Isolation of sialic acid from casein and reclamation of residual casein.

Crystalline sialic acid was isolated in 50 to 60% yields from a commercial grade of dry isoelectric casein by Warren's hydrolysis procedure. One part of casein was heated at 80 C/60 min in ten parts of .18 N sulfuric acid (pH 1.5). A crude sialic acid preparation (7.8%) was obtained from the hydrolysate after addition of calcium hydroxide to remove protein at pH 4.6 and sulfate as calcium sulfate at pH 7.0 followed by concentration and lyophilization of the soluble material. Highly purified sialic acid (95%) was obtained by chromatography on Dowex-1 X 8 with a gradient of 0 to 2 M sodium acetate. The sialic acid positive fraction was passed through Duolite C20H followed by lyophilization. Slow evaporation of a concentrated solution of this material produced typical crystals of N-acetylneuraminic acid. The study also included an examination of the properties of the residual sialic acid-free casein. Electrophoretic patterns, solubility properties, rennet sensitivity, and viscous character revealed that the residual casein was remarkably similar to intact casein. However, the calcium sensitivity of this modified protein was increased in the presence of .1 M calcium chloride.