The Isoelectric Point and the Median Minimum of Growth.

The typical curve which has been secured for growth of organisms in varying hydrogen ion concentrations is platycurtic and bimodal. Jacques Loeb 1 secured similar curves for the viscosity, osmotic pressure, potential difference, and swellability of gelatin and other proteins. He explained the bimodality of these graphs upon the basis of the Donnan's equilibrium, and arrived at the conclusion that the median minimum lying between the acid and the alkaline optima respectively is located at the isoelectric point of the protein in each case. In physico-chemical investigations, then, one seems quite warranted in regarding the median minimum of curves plotted along the pH scale as a sound criterion of the isoelectric point of the protein involved. A number of biologists, including Robbins 2 and Pearsall, 3 have undertaken to utilize this relationship in the interpretation of similar graphs obtained in the study of plants and animals, that is, concluding, for instance, that the median minimum for growth curves is an index of the isoelectric point of the constituent proteins.

Michaelis 4 has pointed out that “the isoelectric point is not deflected by true salt formation” (p. 145). If the median minimum of biological graphs plotted along the pH scale is at the isoelectric point of the proteins composing the tissue concerned, then it would seem that this medium minimum should be located at approximately the same hydrogen ion concentration irrespective of the molar concentration of the salt solution bathing the tissue. If, on the other hand, the median minimum is found to shift toward the acid or the alkaline side with alteration of the salt concentration, there would be definite evidence that the median minimum of these biological curves is determined by some other factor than the isoelectric point of the organism.