An Ultracentrifugal Study of Crystalline Pepsin

Abstract

WE have examined in the ultracentrifuge crystalline pepsin, prepared by a method closely resembling that of Northrop¹, using both the velocity and the equilibrium methods². The velocity method gave a sedimentation constant of 3.3 × 10¹³, as a mean of seven determinations having an estimated standard deviation of 0.15 × lO¹³. The equilibrium method gave a molecular weight of 35,500 as a mean of two determinations, the individual figures being 34,400 and 36,600. Comparison of the molecular weight with the sedimentation constant shows that the molecule is spherical. Although, owing to the small number of determinations, these figures are only provisional, they lead to the same conclusion as Northrop's osmotic and diffusion data, namely, that as regards molecular weight and shape, Northrop's crystalline pepsin belongs to the same class of proteins as ovalbumin, Bence Jones protein and insulin. This conclusion is independent of whether the peptic activity is a property of the protein molecule or of an active group adsorbed on its surface.