Isoelectric focusing of native urinary uromodulin (Tamm-Horsfall protein) shows no physicochemical differences between stone formers and non-stone formers.

Recent studies have suggested the occurrence of an abnormal form of uromodulin in stone formers which may be related to a reduced sialic acid content of the protein in these patients. Previous attempts to demonstrate these differences have required extensive sample processing prior to analysis, which may in itself alter the nature and behaviour of uromodulin. By employing a sensitive detection system of enhanced chemiluminescence on Western blots following isoelectric focusing we have been able to study the physicochemical properties of uromodulin in whole unprocessed urine from 50 idiopathic calcium stone formers and 15 non-stone formers. Uromodulin, desialated in vitro with either acid or neuraminidase was also analysed using the same system. All urine samples analysed from stone formers and non-stone formers showed a single band of pI 3.5 after isoelectric focusing. Desialated uromodulin showed a series of bands ranging from pI 4.0 to 5.1 reflecting different amounts of sialic acid removed. We conclude there are no charge-related differences in native uromodulin between stone formers and non-stone formers, in particular none relating to the sialic acid content of the protein.