Effect of methotrexate on folate binding to a folate binding protein in cow's milk.

Folate in cow's milk was strongly bound to a minor whey protein. Methotrexate inhibited the folate binding in a log dosis dependent manner, but was a rather weak inhibitor. The binding mechanism of folate changed from a cooperative to a non-cooperative type in the presence of methotrexate. This model system could be used in investigations on human body fluids and tissues.