Chinese Journal of Physiology

DENATURATION VERSUS COAGULATION 3

by shaking is also insoluble. There is thus a sharp line of demarkation between the flocculated and the coagulated egg albumins with regard to solubility in concentrated urea solution.

The difference between flocculated and coagulated proteins is not always so marked as in the case of egg albumin. Thus, hemoglobin coagulated by shaking dissolves readily in dilute alkalies and is somewhat soluble in concentrated urea solution. Serum albumin, coagulated by alcohol or by heat, unless the heating is prolonged, is soluble in dilute acids and alkalies and in concentrated urea solution. Coagulated proteins are, however, always less soluble than the flocculated proteins. We must conclude, therefore, that coagulation is not merely denaturation followed by flocculation, but an entirely different process.

A question of prime importance raised by the above consideration is this: Is denaturation a necessary preliminary to coagulation? In other words, can natural protein be coagulated directly without first undergoing denaturation? Since denaturation and coagulation are brought about by agents of opposite nature and under essentially opposite conditions, there is no theoretical reason to believe that denaturation is a necessary preliminary of coagulation. Experimentally, there is some evidence that coagulation can be independent of denaturation.

In heat coagulation the agent which causes coagulation also accelerates denaturation. Consequently, when a protein solution is heated for the purpose of coagulation some denaturation may also occur, unless the protein is far more susceptible to coagulation than to denaturation. However, at the isoelectric point where the rate of coagulation is at the maximum and the rate of denaturation at the minimum*, coagulation may take place without denaturation. It has been shown that denaturation of egg albumin is accompanied by an increase in acid and base binding power and the amount of this increase depends on the pH of the solution at which denaturation occurs. At the isoelectric point it cannot be experimentally determined, but by extrapolation it can be shown to be zero (9). If we assume that the change in acid and base binding power is a primary result of denatura-

* According to Lewis (6) the point of minimum denaturation coincides with the neutral point of water at the temperature of heating. According to our work reported in the following paper, the point of minimum denaturation uncomplicated by coagulation would seem to be at the isoelectric point of the protein, both points being referred to the room temperature. It should be noted that Lewis did not study denaturation alone but denaturation and coagulation together,