A high molecular weight fraction of a soluble Marsh muscle-relaxing preparation has been shown to contain a calcium-complexing substance. By examining the nature of the competition between this fraction and chelex-100 for calcium at various total calcium concentrations it has been possible to calculate the concentration and calcium stability constant of this calcium-complexing substance. Taking into account dilutions which occur during the preparation of fractions containing this substance its concentration may be estimated at about 2·10-4 in muscle and its calcium stability constant was found to be about 1.5·105 M-1. Preliminary evidence suggests that the calcium-binding substance is a protein.

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