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Summary The anti-Pasteurella pestis factor (APF) from the organs of mice and guinea pigs contained 75 to 87% protein, 1% or less of carbohydrate and phosphorus and from 2 to 12% of cold chloroformsoluble substances. Amino acids constituted 51 to 84% of the proteins, the remainder being unidentified. The chief amino acids were arginine, leucine, lysine, glutamic and aspartic. Since the activity of APF was destroyed by lipase and a chloroform-soluble fraction alone retained activity, the activity appeared to be associated with a lipid. Chloroform-soluble substances contained 1% or less of cholesterol esters, 3 to 10% triglycerides and 18 to 58% fatty acids. The fatty acids were chiefly caprylic, lauric, myristic, stearic, oleic and linoleic. The protein, though apparently inactive against P. pestis, served to solubilize the active lipids and thus facilitate their action in vitro.

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