Division of Biological Sciences, The University of Rochester, Rochester, New York.
J Gen Physiol. 1943 Nov 20;27(2):101-11. doi: 10.1085/jgp.27.2.101.
The extent of urea denaturation depends on the concentration of protein and urea and also on the temperature of the solution. Egg albumin solutions (0.9 per cent) are not denatured by 20 per cent urea, denature slowly with 25 per cent urea, and denature rapidly with 35 per cent urea at room temperature. At a higher temperature 30 per cent urea is rapidly effective. Denaturation of the egg albumin molecule by radiation or by heat is accompanied by structural changes as evidenced by optical rotation values, but is not accompanied by association or dissociation of the molecule in the pH range outside the zone in which aggregation follows denaturation. Denaturation of the egg albumin molecule by urea produces no change in optical rotation until the concentration of urea is high enough to dissociate the molecule. In the presence of urea a urea-protein complex is formed in which the protein is denatured but cannot flocculate because of the dispersive action of the urea. This prevents flocculation of proteins exposed to radiation and subsequent heating to 40 degrees C. as the urea-protein complex is not broken down at a temperature of 40 degrees C. The presence of urea therefore prevents the flocculation of proteins denatured by radiation. The urea-protein complex is broken down by heating to 55-58 degrees C. so that the molecules aggregate at a temperature below the temperature of rapid heat denaturation. This appears to be an acceleration of heat denaturation or a lowering of the heat denaturation temperature, but in reality is an effect of heat on the urea-protein complex which frees the urea-denatured protein and permits its aggregation.
脲变性的程度取决于蛋白质和脲的浓度，也取决于溶液的温度。0.9%的卵白蛋白溶液不会被 20%的脲变性，用 25%的脲变性缓慢，在室温下用 35%的脲变性迅速。在较高的温度下，30%的脲很快就有效。卵白蛋白分子的变性由辐射或热引起，伴随着结构变化，如旋光值所示，但在聚合后变性的 pH 范围之外，分子不会发生缔合或解离。脲变性的卵白蛋白分子不会改变旋光值，直到脲的浓度足够高以解离分子。在脲的存在下，形成脲-蛋白质复合物，其中蛋白质变性，但由于脲的分散作用而不能絮凝。这防止了暴露于辐射和随后加热至 40°C 的蛋白质的絮凝，因为在 40°C 下脲-蛋白质复合物不会分解。因此，脲的存在阻止了蛋白质的絮凝变性由辐射引起。脲-蛋白质复合物在加热至 55-58°C 时分解，因此分子在低于快速热变性温度的温度下聚集。这似乎是热变性的加速或热变性温度的降低，但实际上是热对脲-蛋白质复合物的影响，它使脲变性的蛋白质游离，并允许其聚集。
