2-SEC-BUTYL-4,5-DIHYDROTHIAZOLE IS A LIGAND FOR MOUSE URINARY PROTEINAND RAT ALPHA-2U-GLOBULIN - PHYSIOLOGICAL AND TOXICOLOGICAL RELEVANCE

Mouse urinary protein (MUP) and alpha 2u-globulin are structurally hom ologous proteins that belong to a superfamily of ligand-binding protei ns and represent the major urinary proteins excreted by adult male mic e and rats, respectively. Although a variety of xenobiotics bind to al pha 2u-globulin and produce a male rat-specific hyaline droplet nephro pathy, no endogenous ligand for this protein has been identified. Desp ite extensive sequence homology, MUP does not bind to hyaline droplet- inducing agents. While performing experiments with purified MUP, we ob served that it presented with a strong, distinctive odor reminiscent o f mouse urine. To determine whether this odor was the result of contam ination or degradation or was attributed to an endogenous ligand bound to the protein, the protein was subjected to thermal desorption and a ny released volatile compounds were detected with a gas chromatograph equipped with an external sniff port and mass spectrometer. With this approach, two odorous compounds were detected at the sniff port by a h uman observer, but only one was present in sufficient mass to allow id entification. This compound, which presented with the characteristic o dor, was subsequently identified as 2-sec butyl-4,5-dihydrothiazole (D HT) by GC/MS/matrix isolation IR and NMR analyses. The identification of DHT was confirmed by comparing the chromatographic and spectral pro perties to those of the synthesized authentic compound. in direct cont rast, purified urinary alpha 2u-globulin did not present with an obvio us odor, and no volatile ligands were detected on this protein. Althou gh DHT is a major endogenous ligand for MUP, it was also found to comp etitively inhibit the binding of [C-14]d-limonene-1,2-epoxide to alpha 2u-globulin with relatively high affinity (K-l = 2.3 mu M). When dose d orally to F344 rats, DHT (1 mmol/kg for 3 days) caused the character istic exacerbation of hyaline droplets in male rat kidneys and increas ed renal levels of immunoreactive alpha 2u-globulin about threefold ov er control levels. These results indicate that despite structural homo logy, MUP and alpha 2u-globulin are distinguished by the presence of a volatile endogenous ligand only on the former, a distinction that may reflect differences in the physiological functions of the two protein s. Furthermore, although DHT can bind to both MUP and alpha 2u-globuli n, renal toxicity was only observed in rats, thereby emphasizing the u nique toxicological properties of alpha 2u-globulin in the development of hyaline droplet nephropathy. (C) 1998 Academic Press.