**4.2 UV spectra of cystatin in the presence of donepezil**

The UV measurement of brain cystatin in the presence and absence of drug was made in the range of 200–300 nm, and the inhibitor (cystatin) concentration was fixed at 1 M, while the drug concentration was varied from 0.16 to 1.6 M. Absorption spectra were recorded on a double-beam Shimadzu UV-vis spectrophotometer UV-1700 using a cuvette of 1 cm path length.
