Uromucoid (Tamm-Horsfall glycoprotein) forms different polymeric arrangements on a filter surface under different physicochemical conditions.

Normal human urine cannot be forced through a 0.2 micron filter. To investigate the reason for this phenomenon, uromucoid (Tamm-Horsfall protein) was purified from human urine and its capacity to block a 0.2 micron Millipore filter was measured under different conditions. In the presence of cations (H+, Na+, Ca2+) uromucoid blocked the filter. The blocking varied with cation concentration. Scanning electron microscopy of the filter surface revealed different arrangements of polymerized uromucoid coating the filter surface depending on ionic conditions. In the presence of 100 mmol/l NaCl or 1 mmol/l CaCl2 uromucoid polymers were present in a fibrous arrangement. In the presence of both NaCl and CaCl2 a dense mat of uromucoid polymers was present together with clumps of aggregated polymer. In the absence of ions uromucoid formed a homogeneous coat on the filter surface (as demonstrated by scanning electron microscopy, Western blotting and 125I-uromucoid binding studies) but did not block the filter. Similar fibrous and highly aggregated arrangements of uromucoid polymer were seen in hyaline casts from urine. These data are consistent with the concept that the uromucoid glycoprotein can exist in several different polymeric forms under different ionic conditions.