Characterization of Bovine Anterior‐Lens‐Capsule Basement‐Membrane Collagen: 1. Pepsin Susceptibility, Salt Precipitation and Thermal Gelation: a Property of Non‐collagen Component Integrity

Basement membrane collagen was extracted sequentially from bovine anterior lens capsules by repeated pepsinization at very low enzyme‐to‐collagen ratios. The earliest extracts, derived from 1–7‐days pepsinization at 4°C showed that a substantial fraction of basement membrane collagen material did not enter a 6% or 3% sodium dodecylsulfate/polyacrylamide gel. The material that did penetrate the gel ran as discrete bands with molecular weights between 220000 and 140000. The relative proportions of these components could be controlled solely by the duration of pepsinization. Furthermore, as pepsinization was continued or as the quantity of pepsin or the extraction temperature was increased basement membrane collagen peptides with molecular weights between 115000 and 30000 were produced, with little material not entering the gel. The extracts containing collagen chains with M_r > 130000 were precipitable from acetic acid solution at NaCl concentrations of 0.3–0.6 M and could undergo a thermal gelation at pH 7.03. At or near the transition to the lower‐molecular‐weight species (M_r < 130000), the precipitability at low NaCl concentrations and thermal gelation properties were lost. Correlation of these properties with amino acid composition and with previous electron microscopic studies suggest that strong attractive interactions between pepsin‐susceptible molecular end regions are responsible for the salt precipitability and gelation. These end region interactions may also play a role in inhibiting D‐periodic striated fibril formation in basement membrane structures in vivo.