Additional Functions Attributed to Cystatin

In addition to being an inhibitor of papain-like cysteine proteases, cystatin C has recently been shown be an efficient inhibitor of some of the cysteine proteases of another family of cysteine proteases, called the peptidase family C13, with human legumain as a typical enzyme (C6). Human legumain has, like cathepsin S, been proposed to be involved in the class n MHC presentation of antigens (M3). It has also been shown that the cystatin C inhibitory site for mammalian legumain does not overlap with the cystatin C inhibitory site for papain-like cysteine proteases (Fig. 1) and that the same cystatin C molecule therefore is able to simultaneously inhibit one cysteine protease of each type (A 10). 

Half-lives (t /2, s) of Free (a) Human Cathepsin B and (b) Papain on Interactions with Cystatins in Human Body Fluids 

Calculations were made according to the equation t /2 = In 2/(k+ x [/]), where k+ denotes association rate constant and [/] is inhibitor concentration. The molecular weights used in the calculations are given in parentheses. Cystatin S was assumed to have the same k+ value as cystatin SN. A k+ value of 1.0 x 107 M-1 s-1 for cystatin C was used for calculations of the t /2 for free papain at the cystatin C concentrations found in the fluids. Data from (Al). 

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