precipitation. The lipoprotein complex which binds milk alkaline phosphatase may be disrupted by treatment of the aqueous material (milk or its products) with normal or isobutanol. Removal of the lipid is accompanied by simultaneous release of the enzyme into true soln. No other organic solvent tested was effective in releasing the bound phosphatase.