The denaturation of edestin by acid: T. B. Osborne's edestan

Abstract

The formation of edestan by the action of HC1 on edestin between pH 4-5 was studied. The denaturation change, beginning near the acid side of the isoelectric zone, is relatively slow at pH 5 and almost too rapid to measure at pH 4. Denaturation kinetics are characterised by a rapid initial reaction followed by a much slower one, and departure from the uni-molecular law is due in part to a progressive shift of pH towards the alkaline side. Since the rate of pH change is a function of the denaturation rate, it seems that edestan formation involves the liberation of new titratable groups, or a change in pK of groups already present. In the ultracentrifuge edestan is a monodisperse fragmentation product and the edestin recovered from systems in which edestan formation is occurring is normal edestin. The number of SH groups appearing in edestin-edestan systems is proportional to the conc. of the edestan, and represents 1/4 of the total cystine-S. Whilst the tyrosine content of edestan is unaltered, the total N and tryptophan contents are lower than those of edestin, and although small amts. of soluble N (largely NH3 and tryptophan) are liberated during formation of edestan, the decrease in total N is due probably to hydration of the molecule. The mechanism of edestan formation is discussed.