Detailed Biochemical Characterization of Human Placental Cystatin (HPC)

A low molecular weight thiol protease inhibitor (12,500) purified from human placenta has been characterized in detail. Human placental cystatin (HPC) was found to be stable in the pH range 3.0–9.0 and temperature stability was between 40 and 100 °C. It does not have any disulphide groups and carbohydrate content. There was no cross-reaction between anti-HPC serum and other purified cystatins like HMW kininogens isolated from sheep plasma and phytocystatins isolated from Phaseolus mungo. The kinetics of inhibition of HPC was studied with ficin and bromelain and the comparison was made with our already reported results with papain. The respective Ki values obtained for ficin and bromelain are 8.4 × 10−8 M and 9.5 × 10−8 M, respectively, whereas the value for papain was 5.5 × 10−8 M. The values of association constants (Kass) for ficin and bromelain were 2.9 × 103 and 8.6 × 102 M−1 s−1, respectively, however, the value for papain was 3.4 × 104 M−1 s−1, the respective dissociation constant values for ficin and bromelain were 2.6 × 10−5 and 2.1 × 10−5 s−1, respectively, and the value obtained for papain was 2.3 × 10−5 s−1. These kinetic parameters taken together along with t1/2 values and IC50 values imply that HPC binds more effectively to papain, then ficin and least with bromelain. Far-UV-CD analysis shows that HPC has 21.08% α-helical structure and significant amount of β structure. Near-UV-CD spectra of HPC show positive peak at 280 nm indicating significant amount of tertiary interactions. The partial amino acid sequence analysis shows that HPC has highest sequence homology with chicken cystatin and Gly residue is present at position 11 rather than at conserved position 9, which has also been reported for human stefin A structure. The hydropathy plot of 1–30 amino acid residues shows that most amino acids of this stretch are present in the hydrophobic core of the protein. Owing to low molecular weight, absence of disulphide bonds and carbohydrate content HPC can be placed in type I cystatin family with some resemblance to chicken cystatin as shown by CD studies and amino acid sequence analysis.