The effect of heat-denaturation on the base-binding capacity of beef-muscle press-juice

Abstract

The hydrogen ions absorbed ( ΔB ) on heat-denaturation of dialysed and centrifuged beef press-juice is shown to vary with the pH of denaturation along a smooth curve. The values of ΔB are remarkably constant from sample to sample. The main features of the curve are a positive peak at pH 4·1 with troughs on either side, a plateau extending from pH 6·5 to 8·0, and the assumption of negative values of ΔB from pH 8·5 to 10·5. The curve differs from those of undialysed samples and globulin X preparations. The peaks and troughs bear no direct relation to the isoelectric point or the degree of aggregation. Heat-denaturation results in reduction of buffering capacity in the range pH 3·5 to 5·0, and to a new peak in the buffer curve at pH 6·0 to 7·0. Acid-denaturation at pH 3·60 has a similar but smaller effect. The evidence suggests that imidazole groups are released and terminal carboxyl groups absorbed during denaturation. There is no evidence that other amino groups are released or absorbed. These changes in buffering capacity are unlikely to be due merely to changes in physical configuration, not involving chemical bonds, since they occur whether a true coagulum is formed or not. It is suggested that heat- or acid-denaturation result in a fission of the protein chain at labile linkages involving imidazole, sulphydryl and hydroxyl groups, followed by hydrogen-bond formation between carboxyl and amino groups, to give a new configuration. It is shown that these hypothetical changes would give rise to large shifts in pH, and the apparent release or absorption of hydrogen ions ( ΔB ) in a way similar to the actual results. The marked difference in behaviour of the undialysed samples and globulin X prepara­tions, on the one hand, and the crude myogen fraction on the other, suggests a fundamental difference in structure.