Cellulase from Trichoderma reesei is a multienzyme mixture that hydrolyzes cellulose to glucose. Two enzymes in this mixture, cellobiohydrolase (CBH) and endoglucanase (EG), possess a common structure comprising a distinct cellulose-binding domain (CBD) and catalytic domain. Inhibition of the catalytic domain of cellulases without affecting their CBD function might be useful for structure/function studies of these enzymes. Complexes of the platinum group metals were tested for their ability to inhibit the major cellulase enzyme from T. reesei, cellobiohydrolase I (CBH I). Only palladium complexes inhibited CBH I, inhibition being dependent upon the molar ratio of palladium to CBH I with 1 microM CBH I retaining only 10% of its activity in the presence of 100 microM ammonium hexachloropalladate(IV) and after the incorporation of 28 mol Pd/mol CBH I. Inhibition was irreversible and could be completely prevented by including histidine, cysteine, and cystine in the assay mixture. Although the primary mechanism of inhibition of CBH I by palladium remains to be elucidated, it could involve the binding of palladium to sulfur or cystine residues resulting in their degradation. This is based on the findings that (i) palladium-inhibited CBH I was less thermally stable than native CBH I; (ii) CBH I, chemically modified by the attachment of pentaammine ruthenium(III) to the imidazole-N of either H206 or H228, showed greater sensitivity to inhibition by palladium compared to native CBH I; and (iii) ammonium hexachloropalladate cleaved 5,5'-dithiobis(2-nitrobenzoic acid)--Ellman's reagent. Binding of CBH I to crystalline cotton linters was not affected by palladium.