The morphological basis of glomerular filtration and protein reabsorption in mouse kidney was examined by using mushroom tyrosinase subunits (mol wt 34,500), as an ultrastructural tracer. Almost immediately after injection tyrosinase reaction product was visualized in the glomerulus, and within the capillary lumen extending into the endothelial fenestrae. The entire basement membrane showed accumulations of tyrosinase in the subendothelial and subepithelial layers. The urinary space contained considerable amounts of reaction product, some of which was adsorbed to the cell coat of the podocytes. Reaction product could also be seen in the brush border region of the proximal tubule cells. By 30 min after injection, no tyrosinase reaction product was demonstrable in the glomerulus except for dense vesicles in mesangial cells. Most of the reaction product was localized in absorption droplets in the apical cytoplasm of proximal tubule cells. Occasionally, some tyrosinase reaction product was present within the basal infoldings of these cells. The behavior of tyrosinase in the mouse kidney is in accordance with that of other low molecular weight tracers. The pattern of localization within the basement membrane provides additional support for the presence of two filtration barriers in the glomerulus. The adherence of tyrosinase to the cell coat of the glomerular epithelial cells suggests that this may be an additional mechanism whereby protein is removed from the glomerular filtrate. Tyrosinase subunits may prove to be a useful new tracer for the study of protein transport.

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