Molecular Weights of the Proteins in their Native State

Abstract

MOST of the determinations of molecular weights of the proteins carried out in this laboratory by means of the ultracentrifugal method were made on material which had been subject to purification using ammonium sulphate as a precipitating or crystallising agent.¹ When trying to isolate the lactalbumin from cows' milk, we observed that no product homogeneous with regard to molecular weight could be prepared—the figures ranging from 12,000 to 25.000—and that the molecular weight increased during the process of purification.² A closer study of this phenomenon showed that the condensation is due especially to the action of the ammonium sulphate used for the ‘purification’. The bulk of the native material in cows' milk from which the lactalbumin is formed has a low molecular weight, probably not exceeding 1000. This startling discovery made it an urgent task to re-examine as many of the proteins previously studied as possible without any chemical treatment at all or at least under conditions closely resembling those of their native occurrence. A short summary of the results so far obtained is given below.