Biochemical and structural characterization of membrane fractions from bovine lens

Plasma membranes of vertebrate lens fiber cells contain large numbers of gap junctions that may provide pathways for metabolic cooperation. Characterization of fiber cell gap junctions is thus necessary to understand this function. In this study, plasma membrane fractions were isolated from bovine lens according to established techniques, but without urea, detergents, or proteolytic enzymes. Electron microscopy indicated that isolated plasma membranes with gap junctions form double-membrane vesicles, and gap junctions comprised approximately 35% of the total membrane area in the crude fraction. These vesicles were impermeable to cationized ferritin, suggesting that they were sealed, and may be useful for permeability studies. Treatment of the crude fraction with 2.5% beta-mercaptoethanol or dithiothreitol caused reversible separation of junctional membranes, suggesting that disulfide bonds may be important in maintaining gap junction structure. Fractions with varying proportions of gap junctions were isolated using linear sucrose density gradient centrifugation. The proportional area of gap junction membrane versus total membrane in the various fractions ranged from 10% to at least 51%. The following plasma membrane enzymes were assayed in all fractions: Mg++-ATPase, Ca++-ATPase, alkaline phosphatase, phosphodiesterase, 5'-nucleotidase, and Na+, K+-ATPase. There was no correlation between enzyme activity and gap junction enrichment. This suggests that these enzymes are not associated with fiber cell gap junctions.