Native and activated properdin: interconvertibility and identity of amino- and carboxy-terminal sequences.

The development of a two-step purification procedure of native properdin with good yield has allowed the physical and chemical comparison of native and activated properdin. The two forms of properdin have identical electrophoretic mobility, subunit size, and amino- as well as carboxyl-terminal amino acid sequences. The two forms of properdin can be interconverted by using mild denaturing agents, indicating that the change in biologic activity is conformational. Circular dichroism analysis of properdin reveals a significant variability in the tertiary structure. However, the differences are a result of the method of purification and do not correspond to the biologic activity of the protein, because the spectra of the interconverted forms of properdin do not change. This indicates that the conformational transition that causes biologic activity changes is small, relative to the conformational variations produced by other conditions that do not alter the biologic activity.