STUDIES ON PITUITARY LACTOGENIC HORMONE III. SoLuBILITIES OF Sm~EP AND BEF,~ Hom~om~s* BY CHOH

Northrop and coworkers (1-3) believe that solubility studies constitute the most sensitive and reliable test for determining the homogeneity of proteins and have used them successfully as a test for purity of their crystalline enzyme preparations. Herriott, Desreux, and Northrop (4) were able to show that a number of pepsin solutions that appeared to be homogeneous in electrophoresis experiments contained several protein components as indicated by solubility studies. Steinhardt (5) has recently shown that in the case of some proteins difficulties may attend the application of the test. Cohn and coworkers (6, 7) have carried out protein solubility studies using the salting-out effect of electrolytes in buffer solutions. If proteins are pure, a plot of the logarithm of solubility against ionic strength may appear to be a straight line. Slopes and intersects to the ordinate are used in characterizing the protein. The solubility method is not only useful in determining the purity of a preparation but is a sensitive method for distinguishing species specificity of proteins (8). Our preparations of pituitary lactogenic hormone have been demonstrated recently to act as a homogeneous substance in the Tiselius apparatus (9), and preparations of beef and sheep hormone were indistinguishable (10) electrophoretically. Bischoff and Lyons (11) were not able to differentiate the hormone prepared from beef or sheep pituitary by certain immunological methods. It seemed worth while, therefore, to investigate the solubility behavior of our beef and sheep lactogenic hormones in order to determine not only whether they were pure but