Cation-binding property of choroid plexus peptide IIF.

Preparations of the melanotropic-lipolytic peptide IIF from bovine choroid plexus contain 3% Ca, 1% Mg and less than 0.1% Na and K. Ca and Mg were removed by gel filtration on Sephadex G-10 in 1 N acetic acid. Capacity of the resulting cation-free peptide to bind Ca++, Mg++, Na+ and K+ was examined with the method of Hummel and Dreyer (Biochim. Biophys. Acta 63: 530-532, 1962). Peptide IIF bound a maximum of 3.7 to 4.4 mEq of Ca++, Mg++, Na+ or K+ per g of peptide. For each cation, linear Scatchard plot indicated one class of binding site. Association constants (liters per mole) were Ca++, 4.3 X 10(4); Mg++, 3.6 X 10(4); Na+, 4.8 X 10(2), K+, 1.9 X 10(2). Competitive binding experiments showed that all four cations occupied the same class of binding site.