Evidence that tropoelastin exists as a random coil.

Cells isolated from the aortae of 17-day old embryonic chicks were incubated with [3H]valine in Krebs-Ringer medium and the secreted labeled tropoelastin was isolated. This tropoelastin was studied by zone velocity centrifugation in sucrose density gradient over the pH range 2.5–12.0, and temperatures ranging from 4° to 35°. The sedimentation coefficient was relatively constant under all experimental conditions and at pH 7.4 was 2.1 S20,w. These sedimentation results are consistent with tfie tropoelastin having a conformation of either a very asymmetric rod or largely that of a random coil, but not that of a hydrated sphere. In order to distinguish the first two possibilities, the ability of unhydroxylated tropoelastin to serve as a substrate for proline hydroxylase was used as a probe of conformation. Unhydroxylated, [14]proline labeled tropoelastin was prepared from the same cells by incubating them with the iron chelator, α,α′-dipyridyl. When this tropoelastin was hydroxylated with excess proline hydroxylase at 20° and 37°, 44% of the prolyl residues were hyroxylated at both temperatures. These results mean that a large fraction of the prolyl residues in tropoelastin are accessible to the enzyme. Considering that proline hydroxylase will only hydroxylate prolyl residues in collagen which is in a random coil conformation, these findings suggest that tropoelastin exists in aqueous solution largely in the form of a random coil.