Abstract
αs1-Casein was dissolved in 50mM cacodylate-HCl-70mM KCl buffer containing 0.02% of sodium azide (pH 7.1), and the size and shape of αs1-caseins in the absence and presence of calcium ions were observed with the electron microscope. In the absence of calcium ions, most αs1-caseins existed as spherical particles of which the smallest diameter was 5_??_6nm. The particles were polymerized into bent chains by adding 3mM calcium. It seemed that the smallest particles were the polymerizing units. The mean length of αs1-casein in the absence of calcium was 8.7nm, and it increased as the calcium concentration increased. From these results, it was speculated that αs1-casein in the absence of calcium had one binding site and the calcium-induced conformational changes produced a second binding site. The probability distributions were calculated with the above speculation, and compared with the frequency distributions obtained from electron micrographs. It was suggested from the comparisons that the second binding site produced in αs1-casein might be responsible for the calcium-induced aggregation.
