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Testican-1, a secreted proteoglycan enriched in brain, has a single thyropin domain that is highly homologous to domains previously shown to inhibit cysteine proteases. We demonstrate that purified recombinant human testican-1 is a strong competitive inhibitor of the lysosomal cysteine pro-tease, cathepsin L,withaKiof 0.7nM, but it does not inhibit the structurally related lysosomal cysteine protease cathep-sin B. Testican-1 inhibition of cathepsin L is independent of its chondroitin sulfate chains and is effective at both pH 5.5 and7.2.At neutral pH, testican-1also stabilizes cathepsin L, slowing pH-induced denaturation and allowing the protease to remain active longer, although the rate of proteolysis is reduced. . | Eur. J. Biochem. 270 4008-4015 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03789.x Human proteoglycan testican-1 inhibits the lysosomal cysteine protease cathepsin L Jeffrey P. Bocock1 Cora-Jean S. Edgell2 Henry S. Marr2 and Ann H. Erickson1 1 Department of Biochemistry and Biophysics and department of Pathology and Laboratory Medicine The University of North Carolina Chapel Hill NC USA Testican-1 a secreted proteoglycan enriched in brain has a single thyropin domain that is highly homologous to domains previously shown to inhibit cysteine proteases. We demonstrate that purified recombinant human testican-1 is a strong competitive inhibitor of the lysosomal cysteine protease cathepsin L with a Ki of 0.7 DM but it does not inhibit the structurally related lysosomal cysteine protease cathepsin B. Testican-1 inhibition of cathepsin L is independent of its chondroitin sulfate chains and is effective at both pH 5.5 and 7.2. At neutral pH testican-1 also stabilizes cathepsin L slowing pH-induced denaturation and allowing the protease to remain active longer although the rate of proteolysis is reduced. These data indicate that testican-1 is capable of modulating cathepsin L activity both in intracellular vesicles and in the extracellular milieu. Keywords cathepsin L proteoglycan protease testican thyropin. Testican is a proteoglycan first identified in human seminal plasma 1 . The cDNA was subsequently cloned from the human testis 2 hence the name testican and from human vascular endothelial cells 3 4 and mouse brain 5 . In both human and mouse testican mRNA is prominent in brain and absent in certain other tissues. Two additional human homologues have been identified testican-2 6 and testican-3 7 . The amino acid sequences of human and mouse testican-1 are 94 identical which argues for a significant function for this proteoglycan 5 . Testican is a multidomain protein Fig. 1 including three domains that have homology to inhibitors of three different classes of proteases.