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Binding of 3H-folate to human mammary tumor homogenate was of a high-affinity type (K = 10(10) M-1) and displayed apparent positive cooperativity. Radioligand dissociation was slow at pH 7.4, but rapid at pH 3.5. As compared to methotrexate, 5-formyltetrahydrofolate acted as a strong inhibitor of radioligand binding. Gel chromatography of radioligand-labeled homogenate of tumor tissue revealed three peaks: a small > or = 110 kDa peak and two major peaks of folate-binding activity (M(r) approximately 25 kDa and M(r) approximately 100 kDa). Mammary tumor tissue showed immunostaining with rabbit antibodies against human milk folate binder. A parallel elevation in the concentrations of folate-binding protein and triglyceride in tumor tissue as compared to normal tissue adjacent to the tumor was compatible with the localization of folate-binding protein in the triglyceride-rich fraction of mammary gland homogenate.
Holm et al. (1994) studied this question.