Evidence for an amino terminal extension in high molecular weight collagens from mature bovine skin

Insoluble, mature collagen fibers from bovine skin have been partially solubilized by mild, denaturing, but nonhydrolytic means. The soluble denatured collagen was fractionated by alcohol coacervation, and a fraction rich in high molecular weight α chains was obtained. The heavy α chains were isolated by carboxymethylcellulose chromatography. Renaturation, followed by measurements of optical rotation at 365 nm, showed that stable, in-register renaturation was more readily accomplished in mixtures of heavy α chains than in α1 β₁₁ chain mixtures. Renatured heavy α chain preparations were precipitated in SLS form, negatively stained, and examined by electron microscopy. The SLS precipitates were compared with SLS segments from native soluble collagen and were found to match in band pattern and spacing along their entire length from the COOH terminal region, except for an NH₂ terminal extension of 170 ± 30 Å in the heavy α chain SLS. The heavy α chains correspond chromatographically with those previously reported to be intermediates in the conversion of procollagen to collagen, on the basis of their molecular weight and of labeling studies. The presence of NH₂ terminal extensions, and their existence in mature insoluble collagen, suggest that these intermediates may have a special role in fibril formation.