Reversible peritubular binding of a cationic protein (lysozyme) to flounder kidney tubules

Proteins filtered in the renal glomeruli are reabsorbed by the proximal tubule and catabolized in the lysosomes. On the basis of studies on isolated flounder tubules it has been suggested that, in addition to this catabolism, a transtubular transport of intact protein (lysozyme) also occurs. The present study demonstrates that significant amounts of lysozyme are reversibly bound to the peritubular side of isolated tubules. Electron microscopic autoradiography demonstrates that the protein is located in the basement membrane and intercellular spaces. After in vivo injection, 125I-lysozyme is mainly located in endocytic vacuoles of the first proximal segment, but also over the basal part of the cells. Since a significant peritubular binding of lysozyme is demonstrated in vitro, it is suggested that a similar binding of tracer protein originating from the peritubular capillaries might occur in vivo and that subsequent release of this protein in vitro might simulate transtubular transport. It is therefore concluded that release of tracer protein from isolated kidney tubules does not conclusively demonstrate transtubular transport of intact protein in experimental systems in which peritubular binding of protein can be demonstrated.