The behaviour of pepsin in the ultracentrifuge after alkaline inactivation

IN previous work Philpot and Eriksson-Quensel, 1933 the behaviour of crystalline pepsin in the ultracentrifuge was shown to be that of a homogeneous substance of molecular weight about 35,000, with approximately spherical molecules.The work has now been extended to cover pepsin which has been inactivated by alkali.The facts already known about alkaline inactivation are as follows Northrop, 1930;Goulding et at., 1927:(1) The change is very slow at PH 6 and increases in speed with PH until at PH 9 it is practically instantaneous.(2) Under the conditions used by Goulding et at.and by Northrop a definite fraction of the activity disappears instantaneously even at PH 6, and this fraction increases with PE.(3) When the PH is brought back to below 6, up to 10% of the original activity can be restored.The optimum pH for this reactivation is 5-4 Northrop, 1931.(4) Inactive pepsin is rapidly digested by active pepsin.This is probably one reason for the incompleteness of the reactivation.The study of the changes in sedimentation constant accompanying the above changes is complicated by the fact that the solution in the ultracentrifuge is