Low-Molecular-Weight Protein Competition for Binding Sites on Renal Brush Border Membranes

Immunoglobulin light chains, beta 2-microglobulin, insulin, and lysozyme are low-molecular-weight proteins (LMWP) shown to bind to renal brush border membranes. Competition among these proteins and the role of electrical charge in binding to brush border membranes have not been resolved. To investigate these factors, we performed displacement experiments with [125I]-labeled beta 2-microglobulin (pI = 5.6) using six species of LMWP over a pI range of 4.4-11.0. The inhibition constants, Ki, of these six competing ligands, kappa- and lambda-light chains, lysozyme, insulin, cytochrome c, and myoglobin, determined from the log displacement curves, ranged from 4 x 10(-5) to 8 x 10(-4) M. These experiments show marked cross-competition among LMWP for binding to brush border membranes. There was no correlation between Ki and pI indicating that the molecular structure is a more important determinant of LMWP binding to brush border membranes than net electrical charge.