Subunit association and catalytic activity of uridine diphosphate galactose-4-epimerase from yeast.

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Abstract
UDP-galactose-4-epimerase from yeast, when centrifuged in a sucrose gradient, can exhibit a sedimentation coefficient of approximately 4S, 6S, or 11S. These sedimentation values appear to represent successive stages of subunit association. Numerous cationic compounds, among which polyvalent amines are particularly effective, increase the catalytic activity of yeast epimerase manyfold. These compounds also promote the formation of the US particle from the 6S particle. Formation of the US particle is not, however, required for the increased catalytic activity. Titration of epimerase with pCMB, which abolishes catalytic activity, leads to the formation of the 4S particle. Upon reactivation of the enzyme with mercaptoethanol, a 6S particle is reconstituted. Previous reports have shown that such reactivated epimerase differs from the native enzyme in that (1) it has lost the characteristic fluorescence of the native enzyme, and (2) it now requires added DPN for catalytic activity. In addition, the reconstituted 6S particle can no longer form the US particle in the presence of polyvalent cations.