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The binding ability of bovine milk proteins with mutagenic heterocyclic amines was investigated. Binding was determined with 2 mg of beta-lactoglobulin and 20 micrograms of heterocyclic amine in .4 ml of pH 7.4, 50 mM phosphate buffer, at 37 degrees C, in a shaker for 10 min. The unbound heterocyclic amine in protein-free ultrafiltrate was analyzed by HPLC method. The binding of alpha-lactalbumin, beta-lactoglobulin A and beta-lactoglobulin B were 90.44, 81.38, and 89.18%, respectively, with 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole; 37.85, 34.04, and 43.90%, respectively, with 3-amino-1-methyl-5H-pyrido[4,3-b]indole; and 49.11, 43.25, and 57.44%, respectively, with 2-amino-6-methyldipyrido[1,2-a:3',2'-d] imidazole. Binding of beta-lactoglobulin and alpha-lactalbumin to 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole and 3-amino-1-methyl-5H-pyrido [4,3-b]-indole was higher at pH conditions above 7.4, and binding was lost at pH less than 5.5. Maximum binding of both proteins to 2-amino-6-methyl-dipyrido[1,2-a:3',2'-d]imidazole was at pH 7.4, and binding was inhibited at pH conditions above 8.5 and less than 6.5.